IWIC Worldwide Tropical Weather Discussion - August 27 2006 - 23:00 UTC
Ernesto strengthened into a minimal hurricane earlier this morning but has since weakened back to a moderate tropical storm. This weakening can be explained by land interaction with the mountainous terrain of Hispaniola which has disrupted its inflow and resulted in a rather assymetric satellite appearance. Nonetheless, spurts of deep convection continue to flare around the center. An upper-level anticyclone present over the system is enhancing outflow, particularly in the southern quadrant where land interaction is not an issue. Since Ernesto is moving further away from Hispaniola, and should develop deeper convection over the next 12 hours in diurnal maximum, there will probably be reintensification. The chance to strengthen will be short, as later tomorrow Ernesto will close in on eastern Cuba. Ernesto is expected to traverse the long portion of Cuba for at least 24 hours, so even though it will be able to draw in moisture from both the south and north, there will likely be further weakening. In about 48 hours from now, Ernesto should emerge from Cuba and, once again, intensify over the Florida Straits before moving inland somewhere in southern Florida a day later. Ernesto is expected to regain hurricane status before making landfall in Florida. However, major hurricane intensity is not very likely owing that the storm has been and likely will further be disrputed by land interaction. Historically, storms that have suffered from traversing Hispaniola and eastern Cuba take a long time to regain intensity, if at all. This is not saying Ernesto will not restrengthen, but rapid intensification immediately after clearing land does not seem likely. Nonetheless, it is urged to err on the side of caution and prepare if you are in southern Florida.
Model guidance has become tightly clustered on the aforementioned track. The leftmost outlier is the GFDL, which takes Ernesto into the Florida Panhandle on Day 5. This forecast seems unreasonable given the magnitude of the shortwave trough which apparently is not accurately represented in the GFDL. Furthermore, the GFDL has been swaying back and forth with this storm and so one run, especially an outlier, will not be given much weight. The easternmost models such as the NOGAPS still take it through Florida but more on the east side. So at the very least, every single model takes Ernesto through Florida, and there is no reason to believe otherwise given the synoptic steering pattern. The 2100Z track provided by the NHC/TPC seems reasonable, although it may even be slightly further east given the model consensus is still east of their track. Ernesto has had a tendency to veer right of the model guidance which gives this idea more credence.
All interest in Haiti, Cuba, Bahamas, Florida, and even the northern Gulf coast and southeast coast should closely monitor the progress of Ernesto. This has been, and remains, a rather difficult storm to forecast in regards to both track and intensity.
August 27 2006 - 02:05 UTC
The discussion is brief and late due to time constraints this evening.
There are still serious questions regarding Ernesto's future track. A model consensus has tried to form over the eastern Gulf of Mexico today. The majority of models are indicating a landfall somewhere along the central and eastern US Gulf Coast. However, there is still concern over the possibility that these tracks are flawed. The upper ridging over the southeast United States remains strong, especially in the mid to upper levels. If Ernesto begins to rapidly intensify in the western Caribbean Sea within the next couple days, then it will be steered by mid to upper level flow over the northwest Caribbean. None of the global models depict rapid intensification in the Caribbean. That may be a problem. In other words, a stronger Caribbean hurricane could spell a more westerly track closer toward the northern half of the Yucatan due to the stronger ridging in the mid to upper levels. If Ernesto is more southerly and moving slower than progged by the models in 72 hours, then the upper trough that is forecast to weaken the eastern half of the Gulf Coast ridge may not be strong enough to recurve Ernesto. Such a scenario is being depicted by the BAM models, GFDN, and the WRF. So by no means is the western Gulf in the clear. A western Gulf solution will be favored as long as the global models do not depict recurvature as a major hurricane. Until then, they will remain suspect. Coastal Texas is still a viable target based on this analysis, and even extreme northern Mexico is not entirely out of the woods. With that said, all interests in the Gulf Coast should be on high alert. Ernesto is one of the more difficult storms to forecast.
As for intensity, it still looks very likely that Ernesto will achive major hurricane status, possibly before it even enters the Gulf of Mexico. The distance between the upper low near Cuba and the tropical cyclone continues to increase with time. Ridging aloft is steadilg increasing, and that explains why Ernesto is already beginning to gradually intensify. There are no inhibiting features that stand out over the next 3-5 days. Therefore, slow intensification into a minimal hurricane is likely over the next 24-36 hours, followed by potential rapid intensification once west of Jamaica. Ernesto should have little trouble becoming a Category 3/4 hurricane in the southern Gulf of Mexico.
August 26 2006 - 01:00 UTC
Aircraft reconnaissance flew into Tropical Depression Five and found tropical storm force winds. The surface circulation was exposed to the northwest of the convection and MLC when recon was investigating the system. But the NHC concluded that it was organized enough for an upgrade. So, we're now dealing with Tropical Storm Ernesto. The LLC still looks partially exposed tonight, but it is more difficult to know for sure without visible imagery. If the LLC is partially exposed, there are signs that the convection is attempting to spread over the center. If so, this would be an indication that the westerly shear is lessening very gradually.
The intensity forecast remains problematic. The NHC has shifted their intensity forecast upward slightly, and their discussions allude to the possibility that the intensity forecast may still end up conservative. Yesterday, we outlined the threat of a category 2 or possibly even category 3 entering the southeast Gulf. There is still a reasonable chance of that occurring. No one expected rapid intensification this soon anyway. The two intensity parameters everyone should be monitoring are the two upper lows north and west of the tropical cyclone; one between Cuba and Jamaica and the other located north of Hispaniola. The one near Jamaica appears most important at this time. This upper low is the main culprit behind the strong westerly shear that has been present over the Caribbean the last several days. Several global models have consistently indicated that this feature will begin moving west-southwest. The signs of that happening are highly apparent on water vapor imagery this evening. The shear axis will become more south-north orientated with time as this motion continues. As a result, ridging over Ernesto should gradually increase. Shear should not get any worse than it is now over the next several days, so there is little reason to believe Ernesto will dissipate into a wave axis. Therefore, Ernesto should have little trouble becoming a hurricane in the Caribbean Sea. A Cat 2/3 before the Gulf is still very possible if the distance between the UL and TC increases faster than forecast.
It's no wonder that everyone along the Gulf Coast is a bit concerned about Ernesto due to the model spread, and rightfully so. Furthermore, any Gulf landfall forecast is six days out at the very least. There is large room for major error. Everyone in the Gulf and west Caribbean should be monitoring Ernesto very closely. With that said, IWIC is not official and we can take a stab at some medium range scenarios. There has been a lot of talk about models shifting eastward today. But a lot of that is exagerrated. Yes, the Canadian did shift from Texas to the Florida panhandle. Yes, the 00Z ECMWF did begin to pick up on Ernesto and then turn it north toward the central Gulf coast. The 00Z and 12Z GFS runs also depicted a weak low, which is Ernesto, moving into the southeast Gulf and then recurving it into Florida after nearly stalling. All of the flaws in those runs can be discussed, but since half of them have shifted back toward the west already we're not going to waste any time.
The 18Z GFS made a huge shift from the northeast Gulf to the Bay of Campeche. One can make out a weak low (Ernesto) continuing westward and into the BOC using SLP charts. Such a track can be confirmed by observing the 850MB vorticity maps. It's no surprise that the tropical model suite (including the dynamical GFDL) has also shifted to the west after the latest GFS run. Furthermore, the UKMET is indicating a Texas threat. The European lost Erneso once again at 12Z, and it is not being incorporated in the forecast. The 12Z NOGAPS indicated a stall in the southeast Gulf, but that is because it is trying to determine if Ernesto will be caught by a weakness that will be passing over the southeast next week.
So, the model consensus actually suggests that Ernesto will make it west of 90W in the Gulf of Mexico. In addition, several of the models that show a bend toward the west still take Ernesto directly over Jamaica. However, an extrapolation between the official 5PM and 8PM EDT coordinates indicates that Ernesto could pass south of the island if the general movement remains the same. The current low to mid level steering flow over the central Caribbean remains dominated by easterlies. It is conceivable that the tropical cyclone could pass just south of Jamaica. If it does pass to the south, then the threat posed to the eastern Gulf would decrease even more so.
In summary, slow intensification is expected within the next 48 hours, but the pace is likely to increase thereafter. Ernesto should easily make it to hurrricane status in the northwest Caribbean. Interests in the Yucatan and Isle of Youth also need to acknowledge the possibility of Ernesto strengthening more than forecast, possibly into a Category 2 or 3. A track over or barely south of Jamaica and toward the northern Yucatan rather than Isle of Youth is favored. Once clearing the northern half of the Yucatan, the subtropical ridge over the west-central Gulf Coast may keep Ernesto on a WNW heading toward northern Mexico or coastal Texas. The good news is that interests along the U.S. Gulf Coast and northern Mexico have much more time to prepare while monitoring the storm's progress.
August 25 2006 - 00:00 UTC
Tropical Depression Five has formed in the southeast Caribbean Sea. Aircraft reconnaissance flew into the system this afternoon and barely found a closed surface circulation, but 35mph winds were present in pockets. The name Ernesto would be given to TD5 if it reaches tropical storm status. Latest recon reports indicate that TD5 is near tropical storm strength, with sustained winds nearing 39mph. This evening, infrared imagery shows convection spreading over the surface circulation; a sign of organization. If convection continues to persist directly over the center, then an upgrade is likely within the next 12 hours.
Low to moderate westerly shear over the eastern Caribbean is the only thing that makes an upgrade within 12 hours slightly questionable. Shear is only 15-20 knots, but that is often enough to inhibit strengthening. Nevertheless, the increasing organization on satellite makes a TS upgrade likely within 12 hours. There is little disagreement with the NHC forecast in this regard. Further strengthening should continue at a slow pace until the westerly shear begins to relax.
The biggest disagreement with the official forecast comes in during the three to five day period. The official forecast calls for TD5 to strengthen and eventually threaten the Yucatan Peninsula as a borderline TS/Cat 1 hurricane by day five, respectively. The SHIPS intensity model ramps the intensity up to 81 knots by Day 5, but the NHC took a more conservative approach due to the lack of aggression by some of the global models. However, we see what could end up being major flaws in some of the global model output.
First, we'll start with the 12Z run of the UKMET. The 850MB level UKMET maps show a tropical wave and associated low pressure (which is now TD 5) heading westward, and then merging with a stronger low pressure system in the southwest Caribbean. First of all, the stronger of the two systems is going to obviously be TD 5. Second, the progged low in the southwest Caribbean is actually part of the tropical low that is currently in the eastern Pacific, south of Nicaragua. The EPAC low is slowly drifting west, and it is highly unlikely that it will persist near Nicaragua for four days and then turn into Central America. This seems to be a common problem for the UKMET this year. The UKMET's forecast of TD 5's intensity should probably be thrown out due to all of the aforemention problems with the model.
Second, the 12Z run of the NOGAPS model seems to be suffering the same problem as the UKMET. In fact, NOGAPS is probably more flawed than the UKMET. No forecaster should even question taking the NOGAPS track literally today. The 12Z run shows TD 5 being completely absorbed by low pressure in the southwest Caribbean and the disturbance south of Nicaragua. If that pans out i'll never doubt a global model interpretation ever again. However, the 200MB-850MB shear forecast is somewhat alarming. As mentioned in the NHC discusssion, the upper low over eastern Cuba may continue to slide south and west. If that occurs, the westerly shear over the Caribbean will become more south-north orientated, thus enhancing ridging above the tropical cyclone. This is exactly what NOGAPS is progging.
Third, The 12Z and 18Z runs of the GFS continue to miss or weaken the low pressure system that is actually TD 5. Furthermore, a lot of the runs from the GFS/GFDL and tropical model family have been too far north, which would imply land interaction with the Greater Antilles. In reality, we're dealing with a tropical cyclone over the east Caribbean that is at a low latitude and is caught in the low to mid level easterly flow. There is no chance of interaction with the Greater Antilles (exlcuding the flat Isle of Youth, west tip of Cuba). Once again, this global model run should not be taken literally. On the other hand, the 200MB-850MB shear forecast is just as alarming as NGP forecast. The GFS shows the east Cuba upper low sliding south and west. As a result, upper level ridging increases over TD 5's path.
Finally, there's the Canadian model. The Canadian model is by far the most bullish global model. The CMC is progging rapid intensification (RI) once in the extreme northwest Caribbean. By Day 6, the model is indicating a major hurricane threat to Texas. The Canadian is routinely the most aggressive model, but it may carry more weight than normal this time around. The CMC is indicating that the upper air environment will become more favorable in TD 5's path, which is in agreement with the NGP and GFS upper air forecasts. The difference between the CMC and the remaining global models is that the CMC doesn't maintain the mythical intense low pressure system in the southwest Caribbean. The 12Z run of the ECMWF is not available online at the time of this posting.
The noticeable trend on water vapor imagery lends credence to the more favorable upper air environment as progged by the CMC, NOGAPS, and GFS. The upper low over eastern Cuba is in fact moving west-southwest. Shear remains strong over the western and central Caribbean, but the shear will relax with time if this trend continues. If the distance between the upper low and tropical cyclone is enough, then conditions could become very favorable for strengthening or possibly even rapid strengthening beginning in 3-5 days. That raises the possibility of a Category 2/3 landfall along the northeast tip of the Yucatan peninsula early next week. By the same token, we believe that the forecast of a strong tropical storm landfall along the northern Yucatan has the potential to be too conservative. Hopefully the NHC intensity forecast is correct, however. Nobody wants to see a major hurricane entering the Gulf of Mexico on the one year anniversay of Hurricane Katrina.
The track forecast is relatively straightforward. The subtropical ridge over the Atlantic extends well into the Gulf of Mexico in all layers of the atmosphere. There are no signs of a ridge breakdown. TD 5 should continue on a west-northwest heading and end up near Cancun by Day 5. Once in the central/western Gulf of Mexico, a secondary landfall over northeast Mexico or the Texas coast is likely. The Texas threat depends on how much the western periphery of the Gulf ridge weakens, and how strong the tropical cyclone is once clearing the Yucatan. Nobody should be overly worried in Texas just because of the Canadian model run just yet. All interests in western Cuba, the Yucatan Peninsula, and western Gulf should monitor TD 5 closely.
Tropical Storm Debby still has a good shot at becoming a minimal hurricane in the medium range, but there is no threat to any landmasses.
August 24 2006 - 00:45 UTC
Tropical Storm Debby weakened temporarily as convection became displaced south of the surface circulation. Upper level conditions appear favorable for further, slow intensification over the next 3-5 days. There is little disagreement with the NHC's call for Debby to become a minimal hurricane by the end of the forecast period. The central Atlantic trough that is forecast to recurve is already in place, and it will not lift north in the foreseeable future. Debby is little to no threat to Bermuda and there is virtually no chance of a U.S. threat.
The tropical entity of more significance is 97L. The wave approaching the islands continues to show signs of organization. A strong midlevel circulation intensified today, and banding features are more prominent. The system already has upper level ridging aloft, and the ridge should retrograde westward along with it. The southwest shear over the Caribbean Sea should continue to slowly lift north in response to the ridge's ever growing presence in the region. The SHIPS intensity model ramps the intensity up to 87 knots by Day 5, but more importantly the remaining intensity models are in agreement. The CMC is by far the most agressive of the global models, strengthening it into what appears to be a large hurricane crossing the Yucatan Peninsula. The other globals are less bullish, but all more or less pick up on a low pressure area moving through the Caribbean Sea during the forecast period. It is known that global models often have a hard time handling tropical cyclogenesis, especially with storms around this area. Thus, their exact strength forecasts are not to be taken literally. Nonetheless, the currently and predicted favorable upper-level conditions and the system's organization clearly hint in a future as a classified tropical cyclone. It is not unreasonable to believe that 97L could become a hurricane in the Caribbean Sea over the next few days.
The forecast track of 97L is rather straight-forward. It is currently being guided west-northwestward along the southern flank of the subtropical ridge. The ridge position and angle is expected to remain in place, and therefore this general direction should persist. The GFDL and other GFS-based tropical models are likely too far right in their solutions. These models are not too trustworthy in this kind of situation, and in recent years they have had a noticeably right bias for Mean Development Region systems. The favored path is of a continued west-northwestward movement, taking the storm through the Caribbean Sea. By Day 5 or so, it should be approaching the Yucatan Peninsula. At the very end of the period, the system may be entering the southwestern Gulf of Mexico. If the latest model progs are correct, once in that area, there will not be any significant steering mechanisms to turn the cyclone northward. This is becoming rather speculative, however.
The remainder of the Atlantic is inactive. Hurricanes Ileana and Ioke are both spinning in the Pacific Ocean, but neither are a threat to land.
August 23 2006 - 00:45 UTC
There are several areas in the Atlantic Basin worth discussing this evening. The first is obviously Tropical Depression Four. TD4 continues to show signs of organization, and convective activity has recently increased. Dvorak satellite estimates are steadily rising, and a tropical storm will likely be declared tonight or tomorrow. Steady intensification should continue through at least most of the period, but there is some question as to whether it will reach hurricane status. There is a lot dry air and the Saharan Air Layer dominates the region the depression is heading into. Furthermore, the upper trough that will allow for recurvature well east of Bermuda may enhance southwest shear over the cyclone down the road. Either way, there is little to no threat to the United States or any other landmasses.
Further west, we're still dealing with a persistent area of low pressure over the Yucatan Peninsula. It started over the southwest Caribbean a few days ago, and its now turning west toward the southern Gulf of Mexico and Bay of Campeche as forecast. The convection that fired late last night was strong enough to generate another midlevel circulation. The MLC was visible throughout the daylight hours on visible imagery, and it moved into the Yucatan Peninsula this evening. Land interaction with the Yucatan and mainland Mexico soon after may greatly hinder development, but there is still the possibility of TC formation if it ends up over open waters for 24 hours. Upper level conditions over the southern Gulf are marginal at best due to the mid to upper trough situated over the central and northern Gulf. However, there is a small, developing ridge over the Yucatan, and if it spreads northwest into west Gulf upper air winds may briefly become conducive. All of the global models lose the low level energy over the Yucatan, but several weak Bay of Camepeche tropical cyclones have developed and then treked into MX with little warning in the past. Therefore, interests between Poza Rica and Coatzacoalcos are advised to keep up with weather reports over the next 36 hours.
There is additional trouble aiming for Central America in the medium range. The tropical wave near 47W has been monitored for any signs of development for several days now. The Saharan Air Layer and marginal upper air conditions have kept the system in check. However, several global models are now depicting tropical cyclone development from this feature in the Caribbean Sea. There is weak upper ridge ridging over the wave now, but the ridge is being suppressed by a stronger upper trough over the southwest and central Atlantic. The ridging will likely continue to be supprssed to some extent until the wave enters the central/western Caribbean. At that time, one of the upper lows associated with the trough are forecast to lift north, allowing for the ridge to expand over the warm waters of the west Caribbean. The west Caribbean is where several of the globals really become aggressive in 5-6 days. If a tropical cyclone does form, it is likely bound for the Yucatan Penunsula due to the low to mid level subtropical ridging over the central Atlantic and the secondary ridge over the Gulf Coast.
August 22 2006 - 00:00 UTC
Tropical Depression 04L has developed southeast of the Cape Verde Islands. The system displays a large, healthy upper-level pattern. A very strong equatorial outflow channel is noted, along with a decent poleward channel. Convection is spotty and not consolidated under one solitary area, though it does seem to be increasing over the center this evening. Wind shear is favorable for strengthening, and is forecast to be over the next several days. The main limiting factors will be the Saharan Air Layer and later in the period, cooler ocean temperatures. However, the SAL does not seem to be significantly affecting the system at this time given its impressive satellite structure. Based on this, slow to steady intensification is expected over the next several days. The cyclone very well may become a minimal hurricane in the forecast period. The track should remain west-northwestward until day 5-6, when a turn northward could occur as it approaches a trough.
Scattered convection has continued to flare in association with a tropical wave in the western Caribbean Sea. A mid-level circulation developed yesterday but is less apparent today. The disturbance remains under marginal conditions due to the presence of an upper-level low over eastern Mexico. This is inducing shear, but is also helping to enhance convection in the first place. Model guidance continues to ignore this feature. However, conditions could still become a little more favorable over the next few days as the aforementioned upper-level low drifts further away. Thus, this area will be watched. The future track we have outlined in the past few discussions remains the most likely. The ridge of high pressure over the central Gulf coast is showing no signs of budging, and therefore this disturbance should slide into the Bay of Campeche and stay in the western Gulf of Mexico. The west Gulf coast should still monitor the tropics just in case this system becomes a threat.
Hurricane Ioke in the central Pacific continues to quickly organize, and is now a major hurricane. Further strengthening into a category 4 appears likely, followed by gradual weakening as curves towards the northwest. The West Pacific is pretty quiet, notwithstanding a few marginal Invests.
August 21 2006 - 00:00 UTC
The tropical low the models have developed in the southwest Caribbean Sea is now apparent on satellite imagery. A cluster of convection developed just after midnight and they expanded through the afternoon hours. Convection has weakened some heading into the evening, but a midlevel circulation is now visible. The series of upper lows north and northwest of the disturbance are still causing a lot of westerly and southerly shear, so no development is expected in the short term. While none of the models indicate significant development, there is some chance that upper conditions could gradually become more favorable. So the disturbance will be monitored for any signs of persistence or organization over the next several days. A general track toward the southwest Gulf of Mexico or Bay of Campeche is still forecast. The low to mid level steering pattern is still being dominated by the subtropical ridge in the central Atlantic and a secondary ridge over the central Gulf Coast. Both high pressure areas support a west-northwest motion into the Gulf and eventually Mexico or Texas in the medium range. Interests in the Yucatan Peninsula and western Gulf should simply keep an eye on the tropics heading into the new week just in case the system organizes.
The pattern in the central and eastern Atlantic remains complex. The first tropical wave located near 35W is now intertwined in the Intertropical Convergence Zone. The strong 200MB easterly jet is increasing divergence over the wave and ITCZ, thus enhancing the convection. However, the wave will not be able to organize into a closed surface low until it separates from the ITCZ and lifts north. That remains to be seen. But as mentioned yesterday, this wave may simply be the frontrunner. There is a stronger tropical wave about to exit Africa within the next 12 hours. This new wave will be monitored closely as it does have support from the GFS and there is some upper ridging near the Cape Verde Islands. Unless it dips to the south after entering the Atlantic, it shouldn't have a problem separating from the ITCZ like the first one.
August 20 2006 - 00:45 UTC
Two areas continue to be monitored for signs of tropical development in the medium range; the central/eastern Atlantic and western Caribbean/southwest Gulf of Mexico. For continuity, we'll once again tackle the Gulf first. Several global models have developed a low in the southwest Caribbean and shown it entering the southern Gulf within four days. The disagreement between the models yesterday started once the progged low entered the Gulf in the guidance. Would the low follow the CMC/NGP solution and continue northward toward the Florida pahandle along a weakness in the Gulf Coast ridge. Or would the ridge persist and keep any low pressure area confined to the western Gulf as supported by the GFS, ECMWF, and UKMET? We sided with the western Gulf solution, and it has become a done deal today. The broad low pressure area that developed in the southwest Caribbean today will slowly spread northwest across Central America an enter the Bay of Campeche and southwest Gulf toward the early to middle part o the week. The CMC has shifted dramatically to the west, and it now places the low over southern Texas vs. the Florida panhandle in the medium range. NOGAPS still wants to split the energy and send some toward Florida, but that solution looks even less likely today.
The more westerly solution means that any broad low pressure area will be over land for a good while at the very least. In fact, the GFS and UKMET now keep most of the 850MB energy over Central America and Mexico instead of the southern Gulf. However, there is still an outside chance that a weak low could still form in the extreme southwest Gulf. Just look at the ECMWF and CMC solutions. Both models show a suspect low over open waters in 3-4 days. The potential for development at this point is low, but if development were to occur there would be a threat to northern Mexico and coastal Texas. The persistent easterly flow over the northern Gulf will reverse to a southerly flow over the west Gulf as the ridge weakens.
The potential for development is higher in the central and eastern Atlantic at this time. The east Atlantic wave and the wave over western Africa will be heavily monitored over the next several days. The global models appear to be slowly trying to figure out which wave will become most dominant. It turns out that the first wave (now approaching 30W) may be the frontrunner, and the TW about to exit the coast may play a large roll in what the models are trying to develop. Such an idea does make sense after observing the upper air pattern. The first wave exited the coast at a very low latitude. Thus, the 200MB Tropical Easterly Jet is enhancing the easterlies over the system, and development is not occurring. On the other hand, the secondary wave is going to enter the Atlantic at a perfect latitude for upper ridging. A large anticyclone aloft has formed just south of the Cape Verde Islands within the last 24+ hours. Development would likely be underway had the first wave exited Africa north of 10N. The 18Z GFS, 12Z ECMWF, and 18Z NOGAPS do indicate development, and the Canadian isn't exactly a polar opposite. The UKMET hasn't indicated development much at all this season and that trend continues. Slow development is expected in the central and eastern Atlantic heading into the new week.
If a tropical cyclone does form over the east Atlantic within the next few days, a turn toward the north may quickly commence. The GFS and ECMWF models show a shortwave trough temporarily weakening the subtropical ridge and allowing for a brief northerly turn. The trough can be observed on water vapor imagery near approximatelly 20N, 35W. But the trough will lift north, and the subtropical ridge will once again takeover. Thus, a turn back toward the west is highly likely. The bottom line is there should be a west or west-northwest moving tropical cyclone near 20N, 45W by next weekend. Surely the East Coast threat hype would begin by then, but there is still a reasonable chance of recurvature over the west Atlantic. A longwave trough will still likely be located south of the Canadian Maritimes in seven days. Whether this trough will amplify enough to allow for recurvature is unknown at this time.
August 19 2006 - 01:15 UTC
The lull is about to end, and there's even more evidence now than there was in early August (keep in mind an invest nearly developed near the LAs Aug 9-10). We're heading toward the climatological peak, the North Atlantic Oscillation is neutral with a negative bias, the enhancing Madden Julian Oscillation is just days away from entering the western half of the basin, and several models agree that development is likely in the central Atlantic within seven days.
The first area of interest is the western Caribbean Sea and southern Gulf of Mexico. Some models are hinting at development in the Gulf of Mexico next week; the Canadian model is leading the charge. The CMC develops a broad area of low pressure northwest of Colombia within 36 hours. It then takes the low northwest toward Cancun and into the southern Gulf within 72-96 hours. By day 6, the model shows a tropical storm landfall between Grand Isle and Pascagoula. Due to the ridging currently in place over the central Gulf Coast, upper lows east of Florida are spinning westward and across the central Gulf. One of these westward moving upper lows is supposed to enhance the southerly flow over the western Caribbean and weaken the eastern flank of the Gulf Coast ridge once west of 90W. The NOGAPS is similar, but the southerly flow is diverted more toward the Florida panhandle because it wants to develop another low to mid level low over the east Pacific and El Salvador.
The GFS, UKMET, and ECMWF models show a varying solution, however. The GFS has the Gulf Coast ridge sliding west through 72 hours, but it begins to return to its initial position by Day 5. Such a pattern would imply a west Gulf/Bay of Campeche storm (if something develops) and an eventual path toward Texas or Mexico. The 12Z GFS progged a tropical storm in the Bay of Campecehe at 12Z, but the less reliable 18Z run backed off. The UKMET is similar to the GFS, but it isn't as bullish. Finally, the ECMWF keeps the broad low area confined to the southwest Gulf through Day 5 as well. The low pressure area is less discernible in days 6 and 7, but the 500MB ECMWF maps show a strengthening ridge over the central Gulf Coast.
So, we have the GFS, UKMET, and ECMWF indicating a broad low in the southwest Gulf while the Canadian and NOGAPS are hinting at a low or tropical storm threat to the central/northeast Gulf Coast. The consensus is leaning to the left, the Canadian has a right bias in the Gulf, and the mid level steering pattern has been consistent all season. Therefore, we believe that the west Gulf solution is more reasonable at this time. A more westerly track implies more land interaction and less time over water. The upper low over the west Caribbean will lift north, but any surface low will still likely have to contend with the ULs heading west over the central Gulf. With all that being said, it is still too early to know how much potential this progged low has, but some development in the southwest Gulf next week cannot be ruled out.
The next area of interest is the central and eastern Atlantic. A strong tropical wave recently exited the coast of Africa. Convection and the Tropical Easterly Jet to the south is strong, but the Saharan Air Layer and easterly shear to the north is impeding immediate development. Several models show the upper trough to the north lifting out, thus the level of ridging over the tropical wave is supposed to increase with time. Effects from the Saharan Air Layer would also be mitigated. Such a scenario is supported by the last several runs of the GFS, the latest 12Z run of the ECMWF, and the latest 18Z run of the NOGAPS. There is little doubt that this tropical wave will develop into a tropical storm or more within the next seven days. Increasing moisture, lessening SAL/shear, NAO, MJO, and climatology are all supportive of development.
Any east or central Atlantic tropical cyclone formation will not impact any landmasses within the next seven days. But changes in the steering pattern and its short term track will of course have long term implications. The 18Z GFS develops the tropical wave into a storm and then keeps it on a westward course through 96 hours. But once it hits 40W at 96-102hrs, the 500MB maps show the ridge sliding to the west and being replaced by a trough over the east Atlantic. As a result, a northerly turn is progged within 4-6 days. However, the trough is going to lift out, and that is why the GFS shows a turn back toward the west along 20N by Day 7. The 12Z ECMWF and 18Z NOGAPS runs are vey similar. We'll likely be dealing with a west or west-northwest moving tropical cyclone located near 50W and 20N by the end of the medium range. If the system gains that much latitude within the next seven days, then the threat to the Lesser Antilles will have decreased to some extent. If it develops slower than the models are indicating, then it may miss the interaction with the first mid to upper trough, which may lead to a greater threat. But there is no reason to believe that at this time, and we have plenty of time to monitor it. Assuming A TC is near 20N/50W by Day 7, there is a very real possibility of recurvature before the Bahamas or United States. A second, strong mid to upper trough is progged to be positioned east of the Mid-Atlantic States by Day 7 according to both the ECMWF and GFS. As long as the trough lingers by Days 8-10, then recurvature is likely. Again, this is highly speculative and we have plenty of time to watch it.
August 18 2006 - 00:00 UTC
93L off the East Coast is being sheared apart. Tropical development is not expected. The upper low over the central Gulf of Mexico is showing no signs of mixing to the surface, so no development is expected there either. The GFS wants to develop a tropical wave about to enter the Atlantic from Africa, but there is no other model support at this time. Meanwhile, the Canadian model shows a broad low moving northwest and into the Gulf by the medium range. Again, this is the only model currently suggesting such a scenario. However, when the enhancing MJO does enter the basin there will be a burst in the tropics. Despite the hyped invests that have been sheared apart, the main parameters remain conducive for an above average (avg = 10/6/2) season. For example, there is no El Nino, and subsurface temperatures in the equatorial Pacific indicate that a warm surge at the surface is not imminent.
August 17 2006 - 00:00 UTC
93L came very close to being upgraded to tropical depression status this afternoon. Aircraft reconnaissance found a surface circulation surrounded by pockets of 25 knot winds. However, deep convection was lacking and it was suppressed to the southwest of the center. There is still a chance for 93L to develop into a depression in the short term, but those chances are decreasing with time. A mid to upper trough extends from the Mid Atlantic states to the southeast, and it is moving southeast with time. Ahead of the boundary are increasing westerly winds aloft. 93L will likely be sheared apart within 36 hours as interaction with the westerly winds increase.
The MCS that was over the Gulf of Mexico yesterday evening dissipated.
August 16 2006 - 00:00 UTC
There are a couple areas of interest in the Atlantic basin this evening. First, 93L Invest persists off the East Coast. Little change in organization has been observed over the last 2-3 days. There is a low to surface circulation, but convection remains limited and upper conditions are currently marginal at best. The parameters behind the marginal conditions are two upper lows; one over central Florida and the other east of Nassau. As 93L heads northward along the western flank of the low level Atlantic ridge conditions may become slightly more favorable. The effects of the upper lows will become less of an issue. However, there is also a mid to upper trough diving out of the Northeast USA. Depending on the timing, the trough may induce westerly shear over 93L once near or north of the outerbanks. Some slow development is still a possibility, but there is little threat to the East Coast regardless. Worst case scenario is a minimal tropical depression or storm brush for coastal North Carolina before recurvature out to sea.
The second area of interest is the flare-up of convection south of New Orleans and Mobile. There is no low to mid level circulation at this time. Therefore, it is nothing more than a Mesoscale Convective System. MCSs must persist for at least 12-24 hours over water to have any chance of developing into a tropical cyclone. If the MCS persists overnight, then its chances of tropical development increase significantly. Upper air conditions are very favorable over the central Gulf of Mexico. An anticyclone aloft has developed between the upper low over Florida and an upper trough over Texas. Overnight persistence is key. Convective enhancement by means of barotropic instability will be hard to come by without the presence of a surface circulation. So, any convection overnight would likely have to be shear-induced, and that in itself is also a problem as shear is light. But there is a weak upper low just north of the Yucatan peninsula, so there is some potential. This system will have to be re-evaluated tomorrow. As for the track, there is no other scenario beside a westerly path into Texas or northern Mexico. The is ridging in all layers over the central Gulf Coast, and it will not shift or weaken in the near future. This ridge will push the system into TX/MX, and that is already happening now.
In the long term, the enhancing Madden Julian Oscillation will be entering the western half of the Atlantic basin in approximately one week. This should lead to additional tropical development during the latter half of August and early September.
August 14 2006 - 00:00 UTC
The tropical Atlantic is still generally relaxed, though there are some hints of life tonight. Two Invests, 92L and 93L, were declared earlier this morning. 92L is a tropical wave passing through the Windward Islands. Although it had a small and impressive mid-level center this morning, it has since weakened with convection activity more linear and spread out. Upper-level conditions are forecast to become unfavorable in the central and western Caribbean Sea over the next few days. Thus, development from the wave is unlikely as it tracks west to west-northwestward.
93L is a broad, cold-core area of low pressure situated east of Florida along the tail end of a dying frontal boundary. A ridge of high pressure is building in north of the low, essentially trapping it from escaping seward with its parent front. This type of setup is akin to what led to Tropical Storm Beryl about a month ago. Indeed, global models all show the low becoming warm-core as it lingers under the ridge and sheds itself from the frontal characteristics. Excluding the NOGAPS, all of the globals seem to literally indicate a marginal tropical cyclone. Seeing how they barely picked up on moderate storm Beryl, such intensity outputs should not be taken at face value, but the fact that they agree on something is worth noting. One big inhibiting factor will be an upper-level low soon forecast to pinch off the upper-level trough stretching from the Carolinas eastward. This upper-level low is expected to position itself directly west of the low over the next 72 hours, which will help funnel dry air into the system and limit most convection activity east of the center. After 72 hours, the upper-level low should then scoot westward towards the Gulf of Mexico. If the system is still around at this point, it should be able to become better-organized.
That assumption leads us to wondering the future track. The 12Z global models differ somewhat on where any low will track. All of them show little movement in the next 48 hours. Beyond that time frame, the CMC, GFS and ECMWF track it slowly northward, taking it very close to the Outer Banks around Day 4-5. The CMC then slows it to nearly a halt, followed by a drift south-southwestward close to the Carolina coast. The GFS slows it near the Banks as well but then turns it south-southeastward, gradually increasing distance from the coast through the period though by Day 6 it is in a setup that would lead to some kind of westerly component. The ECMWF is similar to the GFS through the period and on Day 7 actually does have it turning westward again under a rebuilding ridge of high pressure to the north. Meanwhile, The NOGAPS and UKMET take the low slowly northwestward, with the NOGAPS bringing it inland Florida and the UKMET bringing it inland South Carolina, both around Day 4.
Which model camp is correct? It is too soon to say for sure. Given model history this season, not to mention the overall long-term steering pattern, the early favorite is the CMC/GFS/ECMWF solution, keeping it predominantly offshore through the period. NOGAPS and UKMET have both had left biases this year, making their solutions seem even more questionable. Even if the system remains offshore in the next 6 days as the first few models suggest, it could still come close to the Outer Banks. Even more importantly, the 12Z GFS and ECMWF hint that will turn back westward after Day 6 towards the southeast coast. The 18Z GFS actually has it catching a trough after skirting the Outer Banks with no loop or stalling at all, though it remains to be seen if this is just a typical fluke 18Z run or the start of a trend.
Bottom line: Slow development into a weak tropical cyclone is very possible over the next few days off the Florida east coast. A significant system is unlikely, however, given the region and conditions at this time. A general drift northward should commence in the coming days, but if it misses the connection to the trough later in the week, it is not unreasonable to assume we will still be tracking this offshore the southeast coast a week from now.
Elsewhere, the Eastern Pacific may also awaken in the next couple of days. Model guidance suggests at least one significant tropical cyclone will form in the forecast period. The ECWMF actually develops two probable-hurricanes in the East Pacific. If anything does form, it will likely move northwestward well away from land. In the West Pacific, Tropical Storm Wukong is threatening to affect southern Japan over the next few days as a minimal typhoon. A new tropical depression has developed to its south and is forecast to move north-northeastward with gradual strengthening.
August 10 2006 - 00:00 UTC
The wave axis associated with 91L outran the mid level circulation. The original MLC later on dissipated and a new one failed to form along the wave axis. Interaction between the wave axis and the weakening upper low to the north was underrated. There is no longer a tropical cyclone threat from 91L. A tropical wave approaching the Lesser Antilles and a weakening frontal boundary near the East Coast will be monitored, but there are no signs of development in these areas at this time. A one or two week lull is possible before a significant burst of activity occurs once the MJO returns. Just look at the WPAC.
August 09 2006 - 23:00 UTC
The strong tropical wave in the Mean Development Region, otherwise designated as Invest 91L, is approaching the Windward Islands. A well-defined mid-level circulation developed under new deep convection this morning in association with this wave. However, aircraft reconnaissance flew into the system and could not close off a surface circulation, delaying an upgrade to a tropical storm. Furthermore, the deepest convection is out-running the mid-level low. This low is expected to dissipate overnight, leaving the best chance for a surface low to form further west under the fast-moving convection.
The wave is still being moderately sheared by the upper-level low north of the Virgin Islands. This low has flattened since yesterday and is forecast to continue to drift westward and slowly elognate southwest-northeast. The precise speed and position of this low over the next several days will be critical as far as the conditions will be surrounding the disturbance. In the next 36 hours, the upper low will be moving at a slower clip than 91L. This will allow 91L to position itself more to the south of the upper low as opposed to the south-southeast. Such an alignment will subject 91L to stronger shear in the short term. While this is not exactly a favorable setup for significant intensification, the increased diffluence aloft will likely flare stronger convection in the middle of the disturbance, which in turn could aid the formation of a closed low. Since such a setup may very well occur in tandem with the next diurnal maximum, it is conceivable that a tropical storm will form tomorrow. Even if this occurs it will still be in a hostile environment for strengthening.
Beyond 48 hours lead time, conditions aloft should become much more conducive for strengthening. At this point, the upper-level low is expected to separate itself enough from 91L that any role it plays on the intensity will be via forcing an upper-level ridge over the system. This would be favorable and would increase outflow, especially in the northern quadrant. If the system becomes a tropical cyclone in the near-term and approaches the western Caribbean Sea several days from now, conditions may be conducive enough to allow for a hurricane. This is consistent with SHIPS and DSHIPS, both taking it to category 1 hurricane status in the forecast period. Although only minimal weight should be put in those models, if conditions evolve as planned, is it not an unreasonable assumption.
Having said that, the intensity of 91L from Day 2-6 is logically dependant on its location. The system is currently accelerating west to west-northwestward under a strong mid-level ridge. The extra speed can be attributed to a strong easterly jet from the Saharan Air Layer. Since it will not fully escape the layer until it reaches at least the central Caribbean Sea, the fast pace is expected to continue until then, followed by a more relaxed speed. The mid-level ridge to the north is not forecast to budge, however. Global model guidance maintain the strong ridge, which will take any tropical cyclone west to west-northwestward through the Caribbean Sea. By Day 5, the probable tropical cyclone should be approaching the Yucatan Peninsula. Beyond this point, it is too uncertain to say where it will head. If it makes it into the Gulf of Mexico, as of now there are no signs that it would shift from the west or west-northwestward motion. The final land point would therefore be Mexico or possibly Texas, but I'm getting too far ahead here.
Bottom Line: A continued west to west-northwestward track through the Caribbean Sea is expected over the next 5 days. The system should still be on that heading towards the Yucatan Peninsula early next week. Conditions will not be favorable for strengthening during the next 2 days, but there may be strong enough convection bursts to allow for a tropical storm to form. Once in the central Caribbean Sea in 2-3 days, conditions will become and remain favorable for at least slow intensification. An eventual hurricane is not out of the question, provided it survives the next couple of days.
August 09 2006 - 01:00 UTC
Convection has increased over the area of low pressure in the east Atlantic this afternoon. This is an indication that surface convergence has increased. However, morning Scatterometer wind data revealed that the surface circulation weakened overnight. The circulation became broader and it may have even opened up, at least temporarily. On the other hand, afternoon visible satellite imagery showed that there was a surface circulation that was at least nearly closed. If convection begins to persist over the low center, then the surface circulation will likely close off again. But there are still some factors inhibiting tropical development. Strong southerly winds in the upper levels are still shearing convection away from the center. The upper winds are being enhanced by a ridge to the east of 91L and an upper low to the northwest. There is also a lot dry air surrounding the system, and as long as the shear remains 20-30+ knots it will still be a problem.
Upper level winds are still forecast to become more favorable for development, but it may take some time. The upper low to the northeast of the Virgin Islands is already showing signs of filling in and weakening. The global models expect it to drift north and west if it doesn't dissipate altogether. Also, the southeast shear being enhanced by the western flank of the anticyclone near Africa is lessening with time. If the distance between the upper low and 91L increases or the impact of southwest shear over the low decreases within a few days, then a new upper anticyclone may develop directly over the surface low. That would result in a much more favorable environment for steady intensification. Again, this may take several days so any development in the short term would likely be slow to occur. Dissipation or no development at all still cannot be ruled out beforehand either. However, a tropical depression or storm threat to the Lesser Antilles is more conceivable than dissipation at this time.
As mentioned last night, there is little data to suggest that recurvature or a turn to the north is likely prior to 75W. All of the dynamical models show low to mid level ridging remaining north of the system through the period. The ridging will force a west or west-northwest heading through the period. Therefore, 91L Invest is likely to effect the Lesser Antilles in some form or another. Since conditions are forecast to become gradually more favorable and 91L isn't terribly far away from depression status, the system may pass through the islands as a depression or tropical storm. Once in the Caribbean Sea things become a bit more interesting. Most of the global model guidance shows little shear in the Caribbean through 120 hours, and that is reasonable given the ridging to the north. This is a big if, but if 91L maintains itself over the next 36 hours, then we could be dealing with a strengthening tropical storm under favorable conditions aiming for the western Caribbean in a few days. That is if it doesn't dissipate soon, so this aspect of the analysis will have to be evaluated on a day to day to basis.
The "where are the hurricanes"? question is beginning to arise in the media and on the internet. "Only" three tropical storms, no hurricanes, and the lowered numbers from CSU and NOAA recently triggered the curiosity. Three named storms by early August is no indication of anything. The 2006 IWIC forecast called for only 1-2 named storms by the end of July, yet we still anticipated 14 named storms, 8 hurricanes, and 4 major hurricanes overall. For those wondering, 14/8/4 is still well above average. The latest numbers from TSR, CSU, and NOAA are in general agreement. We have no reason to doubt our numbers at this time. The season is evolving as forecast, there are no major inhibiting factors, and the peak of the season is will over a month away. El Nino will not severely suppress activity this season. We're heading into mid August and conditions remain neutral with only a warm bias. Sea surface temperature anomalies are still fluctating in all ENSO regions, the Southern Oscillation Index has temporarily flipped positive every few weeks, and subsurface temperatures are not reflective of an impending major El Nino by October. Even if weak El Nino conditions are classified by the Climate Prediction Center prior to Novemeber, the warm Atlantic cycle will maintain the likelihood of an above average season.
August 08 2006 - 00:00 UTC
If 91L Invest is going to develop into a tropical cyclone it's probably going to be slow to do so. Intensification into a tropical depression tonight or tomorrow is somewhat doubtful. Meanwhile, a low level circulation has become apparent, but the amount of surface convergence remains limited. Thus, without any shear in place it will be difficult for 91L to generate its own convection. However, if the LLC remains intact and conditions remain marginally favorable, then slow development will be possible. Currently, there is moderate easterly shear spreading over the low. Therefore, the limited convection is being shunted to the west of the center.
The easterly shear will be replaced by southwest shear due to an upper low centered near 20N/55W. Southwest winds will be enhanced between the upper low and the western flank of the anticyclone over the east Atlantic. If the timing is right, the diverging winds aloft between the two upper features may enhance convection over the low as it heads west, which is something it desperately needs. Thereafter, a lot of the global models suggest that the upper low will weaken or lift out. The low would then need to maintain itself after geting a kickstart essentially. None of the global models show this panning out, but it is difficult for them just to initialize such a weak and small low level vortex. The models don't always pick up on this type of development, and Chris is a good example. So, slow development is very possible but it may take a few days.
All indications are that 91L will head into the Caribbean Sea. The subtropical ridge is strong in all levels, and the models suggest that ridging will remain north of 91L through the period. 91L will continue to move westward at a decent clip at least until it gets into the eastern Caribbean. I see no reason why 91L would recurve or turn north east of 75W, but lets not get too far ahead.
There are no other areas of interest in the Atlantic basin at this time.
August 07 2006 - 00:00 UTC
Since Chris is history we can finally focus on the threat of development in the east Atlantic. The discussions over the last two weeks have noted that conditions would become favorable for tropical development in the east Atlantic between Aug 1-10 and that a TC would likely form. Easterly shear remained a little stronger than forecast, so upper winds could be better. However, conditions were favorable enough for 91L Invest to be officially classified this morning. The easterly shear is very apparent on satellite imagery as all of the convection associated with 91L is west of the mid level center. Scatterometer wind data confirms that there is no surface circulation at this time. The easterly shear should lessen as the low heads west and away from the ridging over the northeast Atlantic. The low will then become more susceptible to being sheared by southerly diving upper troughs, but it certainly won't develop within the easterly shear zone.
The biggest question regarding development potential is how much energy will 91L leave behind in the east Atlantic? Most of the global models indicate that the region of greatest surface convergence and low pressure will remain in the east Atlantic, thus any low that detaches itself and continues westward may lose some of its punch. With that said, it is not uncommon for global models to have trouble with small scale low pressure area. Slow development is possible as 91L heads into a more condusive environment.
August 04 2006 - 01:00 UTC
We're seeing another round of shear-induced convection southeast of the exposed surface circulation this evening. The upper trough responsible for essentially destroying the tropical storm last night is inching closer to the tropical cyclone, thus making it easier to set off convection. As long as shear remains high, don't expect this convection to fully wrap around the center. Despite what has occurred over the last several days, some online discussions include talk of "conditions becoming better for development" and that development or regeneration is likely. The high SHIPS intensity forecasts also still seem to be getting a lot of attention despite the fact that it totally missed the decoupling last night. We respectfully disagree, and argue that the proof can be seen on water vapor imagery. It is unlikely that upper winds will become miraculously more favorable when the distance between the two inhibiting upper trough features (one west and the other east of Chris) have been steadily decreasing for two days. The area that was somewhat conducive for upper ridging to form has been shrinking with time, and it's almost gone. Evidence that the center will pass over or ride along the Cuban coastline is also increasing. The subtropical ridge is holding steady, the LLC has been moving in a general westward direction, and there is now a model consensus for such a track. Therefore, it is highly unlikely that strengthening will occur within the next few days, if ever. If the surface circulation remains intact, then it may have a slightly better shot in the central/western Gulf. This is soley because there are still some questions regarding what the upper wind pattern will look like by that period. In summary, Chris has a better shot at dissipating in the short term than regeneration, and if any regeneration does occur it would probably happen in the Gulf. Either way, the energy is bound for lower Texas or northern Mexico.
The probability of tropical cyclone formation in the central or eastern Atlantic within the first 10 days of August has been discussed here for well over a week. There is still a lot of evidence that supports this idea even though there's only seven days left. The MJO is still enhancing convection as it spreads into Africa and the NAO will still be negative over the next few days. Meanwhile, several of the global models have been inconsistent. For example, the ECMWF, NOGAPS, and UKMET have often shown potential development, but have tended to back off in the following run. The Canadian CMC has been a little more consistent, and has been rapidly intensifying a low pressure system near the Cape Verde Islands over the past few runs. The GFS has been the most consistent model out of the bunch. The majority of GFS runs have been progging tropical cyclone formation in this area within the first 10 days of August since the talk of improving conditions began July 24th. Since there is still some model support and the global paramaters are still conducive it is too early to back away from the initial forecast.
August 03 2006 - 01:15
Tropical Storm Chris has been one of the more difficult storms to forecast. First, we did not expect Chris to become a classified tropical cyclone. Second, we were hesitant to call for much in the way of intensification once it was classified. Yesterday evening all of the scenarios were outlined and we said we'd give it another 24 hours before another call is made. Due to the recent intensity forecasts that ended up too conservative and the official call for moderate strengthening, tonight's evening discussion may be controversial to some.
The negating intensity parameters that made development questionable to begin with are still in the vicinity of Tropical Storm Chris. The upper low generating southwest shear is still just west of the cyclone, an upper level shear axis to the north and northeast, proximity to Hispaniola, and dry air are still potential threats to Chris. One may ask why there is still concern about these features even though they have failed to prevent Chris from intensifying over the last 36 hours. Well, yesterday the northerly shear being generated by an upper low to the north eased up and was diverated away from the core enough for strengthening to occur. Consequently, the Saharan Air Layer no longer had the ability to disrupt the center without moderate shear in place. Furthermore, the upper level low and Hispaniola to the west remained far enough away to have no impact.
Chris' uncanny ability to dodge these negating parameters may come to an end over the next 48 hours. The upper level trough axis and associated upper low to the north of Chris is suddently beginning to disrupt the northern semicircle. In fact, the interaction between the two systems may already be the culprit for the slight increase in disorganization today. A lot of the model guidance that progged favorable atmospheric conditions over the next few days did not show the upper level energy sliding south this close to the cyclone. Unless the upper level energy slides southeast in a hurry, northerly shear will begin to take a significant toll on the tropical storm. Additionally, if the shear exceeds 20 knots, the Saharan Air Layer may once again come into play. But there's even more. Tonight, the same northerly culprit is forcing Chris on a more westerly trajectory. This is confirmed by satellite, San Juan radar, and recon fixes. In fact, the latest recon fix even shows a slight jog to the west-southwest. Nearly everyone discounted the GFDL and remaining tropical models, but they may have at least been partially on to something.
Any concern that Chris may strengthen more than forecast as upper ridging builds over the cyclone has diminished significantly, at least in our estimates. Instead, the initial concerns that conditions wouldn't be conducive for strengthening or even maintaining a tropical cyclone may come to frution afterall. As long as the upper level trough axis continues to dip ever so slightly to the southwest, it has the potential to open up a can of worms. Even if the inner core remains just barely off the northern coast of Hispaniola, the southerly inflow could be highly disrupted by the 10,000 foot mountains. Disrupted inflow, dry air entrainment, and northerly shear are not favorable ingredients for strengthening. Tonight, we see little to suggest that strengthening is vey likely in the short term. Moreover, there may even be further weakening. If the negating factors come together like they may very well do, then Chris' intensity may be significantly reduced over the next 36-48 hours. The cyclone threat threat to Cuba, south Florida, and most of the Bahamas would also be greatly reduced. However, the threat of heavy rainfall over the Greater Antilles and Bahamas would still exist, especially in high elevation areas. If Chris were to weaken into an open wave axis, it would still have to be monitored over either the Florida Straights or Gulf of Mexico for signs of regeneration. In the long run, Chris or its remnants would continue westward toward south Texas or extreme northern Mexico. There is a dominant ridge of high pressure over the southeast that shows no signs of giving way. Thus, a westerly track should continue through the period.
This is contrary to many online forecasts, including the official one, but we call it like we see it. If the consensus was always followed there would be no point in maintaing our free product. Nevertheless, our audience does have a right to judge our forecasts. So with that said there are ramifications if tonight's call doesn't pan out. Chris would certainly be one of the worst-called storms in our short history. One thing we ask our audience to keep in mind is how difficult this upper air pattern currently is. It can lead to significant changes in the intesity forecasts. But no matter your opinion, the only guidance that should be taken seriously in life or death situations is produced by the National Hurricane Center. As long as their forecasts continue to show a hurricane or tropical cyclone threat to the Bahamas, Florida, Cuba and surrounding areas, then interests there should continue to prepare.
Meanwhile, we're still looking at the potential for development off the coast of Africa within the next eight days. Not much has changed over the last several days so please read the previous discussions for more details. Slow development is still probable.
August 02 2006 - 01:00 UTC
As one can see, the intensity forecast is not playing out. The upper low to the north of Chris was forecast to at least hinder the northerly semicircle and poleward outflow channel for an additional 24 hours. But the northerly flow is not nearly as strong as expected because the Bahamas upper low is diverting the flow to the east of the cyclone with its associated westerlies. This is why intensity forecasting is nowhere near as advanced as the tracks. One shift in the upper flow can bring dramatically different atmospheric conditions with it. Instead of moderate northerly shear, an upper anticyclone is trying to form north of Puerto Rico. The Bahamas UL is enhancing the southwest flow while the upper low to the north is dropping the anchor behind the storm with northerly winds. This is a very favorable pattern for at least moderate intensification through the period. Although it cannot be ruled out, the southwest winds ahead of Chris probably will not move westward fast enough to allow for rapid intensification. This aspect of the intensity forecast will have to be monitored closely. In the meantime, Chris is still getting its act together. The surface circulation remained partially exposed during the morning hours, but convection has since wrapped around the center. It looks as if the dry air surrounding the system will no longer have any effect on the intensity as long as vertical shear doesn't disrupt the circulation.
There's a lot of uncertainty tonight, and some of the reason for that has already been briefly noted. If the core remains far enough north from the Greater Antilles (primarily Hispaniola due to its 10,000 foot peaks), then the five day intensity forecast will largely depend on the upper low north of the Bahamas. All of the global models agree that the upper low will slowly weaken as it heads west-northwest into the northcentral Gulf by Day 6. If the guidance is correct, then the strongest southwest shear may remain west of the tropical cyclone through the period. Such a scenario would only enhance the upper ridging over the tropical storm as it inches closer to the Gulf Stream, thus possibly causing more intensification than officialy forecast. A Cat 2 type hurricane would not be far fetched at all for southern Florida or northwest Cuba. If the upper low begins moving even faster than forecast, then even that may be too conservative. But i'm not willing to call for anything of that magnitude until I see more support.
On the other hand, perhaps the best scenario for the United States, Cuba, and Bahamas is that the storm gets caught over the mountains of Hispaniola. We've seen several storms in the past that were hyped and destined to hit the southeast USA (most notably Debby 2000) as a moderate or major hurricane before being significantly disrupted by Hispaniola. Such a scenario cannot be ruled as a result of the consistent GFDL and tropical model guidance. It will be a close call. The low to mid level steering flow will keep Chris on a generally westerly heading, and several or most of the models even show a slight southerly dip in the medium range track. The key behind this aspect of the forecast is the strength of the westerly flank of the subtropical ridge, which extends well into the southeast USA. If the ridge becomes as suppressed or builds as far southwest as lot of the guidance is indicating, then a more westerly track is to be expected.
In conclusion, it's too early to make a confident forecast, especially in light of the recent intensity bust. The lack of a G-IV flight and failure of the models to register a tropical cyclone does not make the forecast any easier. Until the models attain a better grasp on the tropical cyclone, then we're not left with much more than water vapor imagery and observation of upper air trends. What complicates matters even more is that the forecaster has to determine exactly how much interaction there will be with the upper low and Hispaniola. If the analysis isn't correct, both the intensity and track forecast will bust. The National Hurricane Center also seems to be playing it safe. They're just north of the model consensus and the intensity forecast can be considered conservative given one of the scenarios. I don't think anyone can disagree with that general assessment at this time. We'll see what happens over the next 24 hours.
The east Atlantic isn't getting much attention due to Chris, but the call for a tropical cyclone formation in the central/east Atlantic within the first 7-10 days of August still looks spot on. The favorable indications have been outlined here for over a week. The NAO and MJO (overall pattern) is favorable. The Saharan Air Layer has already taken a major hit. Satellite and water vapor imagey shows that the Atlantic is very moist, convectively active, and there is a strong 200MB easterly jet between the equator and 15N. There is more model support than ever. The GFS is sitll progging for the formation of a strong tropical cyclone. Meanwhile, the CMC, NOGAPS, UKMET, and ECMWF are all more aggressive today. Debby is likely 48-120 hours from forming off the coast of Africa. It will probably have to be monitored by the Leeward and Virgin Islands as soon as Chris leaves the area.
August 01 2006 - 01:00 UTC
Note: The bulk of this evening's discussion was written before indications of an NHC upgrade became apparent. The possible upgrade still does not change the general forecast. Winds in excess of 30-40mph are likely over the Windward and Virgin Islands, but the system should remain weak through the period.
Northwest shear-induced diffluence and the formation of a low level circulation have substantially increased the magnitude of convection associated with 99L Invest. Make no mistake, there is no closed surface circulation immediately at the surface at this time. Scatterometer wind data confirms that sustained winds are at least 20-25 knots, but it does not confirm a closed surface circulation. The argument against development over the last few days has been that conditions are not conducive for surface circulation formation. The Saharan Air Layer is still present, and it will continue to hinder development for at least another 36-48 hours. The second inhibitor has been shear. Water vapor imagery reveals an upper level trough axis directly impinging on the northern extent of the wave axis. The proximity of the the two axes has made convective bursting a lot easier by means of diffluence. The same upper level winds are shearing the convective bursts to the east of the low level center. Visible imagery depicted a partially exposed center to the west of the convective activity throughout the day. As long as the upper low remains north of the wave, the northwest shear will make further organization highly difficult, despite the increase in convective activity.
All global model data (GFS, CMC, NOGAPS, UKMET, ECMWF) are in agreement that 99L will continue west-northwestward over the next several days. The low level center may even pass over the Leeward and Virgin Islands. Interests in those areas should prepare for increased shower activity and gusty winds wind excess of 30-40mph. By Day 6, all of the aforementioned models indicate that the remaining lefover 850MB energy associated with the wave will slide into south Florida or Cuba. However, several of the models also keep an upper low over the Bahamas/southern Florida while the upper low currently north of the wave axis also begins to retrograde to the west. So, as the atmosphere becomes increasingly moist, it will still be difficult for upper level winds to become favorable for extended periods of time due to the proximity of the TUTT lows. Therefore, the strength of 99L should remain status quo over the next several days. Another flare-up of convection like we're seeing this evening is likely once the wave passes over the southwest Atlantic or Bahamas. Whenever a strong wave interacts with an upper level trough, one can expect a sudden convective burst to occur. In conclusion, while these convective bursts look impressive on infrared imagery, they aren't necessarily benefiting tropical cyclogenesis. The western extent of the SAL and close proximity to the west Atlantic upper lows should keep the system at or below tropical depression status over the next week or so.
The tropical wave that exited the coast of Africa yesterday evening was the first of several waves that will be monitored as they enter the Atlantic over the next 7-10+ days. The latest wave has not shown any signs of organization over the last 24 hours, and convection is still marginal. There is still too much easterly shear. The upper level anticyclone over the Cape Verde islands is still too far north to aid in the development of these waves. Instead, it's enhancing upper level easterly winds over them. Nevertheless, several more rounds of low level energy are about to enter the Atlantic, and it is likely that ridging will begin to increase at lower latitudes.
There is a growing model consensus that development in the east Atlantic will hold off for at least another 72-96 hours. The first wave of interest will probably continue westward with little signs of additional development. It's the one or two waves that are still a couple days away that likely have the greatest potential for tropical cyclone formation. The GFS, ECMWF, and to some extent the Canadian CMC agree with this solution. The UKMET has yet to show many signs of development, and the NOGAPS remains inconsistent. Meanwhile, the enhancing phase of the Madden Julian Oscillation is now completely covering the Mean Development Region, and it will soon spread into western Africa. The North Atlantic Oscillation will remain in negative territory for at least the next several days. The enhancing global oscillations, combined with increasing signs of activity, upper level ridging over the east Atlantic, and a developing model consensus are enough to maintain the forecast that a tropical cyclone will form in the central or eastern Atlantic within the first 7-10 days of August.
July 31 2006 - 02:00 UTC
The true African wave train seems to have left the station this evening. A vigorous tropical wave has exited the coast of Africa over the last six hours. A low to mid level circulation exists along the wave axis, but since there is no visible or scatterometer wind data it is unknown if there is anything at the surface. It is unlikely that a surface circulation exists this early in the game, but if convection begins to consolidate one could form. Convection is prevalent, but it is not organized at this time. The Saharan Air Layer has taken a hit over the eastern Atlantic the last couple days, so there is an abundance of moisture. There is moderate easterly shear over the wave axis since the upper level anticyclone to the north isn't vertically stacked over the wave completely. If the upper anticyclone can stack itself directly over the wave before the low to mid level circulation becomes disrupted by shear, then some slow development will be possible. The anticipation of at least one tropical cyclone forming within the first week to 10 days of August does not ride entirely on one wave alone. The current wave is one of several that will have to be monitored for tropical cyclone formation as they exit Africa over the next 10+ days.
The global models disagree one what wave will become the most dominant and have the better chance of tropical development. The GFS, for example, is progging development sooner than any other model. The GFS develops the wave that exited the coast tonight, while the CMC and NOGAPS are hinting at development of a wave behind it. The ECMWF is more in agreement with the CMC and NOGAPS tonight. For the first time, the ECMWF is developing a tropical cyclone near the Cape Verde Islands in 72-96 hours. The UKMET is the slowest, but is hinting at an intensifying low pressure system over the eastern Atlantic in 5-6 days. In summary, the models are becoming increasingly aggressive.
This is all a part of the changing weather pattern that has been analyzed here for the last seven days. The North Atlantic Oscillation is still becoming increasingly negative, and it will remain negative for at least another week or so. The enhancing phase of the Madden Julian Oscillation is already spreading over the central Atlantic, and it will make it into the eastern Atlantic and Africa over the next five days. The eastern Atlantic will remain moist, the Saharan Air Layer will be limited, and upper level winds should be conducive enough for at least one of these waves to become a tropical cyclone within the next 7-10 days.
99L Invest and the tropical wave over the central Caribbean almost deserve no mention tonight. 99L has become highly elongated from southwest to northeast. It has run into a wall of very stable, dry air that will limit convection and hinder surface circulation formation altogether. Upper level northwest shear doesn't make conditions any better. Meanwhile, the tropical wave over the central Caribbean is being sheared to death. Typically, no wave ever gets this much attention when upper level winds are that extreme.
July 30 2006 - 00:30 UTC
99L Invest continues to show no signs of organization. Satellite imagery has confirmed the existence of a low to mid level circulation for two days, but there's still no sign of surface development. Scatterometer wind data also supports this conclusion. The ITCZ related convection that was interacting with the wave axis yesterday is no longer in close proximity to the MLC. The ITCZ axis extends westward into South America, and that is the route in which the convective burst to to the west of the MLC is taking. The only positive for this wave is that the mid level circulation is slowly gaining latitude, and it is detaching itself from the ITCZ. But this is where the Saharan Air Layer and easterly shear problems arise. The SAL remains strong to the north and west of the circulation, and it is still being entrained into the center. Meanwhile, easterly shear over the southern flank of an anticyclone located north of the wave is still producing moderate easterly shear. Thus, the majority of convection has been located west of the MLC. So, easterly shear and dry air continues to prevent a surface circulation from forming. There are no obvious signs that conditions will become much more favorable than they are now.
Also, if the remnant of 99L were to make it into the eastern Caribbean, westerly shear resulting from TUTT lows and the ridging over the central Atlantic would likely eliminate the possibility of any comeback. You can already see what would be the outcome of 99L in those westerly winds by observing the wave out ahead of it. Convection is prevalent in the east Caribbean, but conditions needed for organization are very hostile. Tropical cyclone formation from this system is not expected.
This is day six of the early August, east Atlantic tropical cyclone formation countdown. All of the global parameters that have been observed over the past week or so are progged to align in a manner that is conducive for tropical cyclogenesis in the east or central Atlantic, beginning particularly in the first week or 10 days of August. As noted over the last few days, conditions should be favorable enough for the first Cape Verde tropical cyclone to form within that timeframe. The paramaters are analyzed on a daily basis to see if the progged conditions are reversing. Today's look at the Madden Julian Oscillation guidance reveals that both the GFS and EWF models agree that the enhancing MJO phase will be present over the central and eastern Atlantic through at least mid August. Upward motion charts indicate that the phase is already spreading across Central America, so the timing is reasonable. Also, the North Atlantic Oscillation is currently heading into negative territory, and it should remain negative through the first week of August. The southern flank of the subtropical ridge is once again expected to weaken in the medium range according to all global model guidance. This is also reasonable since larger-scale parameters are aligning in a way that would support lower heights near Africa.
An increasing number of global models are beginning to hint at the same possibility of tropical cyclone formation that has been discussed for the last several days. The operational GFS is not backing off. In fact, the 12Z run was one of the more aggressive runs of the 00Z and 12Z packages. The 12Z GFS develops a tropical cyclone as early as 48-72 hours, but that may be too soon. The NOGAPS is a little more aggressive today. It is beginning to consistently prog a weak tropical cyclone off the African coast in 96-144 hours. The Canadian CMC is also more aggressive with a progged tropical cyclone forming immediately off the coast in 120-144 hours. The UKMET and ECMWF have yet to show tropical cyclone formation, but over the last 24 hours they've begun to show lowering heights in the area. Overall, the global model guidance is slowly becoming increasingly bullish. We see no reason to back away from our original forecast at this time. A tropical cyclone is likely to form within the first week or 10 days of August in the central or eastern Atlantic.
July 29 2006 - 00:30 UTC
The tropical Atlantic is still relatively quiet at this time. That may sound ludicrous given the attention that 99L INVEST has received from other sources over the last 24-36 hours. The official tropical weather outlook began mentioning the central Atlantic tropical wave yesterday afternoon. By yesterday evening 99L invest was declared. The tropical model suite was run at that time, which is protocol. The TWO basically shows that the NHC is monitoring the system for signs of development, and that is a good thing. But a mention doesn't mean development is more likely than no development at all. For example, the TWO has been left blank only 12 out of 58 days this season. Two tropical storms have formed. The same can be said about invests. Several ill-defined invests have been declared that have had little chance of development. Therefore, it doesn't mean a whole lot when the tropical model suite is run. So lets get into the details of 99L to see if this system is any different than the several invests that have not developed into tropical cyclones.
Yesterday evening, the tropical wave exploded with convection. The wave has been one of the more convectively actives waves to roll off the coast of Africa this season. But that doesn't necessarily give the wave a far greater probability of development than the waves that came before it. CIMSS upper air and water vapor imagery depict upper level ridging centered just north of the Cape Verde islands. The western flank of the ridging extends westward to 40-50W longitude. However, since the ridge isn't centered closer to or directly over the wave, it is not making conditions very favorable. Instead, it has maintained the strong easterly and northeast flow over the Mean Development Region (aka easterly shear). The moderate upper level easterlies is one of the reasons why this wave in particular is so convectively active. The second reason is that the wave axis hasn't detached from the Intertropical Convergence Zone. Visible and Scatterometer Wind data clearly show northeast winds circulating around the north Atlantic subtropical ridge and southerly winds entering the picture from the equator. These winds are intersecting along an axis (ITCZ) that the wave is still intertwined with. Visible and QuikSCAT imagery also confirm that there is no true surface circulation along the wave axis. With that being said there is no surface forcing, the axis is being enhanced by the ITCZ, and marginal easterly shear. This is not a sign of tropical cyclone formation.
Until a surface circulation forms, any convective burst will not mean much in the longhaul. To top things off, the northeast winds streaming into the wave around the subtropical ridge is causing dry air or Saharan Air Layer entrainment. A surface circulation is unlikely to form when the SAL is being fed into the system. The SAL and associated dry air enhances surface divergence, which is exactly what a tropical cyclone does not need. Furthermore, the global models can barely even detect this system. The global models don't seem to get any attention until they prog tropical development. Meanwhile, the SHIPS intensity model run is being hyped because it is calling for a 70 knot hurricane in 120 hours. The SHIPS intensity forecast needs to be completely thrown out. In order for SHIPS to even make an intensity forecast, it requires the meteorologists in charge to initialize the model with a tropical cyclone already exists. Essentially, a tropical depression or storm with a closed surface circulation must be initialized by the model in order for it to provide an intensity forecast. Since a surface circulation has not formed, and conditions aren't exactly conducive for surface ciruclation formation, it becomes easy to understand why the SHIPS guidance currently carries no weight. Tropical cyclone formation from 99L INVEST is not expected as it heads westward toward the Lesser Antilles.
The tropical waves that will be rolling off the African coastline in the first week to 10 days of August are still expected to have a better probability of tropical cyclone formation than all the previous waves thus far. The operational GFS runs continue to hint at tropical cyclone formation during this period, and today it has some support from other global model guidance. Today, the 12Z Canadian CMC and the last two to three runs of the NOGAPS have begun to prog lowering heights near the Cape Verde area beginning in 120 hours. The UKMET and ECMWF models have remained bearish. The aforementioned models are not overly aggressive, but other global parameters, which have been mentioned for the last five days, are still expected to be conducive. Model guidance is still indicating that the North Atlantic Oscillation will turn negative for at least a week or so, and the enhancing phase of the Madden Julian Oscillation will pass through the central Atlantic in early to mid August. The negative NAO and enhancing MJO will help to lower heights, increase convection, and lower shear over Africa and the Mean Development Region. The only concern regarding potential development, is that the NAO may shift neutral/positive at the end of the period, and if a TC hasn't formed by then any waves may temporarily become suppressed to the south yet again. In summary, a tropical cyclone is still likely to form in the Atlantic within the first week to 10 days of August.
July 27 2006 - 19:00 UTC
The tropics are quiet at the moment. In the Atlantic Basin, no current areas pose any threat to ever become a tropical cyclone. The tropical wave moving through the eastern Caribbean Sea and Windward Islands is contending with strong upper trough-driven westerly shear. This will continue to enhance convection through diffluence, but simultaneously limit vertical stacking and organization. The tropical wave behind in the central tropical Atlantic will approach the same environment in a day or so. Thus, development of either wave is highly unlikely. A frontal boundary is located off the US east coast, but is expected to move seaward over the next few days. Model guidance suggests that if any low forms it will remain frontal and non-tropical in nature.
The signs of early August activity off Africa remain present. The GFS has been consistent on the idea of some type of low pressure(s) exiting the coast of Africa starting around Day 6. It has been less consistent with the exact strength and track of any such lows, with the 06Z and 18Z tending to be most bullish on strong tropical cyclone development and the 00Z and 12Z only showing slow strengthening. The latter is probably a more likely scenario given the better reliability of those runs as opposed to the intermediate ones. Other model guidance is slowly trending towards this idea as well, through Day 6 at least. The 12Z NOGAPS and ECMWF both show a very weak closed low off Africa at the end of the forecast period. The CMC and UKMET do not yet show a low but do indicate a lowering of sea level pressures towards the aforementioned time frame. Our expectations have not changed, and at least one tropical cyclone is expected to develop east of the Lesser Antilles the first week of August. It is too soon to really say with confidence what type of track any system will take. The best early bet would be eventual recurvature east of 80W, given the large-scale synoptic pattern that has held fairly well for the past few months.
July 27 2006 - 00:00 UTC
The low pressure area in the northwest Gulf moved into Texas before a surface circulation was able to form over open waters. The heavy rainfall threat will not diminish until tomorrow. Currently, several counties in Texas and southwest Louisiana are under flash flood warnings. But now that the tropical cyclone threat is over we can focus on what may lie ahead.
The second area of disturbed weather that has gotten some attention over the last few days is the central Atlantic. A weak tropical wave passing through the northern Lesser Antilles is interacting with upper level winds. Without shear-induced divergence, this wave would have a lot less convection associated with it. These winds, which are unfavorable for tropical cyclogenesis, should not let up over the next several days. An upper low to the wave's northwest and a secondary upper low near Jamaica will maintain the strong westerlies. There will be no development from this wave over the next several days, if ever. A second high amplitude wave is approaching 45W, but convection is nonexistant.
This is the third day since signs of early August activity off the coast of Africa were noted in these discussions. The signs are still there, and the idea seems to be catching on at other areas of the net. The North Atlantic Oscillation will semi-tank negative over the next seven days, and the subtropical ridge will at least temporarily weaken. As a result, heights will lower over western Africa and the eastern Atlantic, thus lifting the Intertropical Convergence Zone northward. The Saharan Air Layer will also lose some of its punch as the ridge weakens. Additionally, the enhancing phase of the Madden Julian Oscillation is forecast to enter the central and eastern Atlantic by early August. Nearly all MJO model forecasts are in agreement. This will help to decrease shear and enhance convection. It's no surprise that the operational GFS is still spinning up several organized low pressure systems in light of the aforementioned prognostications. At least one tropical cyclone development in the central or eastern Atlantic by August 10th appears likely.
July 26 2006 - 01:00 UTC
Despite the original surface circulation being located inland, the broad low pressure area over the Gulf of Mexico has become better organized. The surface circulation has been inland just south of Brownsville, but a mid level circulation is still located offshore. Convection has persisted over the MLC over the last 12 hours. If this trend continues and the MLC stays over open waters for another 12-24 hours, then a tropical depression or storm may form. Upper level winds have weakened slightly over the last 24 hours since Tropical Stom Emelia didn't intensify as officially advertised and is slowly moving northwest. Sea surface temperatures are more than warm enough and there is little to no dry air to speak of. If a tropical cyclone does form east of Brownsville within the next 24 hours, then a northerly track into the Corpus Christi area is likely. The dominant steering factor is the Atlantic subtropical ridge, which has extended as far west as the Yucatan peninsula. There is nothing but southely flow in the low to mid levels in the Gulf west of 90W. That means all of the low level moisture in the Gulf will stream into Texas and Louisiana before all is said and done. The entire Houston metro is under a flood watch. Fortunately for New Orleans, some of the model guidance has shifted the heaviest rainfall estimates westward. That is likelly because some of the troughing over the eastern U.S. that was enhancing a southwest flow over SE LA is lifting out. There will still be a good chance of rain tomorrow, but hopefully not even to cause major street flooding.
There's been no change in our long range ideas over the last 24 hours. The operational GFS runs are still consistently progging a more active ITCZ within 5-15 days. The North Atlantic Oscillation is still expected to go negative for at least the next seven days, which will set the stage in the Atlantic. Additionally there's the possibility that the Madden Julian Oscillation may pass over Africa during this time period, further enhancing the African wave train. See yesterday evening's discussion for other details.
In the eastern Pacific, a sudden and uexpected increase in easterly shear rapidly weakened Hurricane Daniel over the last 24 hours. The upper level ridge detatched from the hurricane in the upper levels, and drifted north. The anticyclonic flow around the ridge increased the easterly winds to its south. Furthermore, an upper low to Daniel's southwest contributed to the shear. Hawaii can breathe a sigh of relief. Tropical Storm Emilia is forecast to hug the Baja Peninsula over the next few days, but a direct hit is not likely. The primary threat is heavy rainfall from Emelia's feedarbands.
July 25 2006 - 00:00 UTC
Today's scheduled aircraft reconnaissance mission for the broad low in the southwest Gulf of Mexico was cancelled. There has been no shortage of convection, but organization is still lacking. The slightest hint of a circulation is already partially inland and mid level. The hurricane center apparently agrees with that assessment since the 18Z tropical model suite was initialized on the Mexican coast. It's safe to say that land interaction is a problem. Also, there has been a lot of talk about upper level conditions becoming more favorable over the last few days, but until Tropical Storm Emilia (EPAC) moves out of the area that is not going to happen. The problems with that idea is that Emilia will slowly move northwest, parallel to the Mexican coastline. The surface trough will be completely inland within 36-48 hours. So, the broad low in the Gulf will have to contend with strong upper level winds streaming out of the south and west until it dissipates completely over land later in the period. It is difficult to be optimistic about tropical cyclone formation under these conditions.
However, there's always a possibility that a new mid level circulation forms just east of Mexico and spins down to the surface within the next 24 hours, so a weak depression or storm cannot entirely be ruled out. The NHC proved in 2003 that anything is possible when they upgraded a tropical wave in the west Gulf before recon even had a chance to verify that a closed surface circulation existed. That wave became known as Tropical Storm Grace, and today's satellite signature and situation do have similarities. This system shouldn't be treated like anything more than a heavy rainmaker, however. Heavy rainfall will be in the forecasts across southwest Lousiana and all of southeast Texas over the next two days.
The real tropical weather threat may be approaching the horizon. We may have a better chance of seeing "Chris" in the central Atlantic than the Gulf, but the current waves that are barely visible on satellite imagery are not the ones of interest. The Intertropical Convergence Zone and associated convection has been suppressed all hurricane season due to anomalously strong subtropical ridge. The evidence is in the daily North Atlantic Oscillation values for the last two months. Some of the waves that have been called "impressive" were actually quite typical in previous seasons such as 2003 and 2004. But there are signs of change, and we'll likely begin to see the results within the next two weeks. The 7-14 Day GFS North Atlantic Oscillation Forecasts indicate that the NAO will remain neutral (just recently turned neutral) or even slide into negative territory. Meanwhile, all of the operational GFS charts show lowering heights across the entire Atlantic over the next week or so. This is to be expected in light of the GFS NAO prognosis. The CMC, ECMWF, NOGAPS, and UKMET are all in completely agreement with the lowering heights solution. Furthermore, in response to the lowering heights over the deep tropics, the operational GFS has been progging the emergence of a much more active ITCZ for two consecutive days. Nearly every run, barring the July 23 18Z run, has depicted at least one tropical cyclone in the Mean Development Region beyond Day 10. Today's 00Z and 12Z runs began to show two separate tropical cyclone entities in the long range. The 18Z has backed off on the second cyclone, but that is not a major issue at this time. Our use of long range operational GFS guidance has been successful dating back to 2003. The main point is that the things one should look for prior to an increase in deep tropics TC activity are now beginning to appear. Within 5-15 days, we can anticipate a much more active tropical Atlantic, with waves that truly have the potential to become threatening tropical cyclones.
In the eastern Pacific, Hurricane Daniel is still heading in the general direction of Hawaii. Daniel has been weakening, but it should continue to do so in a gradual manner as upper level winds remain light. The ridging over the northcentral Pacific should continue to keep Daniel on a west or west-northwest heading. However, there is some chance that Daniel's core may just barely pass south of Hawaii. A threat to the islands north of Hilo is looking less likely with time. Tropical storm force conditions over Hilo is still a very real possibility. The islands will have to contend with high surf at the very least. Further east, Emelia is still battling it out with an upper low to its north. The upper low will not allow for Emilia to become a hurricane. Upper level SW winds are too great. The low to mid level steering will keep Emilia offshore, but the upper level winds will enhance shower and thunderstorm activity along coastal regions near Emelia.
July 19 2006 - 22:00 UTC
Tropical Storm Beryl has strengthened today. Max sustained winds are currently at 50KT. The storm is characterized by a symmetrical circle of moderate convection over the center and slight outflow in the eastern quadrant. Beryl has about 24 hours to strengthen because after that point it will approach cooler water temperatures and encounter strong westerlies ahead of a shortwave trough. This trough can be seen digging southward towards Beryl on water vapor imagery, and will also be the prominent steering mechanism once the two features meet. When this occurs, Beryl will turn northeastward, weaken, and lose tropical characteristics.
There is some question as to the timing of this turn, since a later turn will result in a closer passage or even landfall in southeastern New England. The tropical models, including the GFDL, can be ignored in this case given their poor performance in the subtropics and clear bias towards the GFS solution. Most of the global models have had a difficult initializing the storm, leading to a possible leftward bias since a weaker storm would not easily curve ahead of the trough. Taking everything into account, Beryl is still expected to miss Cape Cod to the east as a transitioning extratropical cyclone. Interest in New England should still monitor the storm since it could still deliver inclimate conditions, regardless if the center stays offshore.
The rest of the Atlantic Basin is very quiet, and there are no signs of development in the next 5 days. Hurricane Daniel continues to slowly intensify in the East Pacific and could become a major hurricane over the next day or so. Daniel is no threat to land. Invest 92E, behind Daniel, has become a little better organized. The ECMWF and GFS both develop a tropical cyclone from this area later in the forecast period, and thus it will be watched. In the West Pacific, Tropical Storm Kaemi is also strengthening and is forecast to become a minimal typhoon as it continues west-northwestward. Kaemi could threaten Tawain and the China coast in the coming days. Another Invest in the South China Sea is not showing any signs of organization at this time.
July 18 2006 - 22:00 UTC
The East Coast low wasn't expected to become better organized last night. But it did, and that's why advisories are being issued for Tropical Storm Beryl. Convection began to burst in the early morning hours, and upper air conditions were marginal enough to allow for a surface circulation to form and detatch from the frontal boundary. In fact, our upper air wind ideas were quite flawed. Since the low to mid level steering flow is supportive of a northerly track and upper level winds are streaming from the south, the quasigeostrophic flow is reducing shear to a minimum. Conditions will be favorable for slow strengthening until it begins to enter cooler waters.
The global models have not handled Beryl well at all. The intensity initalization is quite poor in nearly all of the models, and several phase the low with an upper trough over the northeast too soon. Nevertheless, all models keep the low level vort max east of the Mid-Atlantic states, and that does look right based on water vapor imagery. The subtropical ridge is building westward toward the East Coast, but a shortwave is also sliding east over the U.S. Northeast. A track slightly east of the coast is likely, and that is in agreement with the official track. The outerbanks and Cape Cod must still play attention to Beryl as the potential for a brush is there. That isn't in the forecast, but it is something that cannot be entirely ruled out. Beryl will likely become nontropical or pass south of the Canadian Maritimes, so a tropical threat to that area appears low.
The rest of the Atlantic is quiet today. In the east Pacific, Hurricane Daniel is strengthening but is no threat to Mexico. To the east of Daniel, there are several disorganized areas of shower and thunderstorm activity. There is still some potential for gradual tropical cyclone development in this area.
Ironically, the global models are not handling newly-formed Tropical Storm 06W (West Pacific) much like Beryl in the Atlantic. Conditions appear favorable for gradual strenghtening, and the Joint Typhoon Warning Center expects 06W to become a typhoon in a few days. Their forecast takes the typhoon in the general direction of Taiwan and China, but it is too early to speculate what will happen beyond that period. The poor model guidance does not help.
July 18 2006 - 02:00 UTC
The broad area of low pressure off the East coast continues to show no signs of tropical cyclone formation. Convergence is limited, and the little convergence that is visible on satellite is in the low to mid levels, not at the surface. Second, convection is also limited. Third, the low pressure area is still intertwined within the nontropical, frontal boundary. Water vapor imagery shows dry air impinging on the northern extent of the low as the upper trough digs south. This impingement will make it more difficult for surface circulation formation without the aid of the nontropical boundary. Additionally, the positioning of the trough will keep upper level winds marginally favorable at best. A recon is scheduled to investigate this low tomorrow, but unless conditions and the overall structure of the system change dramatically within the next 24 hours, a tropical cyclone being found is unlikely. It is also noted that no global model guidance is indicating tropical cyclone development.
The rest of the tropical Atlantic is quiet. The convection over the western Caribbean is being enhanced by strong upper level winds. A somewhat impressive wave that exited the coast of Africa will soon face hostile dry air and shear. In the eastern Pacific, Tropical Storm Daniel has formed. As mentioned last evening, conditions ahead of this cyclone are favorable. Daniel will likely become a moderate hurricane, but fortunately it will remain over open waters. Another low south of Acapulco has the potential to slowly organize into a tropical cyclone. Nevertheless, this low is highly disorganized at this time.
July 17 2006 - 02:00 UTC
The broad area of low pressure located off the East Coast is associated with a dissipating frontal boundary. Conditions and water temperatures are marginally favorable for slow tropical cyclone formation. However, there is only a 24-36 hour window for tropical development to occur before extratropical transition begins. Convection has started to diminish over the last few hours, so a strong surface circulation approaching tropical depression or storm status is not likely for quite some time despite the marginal conditions. Due to the limited oppurtunity of development and current lack of organization, tropical cyclone formation appears very low. Regardless, there is no threat to the Carolinas or New England. This system will head northeast out to sea as troughing remains over the northeast United States.
In the eastern Pacifc, the tropical low near 110W has the potential to slowly develop. Upper level winds and water temperatures will be conducive over the next several days. Several global models have also formed a consensus that a depression or storm will come out of this low pressure.
July 16 2006 - 01:00 UTC
There's still not much happening in the Atlantic tropics. Satellite imagery shows no organized areas of low pressure, and the limited amount of model guidance developing a low in the southwest Gulf has backed off. The enhancing phase of the Madden Julian Oscillation is beginning to pass through the Atlantic Basin, but with no organized weather systems in place climatalogy may win out. In the east Pacific, the enhancing phase may generate one more tropical cyclone south of Acapulco. This system would be no threat to Mexico if it does develop.
July 15 2006 - 01:00 UTC
In the Atlantic, there are no signs of development in the near future. 96L is no more than some weak low level energy passing through the central Caribbean and the surface trough over the Gulf is producing minor, daily shower activity. The east Atlantic wave already weakened considerably as it has encountered unfavorable dry air and strong upper winds. The only feature of some interest is the disturbed area of weather near Panama and Costa Rica. The 00Z and 12Z runs of the Canadian CMC take this broad low pressure area into the southwest Gulf and Bay of Campeche by day 5. The 00Z NOGAPS was similar, but has backed off in later runs. The remaining global models keep most of the energy south and over the east Pacific.
July 14 2006 - 01:00
The Atlantic is quiet. Hostile upper winds are destroying the east Caribbean wave and the trough over the southeast Gulf. Tropical development is impossible in such conditions. The tropical wave that recently exited Africa is also beginning to encounter hostile conditions. It will have contend with dry air, and hostile winds that may even be less favorable than they were when 96L passed through the central Atlantic. No development is expected for at least the next several days. This is completely normal for July. Indications of an above average season (greater than 10NS) probably will not arise until August. Hurricanes Bud and Carlotta are moving out to sea in the eastern Pacific. Tropical Storm Bilis is bringing heavy rainfall to Taiwan and China, but there will be little wind impact, if any.
July 13 2006 - 00:45 UTC
There hasn't been much change across the tropical Atlantic and eastern Pacific over the last 24 hours. Hurricane Bud still has some chance of briefly becoming a major hurricane before entering cooler waters, but the window of oppurtunity is beginning to close. Bud is no threat to land. Further east, Tropical Storm Carolotta formed last night. Carlotta is intensifying and should soon become a hurricane. However, Carlotta will also remain offshore. Meanwhile, the Atlantic remains devoid of any classified tropical cyclones. The central Atlantic tropical wave is now passing through the southern Lesser Antilles. The wave is showing no signs of organization. Westerly winds aloft are enhancing convection along the wave axis, which is not a sign of tropical development. The wave is heading into the eastern and central Caribbean, where upper level winds remain very strong. Tropical development is not expected. The weak surface trough over southern Florida enhanced daily thunderstorm activity, but there are absolutely no signs of tropical cyclone formation. An upper level low over the Gulf of Mexico is enhancing divergence over the area, which is also enhancing the daily thunderstorm frequency. This feature will trek across the Gulf over the next several days, but upper level winds will inhibit any tropical formation. No development is expected.
Tropical Storm Bilis is falling apart just east of Taiwan much like Ewiniar as it arrived at the Korean peninsula.
July 11 2006 - 22:45 UTC
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The Atlantic basin remains quiet. A mid to upper trough extends into the southeast Gulf, west Atlantic, and northwest Caribbean. The convection across this area is nontropical and no tropical development is expected. The remainder of the Gulf of Mexico and Caribbean Sea is devoid of any tropical waves worth watching. The only wave of some interest is the same one we've been monitoring over the central Atlantic the last several days. The low to mid level vorticity max became exposed as convection weakened overnight, thus making it increasingly visible on satellite. Over the past few hours, a small burst of convection has formed near the center of circulation, but it is for the most part shear-induced with only minor help from surface convergence. Water vapor imagery depicts increasingly dry air surrounding the wave. The increasing subsidence will make the life of the vort max increasingly difficult as limited convergence will become more of an issue. Furthermore, upper winds over and immediately east of the Lesser Antilles remain strong. This is not a favorable pattern. The subtropical ridge has been anomalously strong in the deep tropics, which has suppressed the intertropical convergence zone and pumped sinking air into the Mean Development Region. TUTT-related upper lows have also been enabled to slide southwest out of the northcentral Atlantic, making upper winds near the islands hostile. It's no surprise that all global models agree with maintaining or even increasing the high shear over the eastern Caribbean. None of the global models indicate development. With all this in mind, 96L should become no more than a sheared wave that is barely recognizable on satellite imagery within a couple days, if it isn't that already.
The opposite can be said about the eastern Pacific. One of yesterday's potential disturbances for development has rapidly intensified into a minimal hurricane. Hurricane Bud is approaching 115W, and is no threat to land. Upper level winds are favorable, so additional strengthening is expected over the next few days. Bud may even strengthening into a major hurricane briefly before entering cooler waters. The second disturbance near 105W still has some potential to become a tropical depression over the next day or so. The low to mid level circulation needs to tighten before strengthening. It now looks more likely that this system will remain offshore.
July 10 2006 - 23:30 UTC
The Atlantic remains devoid of any areas of interest. As expected, the tropical wave that flared yesterday has lost all organization today. This can be largely attributed to strong westerly, induced by an upper trough to the west. Furthermore, there is dry Saharan air wrapped around the wave, capping sustained convection and restricting low-level convergence. Both of these factors combined make for a very unfavorable environment. Strong shear should persist across the entire Caribbean Sea and southern Gulf of Mexico over the next week. Given the current and anticipated conditions, it is no surprise models do not do anything with the wave. Development remains unlikely.
The East Pacific seems to be awakening. Two areas of low pressure are gradually becoming better organized well to the west of the Mexican coast. The westernmost, INVEST 99E, is picked up by all models and should become a weak to moderate tropical cyclone over the next few days as it moves west-northwestward. The system to the east, INVEST 90E, is progged by many models to strengthen into a more powerful system, probably a hurricane. Most models guide it on a northwestward track roughly parralel to the Mexican coast, but well offshore. Given the conditions, at least some development of both disturbances seems probable.
July 09 2006 - 23:30 UTC
The tropical Atlantic remains very quiet this evening. The convection in the Gulf of Mexico has fizzled and shear remains strong. The isolated convection north of Puerto Rico and Hispaniola is associated with a nontropical, upper low. No development is expected from that area. The wave in the central Atlantic has shown no signs of organization even though it has been classified as an INVEST. Convection remains widely scattered and surface convergence is still limited. Shear is high near the Lesser Antilles, and it would have to lessen significantly before the wave's arrival for it to have a chance. We've seen that happen before, but it is not likely this time around.
Some of our readers questioned our contrast between 2006 and 2005 in yesterday evening's discussion. We were not comparing 2005 to 2006 at all. There is a key difference. It is important to notice the differential between the two seasons after half a year of unrelenting 2006 season hype from the media. Hurricane seasons will not "only get worse." We're in an active cycle that may last an additional 10-20 years, but seasons rivaling 2005 will still be very hard to come by. This year, there are no signs of a recording setting season in terms of net tropical cyclone activity. Some may fire back by saying no one saw 2005 coming, but had there been a modern day 28NS benchmark, then forecasters would have been at least somewhat more aggressive. Although sea surface temperature anomalies in the northern Atlantic have recently become well above average, anomalies in the deep tropics, including the Mean Development Region, don't even compare to 2005. Also, other global parameters are causing upper level conditions to be slightly less favorable than they were one year ago. The proof is in the pudding. As shown in 2005, almost nonstop activity all season is required to reach a total of 28 named storms. By July 12th, we'll be 4 named storms behind 2005's record setting pace, and that drives home exactly what we've been stating even before the start of the season. Our 2006 seasonal forecast called for only 1-2 named storms total by the end of July, and 14 named storms by November 30th. 2006 would be a very average season in contrast to 2005, and the forecast of 14NS is even below several other agencies' forecast totals. 2006 is following the forecast so far, and that was the point we tried to make yesterday.
I should also mention that the east Pacific lull may come to an end this week. There are two areas of potential: one low approaching 110W and another near 100W. It looks reasonable to say that at least one of these low pressure areas will develop over the next several days. The second system has a better chance of impacting the Mexican riviera, but we'll have to give it another look tomorrow.
July 08 2006 - 23:30
The forecast lull of tropical cyclone formation has panned out. Other than the disorganized low that passed over eastern North Carolina, there has been little activity since Alberto. 2006 is currently three named storms behind 2006's record setting pace. 2006 will be 4NS behind if a storm does not form by July 12th, the anniversary of Emily. Today, there are still no signs of imminent development. The only games in town are a weak wave over the central Atlantic and a trough over the Gulf of Mexico. The CATL wave is lacking surface convergence, bursting convection is lacking, and the wave overall is highly disorganized. Upper level shear will become slightly stronger over the wave as it heads westward. Model guidance doesn't want to do anything with the wave either. The GFS is most aggressive, if you can call it that. The 18Z run shows a weak, inverted trough that isn't closed off. Lowest progged surface pressure is only 1014MB, and keep in mind that is in the deep tropics. Taking it once step further, the GFS has had a tendency to over amplify tropical waves passing over the Mean Development Region. The overly aggressive Canadian model isn't even progging the formation of a closed off low. Meanwhile, the stalled boundary over the Gulf of Mexico is even more appealing for those that want the lull to continue. Shear is borderline extreme over much of the Gulf, with northwesterly winds streaming into the area around the central U.S. ridge. The convection being generated over the open waters is not tropical in nature.
June 23 2006 - 19:00 UTC
INVEST 91L has been declared northeast of the Bahamas. A strong upper-level trough is centered over the Bahamas and is inducing scattered weak to moderate convection along its northern and eastern flanks. Although a few weak vortex maxes are noted rotating around and within the area, no defined low-level circulation exists. The whole system appears elognated from the northwest-to-southeast due to the nature and tilt of the trough.
This is not a favorable synoptic pattern for tropical cyclone development. The upper-level trough is forecast to slowly move westward over the next 2-3 days, maintaining a strong signature in the 300 and 250MB levels. Therefore, the conditions should not change much from how they stand now. The trough will continue to funnel dry air and high shear aloft, and convection will remain restricted south and west of the axis. It should also be noted that none of the global models strengthen this into a true tropical cyclone, but rather only show a very weak low-level vorticity reflection as the trough moves into Florida in 3 days. This is the most likely scenario. Even though tropical cyclone development is not expected, the moisture from this system will become entrained with a longwave trough dragging into the eastern US. Copious rainfall and flooding is a distinct possibility in the mid-Atlantic early next week.
Another area worth noting is a surface trough draped north of the Leeward Islands. Strong easterly flow is present in its vicinity and is currently limiting upper-level divergence. However, the CMC, GFS, and NOGAPS all develop this into a formidable tropical storm over the next few days as it moves northeastward. Around Day 5, the said models turn it west-northwestward under a blocking high pressure, an odd track that could threaten the northeast US or the Canadian Maritimes. On the other hand, the UKMET and ECMWF show no progress from what it is now, a surface trough. This is something that we will watch closely in the models. Tomorrow's runs may give more clues, as it is too soon to say which camp is correct.
June 19 2006 - 22:00 UTC
The tropics remain quiet worldwide. Several tropical waves mark the tropical Atlantic Basin, but conditions remain unfavorable for further organization. An area of convection related to western periphery of an upper-level low is located near the Bahamas. Nearly all of the models were showing some sort of low-pressure to develop from this area at 00Z through the next few days. However, since this morning, all of them have backed off. In the latest runs, the only model that actually develops a tropical cyclone is the NAM-WRF and even it is less agressive. With very little model support at the time and the strong shear likely to continue, development in this area is not expected.
Referring back to our 2006 Atlantic hurricane season forecast issued a few weeks ago, we forecasted 1 to 2 named storms before August. Alberto has already occurred, so if our forecast is correct, the Atlantic may remain rather quiet for a long time. It should be stressed that this would be normal and the amount of activity saw early last season was unprecedented.
June 14 2006 - 22:00 UTC
Alberto has become extratropical and weakened below tropical storm force as it passes through the Carolinas. The remnants will exit the NC/VA coastline soon and head off to the northeast as a gale.
This leaves the rest of the tropics very quiet. A tropical wave is noted in the Eastern Caribbean Sea and is approaching an upper-level ridge being forced by the trough that's sweeping Alberto. However, there is a large amount of subsidence in the vicinity as well, not to mentionb the wave is extremely poorly organized at the time. Furthermore, the low-level steering pattern favors a continued westward motion with very little northerly component, taking the wave towards Central America in about 2-3 days. The CMC is the only model that takes the wave into the Gulf of Mexico and even then it doesn't show any development. The CMC is also likely overestimating the weakness in the subtropical ridge, a common flaw in the model. Having said all this, development from the eastern Caribbean wave is not expected.
Two waves are also traversing westward between Africa and the Lesser Antilles. Both have lost almost all of their convection they originally had yesterday, not surprising given the seasonably strong shear and dry air aloft. The GFS is still trying to spurt at least one weak tropical cyclone in this region, but this is being dismissed due to the likelihood of being related to convective-feedback issues. The lack of support from other models, the apparently unfavorable conditions, and climatology are all against development as well. As with any wave, they will be watched if they get into the Caribbean Sea, but given the ridge positioning which is expected to remain in place for at least a week, they will likely follow in the footsteps of the aforementioned wave and avoid the Gulf of Mexico.
Overall, no development is expected in the Atlantic during the next 5 days at least. With the MJO beginning to shift towards favoring a less-convective Atlantic Basin, and the normally quiet time of the year, there is a good chance a several-week lull is commencing.
The East Pacific is quiet right now. The GFS, NOGAPS, and UKMET hint in development 4-5 days down the road from a weak low pressure currently well south of the Gulf of Tehuantepec. This feature is moving westward and if development occurs, it will be far away from any landmasses.
The West Pacific is also unusually quiet, and no development is expected here for the remainder of the week.
June 11 2006 - 01:00 UTC
Tropical Depression One has finally formed from the disorganized mess plaguing the northwest Caribbean the last several days. Signs of a closed surface circulation and confirmation of