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Archives: 2005
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2006
Note: IWIC forecast products are unofficial. Please read the IWIC Disclaimer. The discussion is posted 1-2 times per day (preferably in the evening) during periods of Atlantic tropical cyclone activity, time permitting. Ocassionally, the discussion is updated during significant tropical cyclone events in other basins. These discussions are geared mainly toward weather enthusiasts that have a basic knowledge of the atmosphere, and are already aware that NOAA is the only source that should be used during life-threatening weather events. However, members of the general public are still encouraged to view the discussions as long as they understand the disclaimer. IWIC Worldwide Tropical Weather Discussion - August 14 2007 - 02:30 UTC We're probably beginning to sound like a broken record, but we'll likely be greeted by both "Dean" and "Erin" within the next 36 hours. So lets get straight to business... 91L Invest was classified in the southeast Gulf of Mexico earlier today. Upper air conditions are gradually becoming more favorable for development as expected. There are signs of gradual organization this evening. There are also signs of multiple vortices. Until one surface circulation becomes dominant, it will be difficult to predict an exact landfall zone despite the fact that a landfall is likely 48 hours away at most. With that said, it will hit the general area of northern Mexico and southern Texas. All models agree with this scenario, and it is also supported by the current steering pattern in all levels. A sudden change in intensity should not result in a dramatic shift in the forecast track. Due to favorable conditions aloft and highly favorable sea surface temperatures, it is not unreasonable to think that the tropical cyclone could briefly attain Category 1 hurricane status just prior to landfall. Timing over water is what will prevent the storm from becoming more significant. Bottom line: a strong tropical storm landfall somewhere near the TX/MX border is a good bet. Now for the biggest news in the Atlantic tropics... The tropical wave in the central Atlantic was upgraded to Tropical Depression Four by the National Hurricane Center earlier this morning. The tropical depression is speeding westward, and it is still having a problem with easterly wind shear. The easterly shear is being enhanced by the stronger than normal mid-level easterly jet (MLEJ) that originates from western Africa. A strong MLEJ often enhances tropical cyclogenesis between Africa and the Lesser Antilles by enhancing their equatorward outflow channel, but only when the storms in question are centered north of the jet axis. Furthermore, strong upper level ridging is often located just north of the MLEJ. As to be expected, a large upper ridge is present north and west of the depression. One can see this using water vapor imagery and CIMSS shear profiles. Tropical Depression Four hasn't gained enough latitude to take advantage of this favorable setup, but all available model guidance depicts a west-northwest motion beginning within the next day or so. Also, as mentioned, the strong easterlies do naturally weaken slightly farther west as the low level flow around the strong Azores High becomes less dominant. With that said, upper air conditions are only going to improve, and it is no wonder why 7/7 intensity models are forecasting at least gradual strengthening into a hurricane through the period. The NHC 5-Day intensity forecast indicates strengthening into a Category 2 Hurricane by day 5, and that sounds reasonable. Keep in mind intensity forecasts are less accurate than track forecasts, and it will probably be too high or too low. I would not be overly surprised if it is on the low side given that sea surface temperatures are more favorable further west and dry air hasn't been an issue. The track forecast is also problematic, but it could be worse. All model guidance is in general agreement that the central and northern half of the Lesser Antilles will be threatened by a hurricane within 4-7 days. There is a lot of disagreement with regards to the timing of the storm's closest approach to the Caribbean, but we should have a better idea within 24 hours. The only potential good news, for some residents to the south, is that the models appear to be narrowing down a potential landfall zone to the islands north St Lucia. Now is not the time for residents of the lower Windward Islands to let their guard down (they should still be preparing for a potential landfall), but there does appear to be a higher risk from Martinique, northward to the Virgin Islands and Puerto Rico. This is a bit of a change from yesterday when even the central portion of the Lesser Antilles were well within what could be considered a high risk zone. Nearly all of yesterday's model runs had indicated TD4 would roar into the northeast Caribbean Sea as a hurricane with little sign of any turn to the north. Such a scenario would signify that a direct hit on one or several members of the Lesser Antilles would be inevitable. Such a track would also likely result in potential strikes on the Greater Antilles and an eventual track into the Gulf of Mexico. The synoptic reasoning behind the westerly model tracks was that a weakness (or mid to upper level trough) currently in the western Atlantic would lift north, well ahead of the hurricane, thus allowing the Azores High and a high pressure system over the southeast USA to merge. This would have resulted in a strong east to west flow in all steering layers of the atmosphere along 20-30ºN, thus pushing the storm well into the Caribbean and possibly the Gulf down the road. However, anyone that read last night's discussion was made aware of the fact that there were earmarks of a more northerly track in the synoptic pattern being outlined by the GFS. It was noted that although the GFS was continuing to push the hurricane well into the Caribbean, the weakness in the western Atlantic was trending stronger with each model update. Had the trough been any stronger in yesterday's 18Z GFS run, it likely would have begun showing a track north of Hispaniola 24 hours ago. Yesterday's 12Z UKMET also depicted the weakness, but the model only provides forecasts through Day 6, so it remains uncertain if the model would have shown a northerly turn beyond that period. It wasn't until the 00Z edition of the global model updates were released when the first significant shift in actual model tracks occurred. The 00Z Canadian, which admittedly isn't the best of models, began to depict an almost immediate turn to the north while remaining east of 60ºW through Day 6. Oddly enough, the abrupt change in the Canadian doesn't appear related to the trend in the progged synoptic pattern of the GFS that was being observed. Instead, the Canadian is waiting for an upper low currently near 50ºW and 30ºN to breakdown a section of the subtropical ridge before it lifts the hurricane north and east of the Caribbean. It is unknown if the Canadian would have shown recurvature ahead of the western Atlantic weakness had it not indicated recurvature along 60ºW. The remainder of the global models remained virtually the same at 00Z. The most significant shift in the model consensus over the last few days did not occur until today's 12Z updates became available. The Canadian model still shows almost immediate recurvature. However, more importantly, the 12Z version of the UKMET, GFS, and GFDL also shifted significantly north and east. All three models indicated that the storm would either pass through the extreme northeast corner of the Caribbean island chain or it would pass the islands just to the north. After observing the forecast setup of the 500MB pattern in the most recent UKMET and GFS, it looks as if the models are responding to both the upper low in the central Atlantic, and the cutoff low (weakness) that has been trending stronger in the western Atlantic. The latest paths outlined by all of these models appear well within reason given that setup makes sense. Therefore, the 5PM NHC forecast track appears on target give or take a few miles. There is still an outside chance that the storm will pass north of the Caribbean, but I wouldn't take my chances. Assuming the latest medium range model shift is accurate, and that a complete Caribbean tracker is less likely, the real uncertainty in the expected path of TD4 may not develop until it is north of Puerto Rico. From there, two primary scenarios exist. First, the weakness in the west Atlantic is just strong enough or persists long enough to pull the tropical cyclone north of Puerto Rico. However, the weakness then proceeds to lift north, leaving the storm behind. As the trough lifts out, the two high pressure systems, one over the northeast Atlantic and the other over the southeast USA, build north of the storm, thus forcing it westward toward Florida. The second scenario is the weakness in the west Atlantic persists long enough that it completely picks up what should a hurricane by then, and recurves it out to sea, east of the US mainland. Thereafter, the Azores High and southeast USA high merge, but it is too late to force the tropical cyclone westward. The 12Z ECMWF supports the Florida scenario whereas the 18Z GFS shows complete recurvautre. Both options are 5-6+ days out so this is highly speculative, but it is interesting to note that the latest GFS run may be more legit than the ECMWF for a couple reasons. First of all, the ECMWF did not initialize the tropical cyclone strong enough. A weaker initialization may be one of the reasons why it is the only global model to show hardly any strengthening through the period. Additionally, a weaker system would be less susceptible to being recurved by a mid to upper level weakness. Second, both the GFS and ECMWF clearly show the same weakness that would be responsible for recurvature. We'll take another look at the guidance overnight and into tomorrow before we even attempt to decide on if either scenario has a significantly higher chance of verifying. There's always the outside possibility the models will shift west into the Caribbean again, but there's no signs of that as of 18Z model depicted patterns. In the central Pacific, Category 3 Hurricane Flossie is still expected to remain south and west of Hawaii. However, a brush is certainly possible. A hurricane watch is in effect for the big island due to the possibility that Category 1/2 conditions may still be felt. Please adhere to recommendations being given by local emergency management officials in the region, along with the Central Pacific Hurricane Center. August 13 2007 - 01:00 UTC While there remains potential for development in the Gulf of Mexico, the highlight of the evening is the low pressure south of Cape Verde. Satellite imagery and QuikSCAT data indicate that the low is gradually becoming better organized and should be classified as a tropical depression by tomorrow. Convection has blossomed closer to the low-level circulation, although it remains mostly exposed to the east. Some upper-level outflow is noted, but is being restricted largely in part to easterly shear over the system. This will abate as the low moves westward and loses its grip on the tropical easterly jet. Analysis of mid-level water vapor content indicates some dry air about 10 degrees ahead of the system and a similar distance to the north. On the other hand, the low is enveloped with moisture, which is preventing the dry air from wrapping into its core. A simple water vapor animation confirms this by showing the dry mid-level air is moving westward in tandem with the low while significantly decreasing in coverage. With plenty moisture, improving shear, and warm ocean temperatures, continued organization is expected over the next several days. In fact, the system will likely become a hurricane by the end of the week. This is in agreement with the majority of the global and tropical models. As the system intensfies, the big question is where will it go? It is currently under easterly steering flow, owing to the strong Azores High to the north. All model guidance indicates that this feature will persist through the next several days, so a continued west to slightly north-of-west motion across the deep Atlantic is likely. This will bring it into the Lesser Antilles by Day 5 at its current pace. Model guidance is tightly clustered on a track through the central or northern Lesser Antilles at that time, probably as a hurricane. Given the steering setup, this seems very likely. It is a bit too early to say exactly which islands will receive the greatest impact but there is little question that this will hit somewhere in the eastern Caribbean. All residents from Trinidad to Puerto Rico are urged to check their hurricane supplies and closely monitor the progress of this storm. It is also much too early to say with confidence where this storm will go beyond the probable eastern Caribbean impact at Day 5. However, there is room for speculation. In typical IWIC fashion, we will analyze this storm's long-term track potential. One might be aware of some disagreement between th ECMWF and GFS. The 00Z ECMWF showed a hurricane completely missing the Caribbean while feeling influence of a trough to the north. On that run, it would seem a recurve east of the United States would be most likely. Yet, the 12Z ECMWF completely loses the system in the near-term. A few things here. First, the 00Z run was not accurate in the depiction of the storm's forward motion; it indicated a much slower storm than reality. This allowed the ridge to erode enough to send the storm on a more northward component. Since the storm is actually moving much faster to the west, a Caribbean miss to the north as that ECMWF run showed is highly unlikely. Second, the 12Z run was inaccurate on the system's current structure. It indicated a highly sheared system with a mid-level center significantly southwest of the low-level center, both fully degenerating within a day or so on the model. In actuality, the low-level center is only slightly exposed to the east, with the system only becoming more vertically stacked in time. Thus, the 12Z ECMWF scenario is highly suspect. Since the ECMWF is no good today, this leaves us with the GFS. Unlike the ECMWF, the GFS has been remarkably consistent with this storm, even in the long-range, over the past few days. All the runs today take the storm westward through the Caribbean Sea, into the Gulf of Mexico, and a final destination of Mexico/extreme southern Texas next week. However, there is another possible scenario based on these very runs. The GFS has notably been trending weaker and further east with the expected mid-level ridge north of this system while in the eastern Caribbean Sea. Moreover, the past two runs have featured a 500mb weakness over the Bahamas during this timeframe, with the 18Z run depicting an especially defined weakness. For whatever reason, the model does not see this as strong enough to pull the tropical cyclone northward. However, the trend is your friend, and this is an important trend to watch. If the weakness becomes more defined, then it would steer the cyclone northward into or just east of the Bahamas. Afterwards, it would be far enough north to feel the influence of the larger mid-level trough over the northeast US and Canadian maritimes that most global models indicate will be present at the time. Thus, the storm would continue northward and remain offshore the east coast of the US. It should be noted that the UKMET also shows this feature by Day 6, but that's as far as the model runs so it is uncertain how it would interact with the tropical cyclone. Again, the other scenario that is more supported by models at the moment is a continued westward motion bringing the storm into the Gulf of Mexico. If that occurs, one cannot pinpoint a specific Gulf location to be the target, although early synoptic reasoning suggests Mexico or Texas would be the most likely to be impacted. There is still plenty of time to watch this developing system. Development remains a possibility in the Gulf of Mexico over the next few days. A tropical wave in the western Caribbean Sea will move into the area in the near-term. Although models have backed off somewhat on actual tropical cyclone formation, several continue to show the emergence of a low pressure. The GFS has actually trended stronger with a low pressure today. If something were to develop, current and progged steering currents suggest a landfall in northern Mexico or southern Texas. A more detailed analysis on this system will be posted if tropical cyclogenesis looks more probable, as right now it is second in concern to the East Atlantic low. Finally, Category 4 Hurricane Flossie remains well-defined as it moves closer to Hawaii, although some weakening will likely take place soon as it moves into cooler water. Current steering analysis of the deep system indicates that it should pass close enough to the big island for its effects to be felt. All residents in Hawaii should be monitoring their local and official information on the strong storm. August 11 2007 - 01:45 UTC The potential for tropical cyclone formation in the eastern Atlantic remains high. The southern Gulf of Mexico also remains a suspect area for development next week. Even if formation occurs in both areas, it looks like the first formation would occur in the eastern Atlantic. Therefore, expect to see Dean in the eastern Atlantic, and possibly Erin down the road in the Gulf. For the sake of continuity, the possibility of Gulf development will continue to be the lead topic of discussion this evening. A series of tropical waves are continuing westward in the Caribbean Sea within the strong low level easterly flow. Although convection across the Caribbean remains disorganized, it has been enhanced by an upper trough over the Greater Antilles. Nevertheless, upper winds across much of the Caribbean remain unfavorable for tropical cylogenesis in the short term. I emphasize "much" of the Caribbean because the favorable upper ridge that was just north of Colombia yesterday has propagated northwestward between Beliize and Jamaica. Upper winds in this area are less than 10-20 knots, which is conducive. However, there currently isn't much low level convergence in this area. Nevertheless, several model do show additional energy spreading northwest along one of the wave axes out of South America. A broad area of low pressure is expected to form near or over the Yucatan Peninsula in approximately 72 hours as this energy spreads northwest. The aforementioned upper level ridge should move in tandem with the broad low center into the southern Gulf of Mexico. If the broad low can move far enough away from the Yucatan and southern Mexico, then development may commence in the Gulf within 5-8 days. There remains a high degree of model disagreement regarding how much moisture makes it into open waters. The GFS and UKMET never have developed this system, and although the GFS showed a weak low in the southwest Gulf for the first time as of the 12Z run, it still doesn't support tropical cyclone formation. Both models may be attempting to develop a tropical system in the eastern Pacific, and that would suppress any formation in the Gulf. Meanwhile, the CMC, NOGAPS, and ECMWF still show minimal tropical cyclone development between the Bay of Campeche and northern coast of the Yucatan. Upper air conditions should be at least marginally favorable as the upper ridge east of Belize spreads into the Gulf. If there's enough instability and surface convergence then genesis should occur. With that said, development is still far from a guarantee. Any tropical system that does form in the Gulf of Mexico within the next 5-10 days is bound for Mexico or possibly coastal Texas. Louisiana and points eastward have little to be concerned about concerning a direct impact from this potential storm. A large ridge of high pressure centered over the central United States will continue to block any tropical system from moving due northward. If the low meanders over the western Gulf well beyond Day 5, then a slight weakness may open over the lower Texas coast. The rest of the US Gulf Coast should remain protected. What may soon become the main story in the tropics is the tropical wave exiting Africa this evening. Every global model, with the exception of NOGAPS, is very bullish on this system. The GFS has consistently depicted development for several days, and the latest runs are not backing off. Additionally, the CMC, UKMET, and ECMWF have become equally as aggressive over the last 24 hours. Based soley on the model consensus, it is not a question of if there will be development, but when the first advisories will be issued. Several models, most notably the GFS and UKMET, continue to show tropical cyclone formation almost immediately once the low level center enters the Atlantic. One must question if such immediate formation is realistic regardless of how favorable conditions in the eastern Atlantic may be. By day 6 and beyond, all of the aforementioned models show a significant hurricane due east of the Virgin Islands. The synoptic setup in the eastern Atlantic does appear favorable for significant development. The Saharan Air Layer near the African coast is retreating northward along with much of the additional dry air or subsidence. the mid-level easterly jet to the south of the wave axis is very strong, and that is often a good indicator of a favorable upper air environment. The easterly flow south of the Cape Verde Islands is even a bit too strong at the moment, but the global model guidance shows those winds relaxing with time. All track guidance also keeps the wave in question well south of the 26ºC isotherm, thus making sea surface temperatures warm enough and favorable for development. We should know if all of this is hogwash or not very soon as we should begin to see signs of actual development within 72 hours at the latest. The three day mark even accounts for the possibility that the models may be developing this wave too rapidly. By Day 7, there's good reason to believe a significant hurricane will be looming in the central Atlantic. Once the wave begins to strengthen, the evergrowing question will become where is it headed? For the next week or so, the GFS will likely provide interests between Texas and the Canadian Maritimes with a lot of entertainment as each 6 hourly model update shifts 2,000 miles give or take a few. The GFS simply cannot forecast whether the United States will be directly impacted by this storm just yet. It's not only the tropical cyclone that it will have a hard time forecasting, but also the entire global pattern that will be responsible for steering the storm in the first place. The GFS Ensembles are not immune to medium range errors either. Furthermore, even though the ECMWF is looked upon as the superior medium range model, it too often has trouble forecasting tropical cyclones with any high degree of skill beyond 5-6 days. A lot can happen. I only have two general ideas that I think are most probable. The best scenario is recurvature near 60ºW before a trough over the central Atlantic lifts out. Unfortunately, if that trough lifts out too early, then the ridge over the southeast United States and the Azores Ridge will merge. The potential hurricane would then bend westward toward the Bahamas, Florida, and Gulf of Mexico. The third scenario is an East Coast Carolinas landfall, but the ridge over the southeast should block any storm from recurving into that region. But even that cannot be ruled out at this time, because it is based on the assumption that the synoptic pattern provided by the models is at least halfway accurate. With that said, the more immediate concern is what threat the northeast Caribbean could be facing within 6-10 days. I'm sure interests in the northeast Caribbean that keep up with the models will be looking for a yes/no answer soon as each model update makes them a bit more uneasy. There's no definite answer even for that region this evening. But now is not yet the time to become increasingly alarmed or concerned. There's still no tropical cyclone on the maps, and any storm is still 6+ days away. The 18Z GFS and 12Z ECMWF have trended closer to the Caribbean, but that is a trend that will have to be given another look over the next couple days. Until then, this is the perfect time to re-evaluate your hurricane preparedness plan. August 10 2007 - 01:00 UTC The potential for tropical cyclone formation in both the Gulf of Mexico and eastern Atlantic remains the central focal point of this evening's discussion. Last night, it was stated that development was not a guarantee, but the probability was increasing for both areas. 24 hours later, we're confident enough to say that Dean will likely form in the eastern Atlantic within 2-3 days, and Erin may soon follow in the Gulf of Mexico. While the first storm in this apparent burst of activity may form in the east Atlantic, the potential Gulf storm will continue to garner the most emphasis for a second consecutive evening due to the certainty that it will impact land. The tropical wave that will be responsible for the disturbed weather in the Gulf next week is still south of the Dominican Republic and in the eastern Caribbean. The wave has become more convectively active over the last 24 hours, but development in the short term still isn't anticipated. A mid to upper trough just north of the Caribbean is producing moderate westerly winds aloft over the region, thus keeping the wave in check over the next few days. Meanwhile, an upper level ridge is present just north of Colombia. If this ridge were over the eastern Caribbean, then the beginning stages of development would likely be underway. However, the wave axis and upper ridge should not become vertically stacked until the wave has entered the far northwest Caribbean or southern Gulf of Mexico. The upper ridge will have a difficult time spreading northwest until the upper low currently over the Yucatan Channel moves into the western Gulf. Once the upper ridge finally moves into position, the low level easterly wave will have caught up with the anticyclone. Once firmly established under the favorable pocket of divergent winds aloft, tropical cyclone formation may soon commence thereafter. The number of models that are indicating Gulf storm formation haven't changed since yesterday, but they have become more bullish. The 12Z NOGAPS model now depicts surface low formation over the Isle of Youth and southeast Gulf Monday and Tuesday (Days 4-5). The NGP then depicts gradual strengthening into possibly a minimal tropical cyclone as the low center turns west and moves inland over Mexico in 7-8 days. The 12Z ECMWF is very similar. It depicts a broad area of low pressure passing over the northern half of the Yucatan peninsula on Tuesday and entering the southwest Gulf on Wednesday. The Euro then shows gradual development into a minimal tropical cyclone before making landfall in Mexico within 8 days. The Canadian model also continues to show Gulf development, but the track forecast remains highly suspect. The model develops two separate low centers, and such features would interact with each other and highly affect the dominant storm's path. On the other hand, the Canadian does seem to have the correct overall idea in that it maintains a mid-level high to the north, which would result in a westerly track. The GFS continues to show nothing more than a broad area of low pressure, most of which remaining south of the Gulf of Mexico. Incorrect steering wind analysis and the resulting amount of land interaction in the GFS runs may be limiting development. The same can be said about the latest UKMET run. In summary, several global models depict tropical development once the eastern Caribbean wave enters the southern Gulf. Furthermore, the same models are indicating any tropical cyclone that does form will turn west and aim for northeast Mexico. Both the possibility of southern Gulf development and a turn toward the west in the general direction of Mexico seem legit after analyzing the overall pattern. The wave in question will move beneath a favorable ridge aloft as it enters the northwest Caribbean and southern Gulf, making tropical development a good possibility. The main steering parameter will become the large dome of high pressure that has been stationary over the central and southeast United States once the system moves into the Gulf. There are no signs that this ridge will weaken or leave the central Gulf Coast through the period. Even the models that don't depict tropical development agree that this ridge isn't going to budge. Therefore, any storm that does develop will threaten northeast Mexico, and we cannot entirely rule out coastal Texas. Formation still isn't a guarantee, but a depression or named storm would be a good bet. It's too early to be throwing around the word "hurricane", especially considering this is a medium range forecast and the model consensus could be better. That of course could change. Elsewhere, the potential for tropical cyclone development in the eastern Atlantic is also increasing. A strong tropical wave is expected to exit the coast of Africa within 48 hours. The wave was approaching 10ºW longitude in western Africa as of the 18Z satellite shot. Satellite data over Africa is limited, but the wave axis does appear to already have an excellent satellite signature. Normally a wave's signature over Africa is not significant since even the most suspect waves often temporarily weaken as soon as they exit Africa. However, several global models have consistently depicted or hinted at the possibility of tropical cyclone formation as soon as the wave enters the eastern Atlantic. Both the GFS and UKMET indicate tropical cyclone formation no later than 60 hours. The GFS has consistently shown immediate development on every run for the last two days. That degree of consistency is significant. By Day 7, the model has a significant hurricane moving west-northwest over the central Atlantic. On the other hand, the last two runs of the UKMET are the first of which that not only show true tropical cyclone formation, but development within 3 days. By Day 6, the UKMET shows a decent tropical storm over the central Atlantic. The Canadian model is a little slower in that it depicts development within 4-5 days. However, this is an improvement from previous Canadian runs that only hinted at the possibility of development. By Day 6, the Canadian now shows a tropical cyclone bordering on hurricane intensity. Perhaps the most notable model update since yesterday evening comes from the 12Z run of the ECMWF. The ECMWF, one of the more conservative models, backed off on developing this wave into a tropical cyclone over the last few days. However, as of 12Z it once again shows formation within 72 hours. By Day 10, the Euro depicts a potential hurricane passing just north of the Virgin Islands, much like the GFS. So why should we believe the bullish global model guidance when so many African waves routinely fizzle as they hit water? First, no tropical wave this season has had this amount of model support. Second, several models are developing this wave within 72 hours. Third, the level of consistency being displayed by the GFS is amazing. Fourth, the conservative ECMWF is now clearly showing development. Fifth, the wave already has an excellent satellite appearance. Finally, conditions appear favorable for tropical development in the eastern Atlantic. The Saharan Air Layer has not been abnormally strong over the past week. Additionally, the intensity of the SAL during the first half of the season hasn't been all that unusual. The parameters required to have an above average Cape Verde season still appear to be in place, and we may begin to see the initial results within the next few days. The latest water vapor imagery reveals that the eastern Atlantic is about as moist as it's going to get, with much of the usual dry air or subsidence retreating well to the north. Sea surface temperatures are below the necessary 80ºF north of 15ºN between Africa and 40ºW, but all model guidance is keeping the potential tropical cyclone south of that latitude. Thus, sea surface temperatures are favorable. Upper level easterly winds are a bit strong at the moment. However, the models that do depict development show increasing ridging aloft over the central Atlantic along with a favorable mid-level easterly jet to the south within the next few days. In conclusion, Dean is on the way. Nearly every global model shows development. Conditions in the east Atlantic look favorable for tropical storm formation. The tropical wave that will likely spawn development looks healthy as it edges closer to the African coast. We may see classification within 72 hours. I should also add that the general track over the next seven days is relatively straightforward. An abnormally strong subtropical ridge will keep this system on a west to west-northwest heading through the period. By Day 7, the storm will still be well east of the northeast Caribbean. Interests there have plenty of time to monitor this storm's progress. It is too early to speculate beyond this period. It is also too early to speculate on how intense this potential system could become, but there is enough reason to assume we could be talking about a hurricane in the central Atlantic. August 9 2007 - 01:15 UTC There will be two areas worth watching heading into next week. The first area of concern will be the northwest Caribbean Sea and southern Gulf of Mexico. The second area is the east Atlantic. Development in either location is not a guarantee, but at least one tropical cyclone formation stemming from these areas is becoming increasingly likely. The length of this evening's discussion should not confuse the reader into thinking that there's alarming news about the tropics just yet. The pattern just happens to be fairly complex, and there's some potential for the tropics to become active soon. The source of any northwest Caribbean or Gulf development down the road would be the low level energy currently passing through the eastern Caribbean. The 12Z surface analysis provided by the Tropical Analysis Forecast Branch indicates that a tropical wave is passing over Hispaniola. The wave is enhancing diurnal convection along 70ºW. The wave is expected to "split", with some low level energy recurving into the Bahamas and west Atlantic, while the lower extent of the wave axis continues westward toward the Yucatan. A secondary wave is passing through the Windward Islands this evening. This wave does not look as suspect on satellite imagery, but the amount of convection each wave is currently producing is not significant. There is too much wind shear over the central and eastern Caribbean for development to take place in the short term. Once all of this low level energy enters the west Caribbean, it will begin to slow down and gradually gain latitude. If one remembers 99L Invest, the last Caribbean wave that had the potential to develop, it ran into Central America (primarily Honduras) before it got its act together. Conditions were becoming increasingly favorable as that system moved inland. The reason why 99L slammed into Central America was because the low level easterly steering flow was strong across the entire Caribbean. The mid-level steering weakness that has been present over the East Coast was not strong or deep enough to arc the low level easterlies into the southern Gulf, thus less time over water and no development. In this case, a stronger ridge over the central United States is allowing weaknesses to travel further down the East Coast, which in turn is bending the low level easterly flow over the Caribbean into the Gulf. With that said, it is unlikely that the wave in question will continue westward into Central America rather than bending slightly north and into the Gulf. The real potential for any development will begin once the wave(s) enter the southern Gulf of Mexico. All model guidance shows the central United States ridge persisting through the period. Upper winds usually become favorable for tropical cyclogenesis just south of such large domes of high pressure. In this case in particular, one reason for the increased favorability are mid to upper level disturbances (or low centers) that will likely branch off of the shortwave troughs digging across the East Coast once they reach Florida's latitude. Thereafter, they will propagate westward toward Mexico underneath the central US ridge. While mid to upper lows often carry the reputation of being tropical cyclone killers, the southwesterly winds along their backside often enhance upper ridging and divergence in areas they leave behind. Thus, in this pattern, we should expect to see the central Gulf of Mexico become periodically favorable for tropical development. The latest water vapor imagery shows one such upper low moving west along the Mexico/Texas border, and another over Cuba that will be taking a similar route. Over the next week or so, we should anticipate seeing a series of similar upper lows moving westward, with pockets of increasing upper divergence and ridging between each low center. If the timing is good, then the wave(s) will enter the southern Gulf in tandem with one of these favorable "pockets". Tropical development would soon commence thereafter. Some models are beginning to latch onto such a scenario. The 12Z ECMWF model run is one of the more aggressive runs regarding Gulf development. It does not depict a full blown Gulf storm just yet, but the fact that one of the more conservative models (the ECMWF) is even showing a well organized low pressure center is worth noting. The ECMWF has also been very consistent over the last few days. It has shown a weak low center forming in the southern Gulf between Day 7 and Day 10 (next Wednesday heading into next weekend) for several runs, and it is becoming stronger with each update. The NOGAPS model is another conservative model that is hinting at the possibility of formation. The NOGAPS guidance is only available online through Day 7.5, but it shows an increasingly broad area of low pressure in the region where the ECMWF is depicting development beyond that period. A less reliable model, the Canadian model, has also shown development as of the 00Z and 12Z runs. The 00Z Canadian run showed a low pressure system forming in the southern Gulf within 10 days, much like the latest two ECMWF runs. However, the 12Z run of the CMC is less credible. It shows two storms in the Gulf, one being a major hurricane. In short, such a setup is not possible, so the run will be discounted. Finally, the UKMET and GFS depict a broad low in the same region, but with little development. In summary, as of today there's a slowly developing model consensus that the southwest Gulf will have to be monitored for tropical development between Day 6 and Day 10. Given the synoptic weather setup, it is easy to see why the models think the Gulf will be an area worth watching. Nevertheless, until there's more model agreement and a disturbance becomes more active in the west Caribbean, the odds of development will not be dramatically high just yet. One thing that hasn't been emphasized, but is relatively straightforward, is the path any Gulf storm would take over the next 10 days. Now a 10 Day forecast path is usually an oxymoron on this site, but there is a high degree of confidence that the central US ridge will remain in place through the period. Any tropical system that forms immediately south of this ridge would be sent westward toward Texas or Mexico. Nobody will be able to narrow it down between one or the other until within just days of landfall, if indeed there is a landfall. This is still highly subject to change, so all interests in the northwest Caribbean and Gulf Coast should continue to monitor this potential storm. The second area to watch is the eastern Atlantic. The GFS has been amazingly consistent in that a tropical wave will exit the coast of Africa within 54 hours, and then begin developing shortly after. The GFS has developed the same tropical wave for at least the last 5 model runs. Furthermore, each run has shown a gradual increase in the progged tropical cyclone's intensity as it travels across the central Atlantic through Day 7. A model typically isn't this consistent with both the timing and rate of development even when they're at their best. What make the GFS runs even more unusual, is that there's still not a whole lot of model agreement among the remaining global models. The reliable ECMWF model was developing this wave several days ago, but has since backed off. The ECMWF still clearly depicts the wave in question, however. Meanwhile, the UKMET and Canadian models have been hinting at development, but they have yet to indicate the formation of a true tropical cyclone. I'd rather continue speculating about the possibility of Gulf development when there's this much disagreement. With that said, it will be very interesting to see what the next batch of 00Z guidance has in store for us. Afterall, the GFS shows the wave exiting Africa within only 54 hours. That is no longer what forecasters determine as a medium range forecast. Furthermore, if the GFS is correct, then we're going to be talking about the first hurricane of the Atlantic season within the next week or so. We'll break the east Atlantic down tomorrow if the guidance remains interesting. August 2 2007 - 00:00 UTC 99L Invest has struggled as it traverses across Caribbean waters, but that was anticipated. There has been too much dry air, westerly shear, and not enough surface convergence. But that is not to say development has been completely out of question. In fact, there is still an outside chance of development. Currently, there is an upper low moving westward over the Yucatan Peninsula. This upper low, combined with an upper ridge over the southwest Caribbean, is producing moderate westerly flow over the wave axis. While this setup is not favorable for organization, it is conducive for thunderstorm development. Over the next few days, the wave will continue heading west in the general direction of Belize. During this time, the upper ridge over the southwest Caribbean will begin moving northwest, which could lessen westerly shear to some extent. However, 99L will still probably be under too much shear and it won't have enough time over water to strengthen into a tropical cyclone before landfall. With that said, the only remaining chance of development would be if enters the Bay of Campeche or southwest Gulf of Mexico, and the upper ridge becomes the dominant force aloft at the right time. So, what are the factors that could limit Gulf development you ask? First, half the models keep 99L south of the Gulf. Second, if it enters the Gulf it won't remain over water for long. Finally, the ridge position only has a 50/50 shot at being in the right position for tropical cyclogenesis by the time 99L enters the Gulf. With all this in mind, the potential for 99L to become our next storm is likely still well below 50%. Even if development were to occur, we are not talking about a hurricane...tropical depression or storm at the most. But as long as it has potential, interests in Honduras, Belize, the Yucatan Peninsula, and the surrounding areas of Tampico, Mexico, should keep an eye on 99L. 99L is NO threat to the United States Gulf Coast. A large dome of high pressure will continue to dominate the northcentral Gulf through the period, thus making recurvature impossible. This type of pattern is not one that usually brings rapid model shifts. 99L will remain south of Texas. Other than 99L, there aren't any other specific areas to monitor just yet. But since we are in the month of August, don't expect any lulls to last as long as the one observed prior to Chantal. The impressive waves exiting Africa will continue to be monitored. The models are also showing an influx of moisture entering the western Atlantic out of the deep tropics, but none are becoming bullish on development just yet. Finally, the 00Z runs of the ECMWF and NOGAPS were indicating that a broad low pressure system would originate out of the southwest Caribbean beyond day 7, but not surprisingly the 12Z runs have backed off. All forecasters must be weary of looking at any model guidance beyond day 5. August 1 2007 - 04:45 UTC As Chantal leaves the picture, the main area of interest tonight is the broad area of low pressure east of the Windward Islands. This system, designated Invest 99L, has been pulsing in organization over the past 48 hours. Currently it is exhibiting a healthier satellite signature with moderate convection bubbling near the center, which itself is tucked within a poorly-defined upper-level outflow pattern. With a generally favorable shear regime, one might expect decent intensification to soon commence. However, this will not be the case due to the stability of the surrounding environment. Dry air aloft is present directly north of the system, while the mid-level Saharan Air Layer is undercutting the system from the east. All this dry air is keeping convection from sustaining itself through the diurnal cycles. Thus, any development over the next 48 hours with this low will be slow to occur. Beyond 48 hours, when the system has entered the Caribbean Sea, it will likely face even more problems. First, global model guidance indicates an increase in westerly shear aloft across the the Caribbean Sea. Second, the upper-level ridging north and east of the Bahamas will persist and continue to usher dry air from the northeast into the region. Moreover, the low is still associated with the intertropical convergence zone. Once it breaks free within a day or so, it may be even more difficult to produce convection, especially without an upper-level low to its west to provide ventilation. Finally, none of the global models develop the system except the CMC, which shows strengthening in the midst of strong shear, a weird (not so weird for the CMC) and unlikely scenario. Based on this data, any low or tropical cyclone is going to have a very hard time surviving the Caribbean trip. If a tropical cyclone were to somehow manage in the hostile environment, the mid to lower level ridging to the north suggests a continued westward motion towards Central America. With strong ridging expected to only strengthen in the Gulf of Mexico over the next few days, any system in the 4-6 day period would not deviate further north than the Bay of Campeche. Bottom line: don't expect this system to develop much, if at all...especially in the Caribbean Sea. Nonetheless, it is a low pressure in the tropics in the month of August...so it is still worth monitoring. The intertropical convergence zone continues to spur areas of convection in the tropical Atlantic between the Lesser Antilles and coast of Africa. This will be the region to watch over the next few weeks. July 27 2007 - 01:45 UTC The tropical Atlantic Basin has been in a lull for nearly two months, but conditions can change in a hurry. The current number of tropical entities worth discussing is evidence that such a transition is underway. A surface trough and associated mid-level circulation is moving north, out of the Gulf of Mexico, and into Texas and Louisiana. The National Hurricane Center hasn't been overly concerned about this feature due to unfavorable conditions aloft. An upper level ridge is developing over the southwest Gulf, but the disturbance is already caught in a deep southerly steering layer, which is being enhanced by the ridge to the south and an upper level low over Texas. Such conditions are not favorable for surface low formation. Furthermore, there are no signs of development at this surface based on latest buoy observations and scatterometer wind data. This feature won't amount to anything more than a heavy rainfall event for the western half of the US Gulf Coast. The National Weather Service will likely issued flood watches later this evening or tomorrow. The strongest potential for tropical cyclone formation in the Atlantic Basin over the next 10 days will be between the Lesser Antilles and the coast of Africa. This possibility has been discussed on our primary page and at the Geo-Earth forum dating back to July 12th. Within the past week, there has been increasing model support to back our original ideas. Now to the nitty gritty... Over the last few days, there has been a lot of online chatter concerning a tropical wave that will exit Africa within 48 hours. This wave will be discussed in detail in a moment, but there are other notable features farther west that must also be discussed. First, a tropical wave and associated 1010MB surface low is located near 45ºW. The NHC only recently mentioned this surface low in their tropical weather discussion. This evening's satellite imagery shows convection developing in close proximity to the low center, and the NHC noted a nearby ship report of a westerly wind of 15 knots. Several NOGAPS, CMC, and UKMET model runs have hinted at surface low formation along the wave axis over the last few days. So while there has been little fanfare associated with this wave, there have been indications that it would become more active. This surface low is one that must be monitored despite the fact that it is being completely overlooked by the GFS, and none of remaining global models depict actual tropical cyclone formation. Although there is some dry air surrounding the wave, upper level winds in the central Atlantic aren't terribly unfavorable. In fact, there's only modest easterly shear in close proximity to the ITCZ to the south. Furthermore, dry air often can't be entrained into a well-defined surface circulation without the presence of moderate to strong shear. This wave deserves another look tomorrow. Afterall, it would be the first African wave to possibly pose a threat to any landmasses due to it's location. A second tropical wave has recently passed 35ºW. Convection flared around a mid-level circulation along the wave axis this afternoon, but activity has since decreased. Water vapor imagery depicts a weak mid to upper level low located due north of the wave. The upper low is likely responsible for the increase in diffluence aloft and convection observed earlier. There appears to be a lack of surface convergence, thus making short term development unlikely. Furthermore, CIMSS Steering Layer charts indicate that the easterly flow in the east Atlantic is relatively weak. Any westerly progression would be very slow. Furthemore, all global models are in agreement that a much larger, high amplitude tropical wave will exit Africa within 48 hours. The larger wave is expected to absorb the low level energy currently over the eastern Atlantic. If you're observing the models, don't confuse the secondary wave becoming the dominant low centerwith easterly motion....no tropical cyclones will threaaten the west coast of Africa anytime soon (humor). Once the second wave becomes dominant, slow tropical development should occur. All of the global models (GFS, ECMWF, UKMET, CMC), with the exception of NOGAPS, have hinted at this scenario over the last several days. Subsidence and the Saharan Air Layer should not be a major issue since the central and eastern Atlantic are becoming increasingly moist. The frontrunner axis near 35ºW will also help to act as a dry air buffer. Upper level winds should also remain relatively light. June 22 2007 - 01:30 UTC There's no change to our thinking in regards to the tropical wave over the western Caribbean Sea. There are no longer any models hinting at the possibility of tropical cyclone formation once the wave enters the southwest Gulf of Mexico. The lack of aggression is due to the wave's proximity to land along with it's interaction with upper level wind shear. A midlevel circulation did become apparent on satellite imagery in the late afternoon hours, but it is unlikely to persist due to the aforementioned reasons. This system will bring additional moisture to Texas and Louisiana, with the main axis moving inland over mainland Mexico early next week. The only other feature worth mentioning in the Atlantic basin is the low over Jacksonville. No development is expected as it is expected to remain over land before it becomes absorbed by an approaching trough. June 21 2007 - 01:30 UTC The tropical Atlantic remains quiet, but there are online rumblings over the possibility of west Gulf development in about five days. The potential source of tropical cyclone formation is the tropical wave currently located in the southwest Caribbean. The 00Z and 12Z runs of the GFS and ECMWF models indicated that this feature would cross Central America and eventually enter the central or southwest Gulf of Mexico. The ECMWF depicted a suspect broad low in the Gulf before moving into Texas or Mexico. The GFS was more bullish, showing almost the equivalent of a minimal tropical cyclone heading north or northwest toward the upper Texas coast and southwest Louisiana. The aggressive GFS runs should likely be disregarded. The model still appears to be suffering from convective feedback. The convective feedback problem is responsible for the progged development of a surface low along the northern extent of the wave axis while over the central Gulf in a few days. This is why the GFS is too aggressive and too far north. The GFS assumes the low will be strong enough and far enough north that it will be drawn into the US Gulf Coast by a minor mid-level weakness over the southeast. The rest of the models aren't nearly as aggressive. The Canadian model, which is often way too bullish, is showing little in the way of any surface low development in the gulf. The NOGAPS and UKMET clearly show the wave remaining confined to the extreme southern Gulf and Bay of Campeche before moving into mainland Mexico. The ECMWF, which wasn't overly aggressive to begin with, shows the same general scenario as the NOGAPS and UKMET. Bottom line: The chance of Gulf development appears to be very low. If development were to occur, it would likely take place in the extreme southwest Gulf or Bay of Campeche. One of the few reasons why development cannot be ruled out completely is due to the fact that the global models struggled to pick up on the rapid formations in this area in 2005. Any tropical cyclone that does form would likely head into mainland Mexico due to the wave's low latitude once it enters the Gulf and its relative lack of intensity. June 10 2007 - 23:30 UTC There's not much to add from yesterday evening's discussion. The tropical wave in the eastern Atlantic is weakening and tropical cyclone formation remains unlikely. Elsewhere, disorganized convection remains spread out across the Caribbean, but upper level conditions will remain unfavorable for tropical development. Also, the upper low near the Yucatan peninsula will quietly drift westward into Mexico or southern Texas with no development. So all is quiet this evening. June 10 2007 - 02:00 UTC An impressive tropical wave, at least by early season standards, rolled off the African coast earlier today. The wave is well-defined in the low to mid levels as a result of favorable winds aloft. QuikSCAT imagery depicts a strongly inverted wave axis, with southwest winds converging toward a weak low center. There does not appear to be a completely closed off surface circulation. A large upper level high extends from central Africa westward to the Cape Verde islands. The longwave trough located between 40-50ºW is having little influence on the wave. But that is likely to change over the next several days. Although the wave is currently well-defined, all model guidance does suggest that it will outrun the favorable upper level conditions that are present over the eastern Atlantic. Once along the western periphery of the high, southwest winds ahead of the trough will increase over the wave axis, thus resulting in less organization. The projected result of the shear on the wave axis is also apparent when one compares the low and mid level vorticity maps. For example, the trough is not as deep in the low levels. With that said, the wave will continue to be guided nearly due west toward the Lesser Antilles under a strong surface ridge. However, the mid level energy associated with the wave will gradually take a more northerly component due to the mid level weakness over the central Atlantic. In other words, the shear should be strong enough to keep this wave from remaining vertically stacked, thus making tropical cyclone formation unlikely. The wave should be monitored for some signs of organization while conditions remain somewhat favorable over the next few days. However, if all of the available model guidance is correct, any short term organization will become irrelevant as a strong trough over the central Atlantic begins to interact with the wave by the middle of next week. Of course this is subject to change, but that is how things look as of this evening. It should also be noted it's very early for development in the far east Atlantic, but several strong waves have already exited Africa this season. Similar observations are made on numerous forums nearly every year, but they may hold merit this year. It's especially interesting since we anticipate a strong Cape Verde season. The rest of the Atlantic basin is generally quiet. A large area of convection developed in the southwest Caribbean earlier, but upper winds should remain unfavorable for tropical cyclone formation. June 09 2007 - 01:30 UTC The return of the discussions are eight days late, but better late than never right? Due to time constraints, the discussions will likely be short and more to the point until a system becomes of decent interest in the Atlantic basin. a tropical wave is located near the Lesser Antilles, but there are no signs of organization. Shear remains moderate to strong. All dynamical model guidance shows the wave continuing westward through the Caribbean with little sign of additional orginization. In the southwest Atlantic, an upper level low is generating isolated convection, but this system is not tropical in nature. The layout or contents of this webpage cannot be copied without proper consent
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