I found this and thought it was a very very good explanation and wonderful discussion of all that is going on here it is:
IWIC Worldwide Tropical Weather Discussion - 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 run. 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.
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- Jason Moreland
http://www.independentwx.com
