#229 Postby ronjon » Fri Aug 10, 2007 7:29 am
IWIC Worldwide Tropical Weather Discussion - August 10 2007 - 01:00 UTC
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.
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