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August 2012

TD 9 Forms – Eyeing the Big Caribbean Islands and the United States

   Posted by Levi at 4:55pm on August 21, 2012

Tropical Depression #9 has formed from Invest 94L east of the lesser Antilles. The system has displayed an increase in convection since last night, unsurprising since it has crossed 50W into an area of warmer water. Convective coverage remains low overall, and the northeastern semicircle of the storm is mostly void of thunderstorms. This should change as the system leaves its genesis area within the eastern Atlantic monsoon flow, and thunderstorm activity should gradually fill out during the next couple of days. TD 9 should steadily intensify as it enters the eastern Caribbean, bringing tropical storm conditions to the northern Antilles as far south as St. Lucia. By the time it nears Hispaniola in about 3 days, I expect it will be a moderate-strong tropical storm. This is not aggressive as the NHC forecast which takes TD 9 to a Cat 2 hurricane near Haiti. I am less bullish with the intensity through Day 3 due to the still less than ideal environment in the eastern Caribbean with continued fast trade winds and sinking air ahead of the storm.

The track forecast reasoning remains unchanged. The subtropical ridge directly north of TD 9 right now should continue to direct it westward or WNW for the next 48 hours or so. Thereafter, this ridge will be shifting eastward and a weakness will develop north of the Bahamas, inducing a more northwesterly motion near or over the island of Hispaniola. This motion should then continue, bringing TD 9 in the vicinity of the Bahamas and Florida within 5 days. The 0z ECMWF takes TD 9 south of Cuba and into the eastern Gulf of Mexico as a major hurricane. This is the western outlier of the model guidance envelope, and is being discounted as too far west at this time. The track has been nudged slightly to the left, and is in best agreement with the 06z GFS ensemble mean.

Overall, this storm is now developing, and we should start to get a better handle on the track as the models become better able to resolve the situation. The northern Antilles, Puerto Rico, Hispaniola, and eastern Cuba could all face impacts from this storm during the next few days. The Bahamas and the southeast United States are very likely to be impacted as well, but there is still uncertainty on the details given that a potential U.S. landfall is still at least 6-7 days away.

Elsewhere….Invest 95L in the western Gulf of Mexico has not developed significantly since yesterday, mainly due to the frontal boundary to the northeast being too strong to allow much consolidation. 95L still has another day or so over water and could still wind up just before moving ashore, but should not be more than a rain maker for northern Mexico and extreme south Texas.

We shall see what happens!