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August 2012

Isaac a Dangerous Storm for Hispaniola – U.S. to get Hit After

   Posted by Levi at 3:55pm on August 22, 2012

Tropical Depression #10 has formed well east of Isaac in the central Atlantic, and is forecasted to pass north of the Antilles islands. This storm can be safely ignored for now while we deal with Isaac hitting land.

Tropical Storm Isaac was named yesterday, and has shown a healthy increase in thunderstorm activity, though the center remains under the northern edge of the main convective mass, and the northeast quadrant remains void of thunderstorms due to light wind shear and a lack of low-level convergence in that part of the storm. Isaac has not deepened during the last 12 hours, and is only up to 45mph winds right now. Only gradual strengthening should occur as Isaac crosses into the eastern Caribbean near Guadeloupe today, and the intensity forecast continues to keep Isaac under hurricane strength on approach to Hispaniola due to the time it will take for the large circulation to tighten, and the fast trade winds in the eastern Caribbean. This is still a less aggressive short-term intensification scheme than the NHC, though their forecast has come down several notches over the last couple of days, and is closer to my forecast now. Isaac should weaken while interacting with the high mountains of the Greater Antilles, but once clear of them, should restrengthen faster than it did in the Caribbean, and could quickly become a hurricane in the vicinity of Florida if it gets at least a couple of days of water time.

The track forecast philosophy remains generally unchanged. The trough currently over the eastern U.S. is lifting out to the northeast, but the southern part of this trough is being left behind as a trough-split over the SE U.S. during the next few days. This will be helping to erode the western periphery of the Bermuda High as it retreats eastward over the Atlantic, and the models generally agree that this will create an open weakness north of the Bahamas in 48-72 hours. The models then diverge on whether Isaac will move right into this weakness. The CMC continues to be the easterly outlier, taking Isaac well east of Florida. The GFS has Isaac approach the weakness but get entangled with the greater Antilles and miss it, not recurving until it reaches southwest Florida. The ECMWF remains persistent that the weakness will close off and a rebuilt ridge will direct Isaac into the central Gulf of Mexico as a major hurricane. Thus, there is still a large amount of uncertainty beyond 72 hours. For now, the forecast track remains close to, but just east of the consensus of the non-ECMWF models, and takes Isaac into the Bahamas just east of Florida with the assumption that Isaac’s large circulation will be able to feel the weakness in the ridge north of the Bahamas. However, there is considerable inherent uncertainty to the west and south of the forecast track due to Isaac’s interaction with the greater Antilles. The high mountains there have a tendency to jerk storms around in an unpredictable fashion, and could throw the track off at any time. Additionally, any further entanglement with the islands could reduce Isaac’s intensity and prolong his recurvature, putting Florida and the eastern gulf coast at greater risk. The forecast cone reflects this, and interests from the Carolinas to the central gulf coast should monitor Isaac’s progress closely.

We shall see what happens!