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Posted at 12:54 PM ET, 10/30/2010

Tomas grows into season's 12th hurricane

* Four straight sunny days: Full Forecast *

The tropical system impacting the Lesser Antilles, just yesterday identified as 91L (pre-depression stage) by the National Hurricane Center, has quickly intensified into Hurricane Tomas. Its satellite presentation is unmistakable in its hurricane characteristics, with very cold (high) cloud tops radiating outward in all directions almost symmetrically.

Maximum sustained winds are estimated to be near 75 mph, and the storm is moving west-northwestward at 15 mph. The center of Tomas is passing near St. Lucia and St. Vincent this Saturday afternoon (see live radar), where hurricane warnings are in effect, and will enter the Caribbean Sea tonight. The National Hurricane Center predicts Tomas will intensify to major (Category 3) hurricane status next week.

For now, wind shear around Tomas is light. And the circulation is moving over unusually warm 85+ degree ocean waters that are capable of supporting a Category 5.

But as so often been the case this year over the Caribbean, an unfriendly environment is not far away. A large counterclockwise swirl associated with an upper trough sits right in the middle of the Caribbean, and is introducing relatively strong wind shear and dry mid-level conditions to that region.

If the weather aloft doesn't improve over the next couple of days as Tomas heads that way, the hurricane may struggle. In fact, if you want to be picky about Tomas' appearance right now in the satellite pictures, one could argue that some dry air is already working into the primary circulation, as indicated by the slight comma-shaped (rather than circular) distribution of tall thunderstorms near the center. Though this is probably something Tomas will be able to overcome in the near term, we will see how the large-scale environment moves with and against the hurricane in time.

Prospects for its longer term survival, beyond a week, particularly if it moves northward toward the Northern Caribbean and the Greater Antilles (e.g., Jamaica; Cuba; Hispaniola), are not especially good. The global weather models continue to advertise a solution that brings a large and very windy upper vortex over the Southern United States later next week. That could very well spell trouble for Tomas.

But in the larger context, the fact that we have a hurricane at this stage of autumn is really not unusual. Officially, we have another month to go the in the 2010 Atlantic season. Historically, 7 of the last 10 years have had named storms in November. Two of those years (08 and 01) saw major hurricanes in the Caribbean. And going back just a couple of years further to 1998, one of the more ferocious storms of modern times, Category 5 hurricane Mitch, tore across the Western Caribbean in the last few days of October.

This season, there's no reason to be convinced that Tomas will be the final one. Though the high-altitude conditions will gradually deteriorate, as hostile intrusions of dry and windy conditions from the north become more frequent with time, the southern-latitude water temperatures stay warm enough to support hurricanes even beyond November.

By  |  12:54 PM ET, 10/30/2010

Categories:  Tropical Weather

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