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Tomas Strengthens Again; Threat to Haiti

Posted by Andy Johnson under Daily updates

Despite seemingly favorable atmospheric conditions today, Tomas continued to be weak most of the day after weakening to a tropical depression at 5 AM EDT, Wednesday, November 3, 2010. Tomas had succumbed to moderate southwesterly shear two day ago and never regained a vertical structure. Winter storms (baroclinic) tilt with height towards colder temperatures. Tropical systems need to remain vertically oriented  in order to intensify. Tropical system are barotropic and do not get their energy from temperature differences as winter baroclinic systems do. That is why hurricane are unique in possessing an eye. Satellites or aircraft can see right into the center of the structure which is vertically stacked. The southwesterly shear that Tomas was experiencing separated the heaviest convection from the circulation center.

Evidence from a hurricane reconnaissance aircraft this afternoon showed that the center of Tomas redeveloped to the northeast closer to the center of heaviest convection. As a result, the NHC upgraded Tomas back to tropical storm strength at 5PM Wednesday. Hurricane watches in Jamaica were changed to tropical storm watches since by the time Tomas begins to intensify, its circulation will be interrupted by the terrain of Jamaica and Hispaniola. However, since Haiti will likely be on the stronger east side of the storm a hurricane watch has been issued for Haiti.

The long range forecast still calls for an east coast trough to pick up Tomas and move him to the northeast. The trough weakens and flattens out by the weekend, however, leaving Tomas in a weak steering environment. As a result, Tomas could meander for days. If he is over open water at that time, it would not be as much of a problem. If Tomas stalls anywhere near Haiti, however, just the heavy rains (even without hurricane force winds) could could major problems with flooding. Haiti has been largely deforested while the adjacent Dominican Republic has not suffered the same fate.

The close-up enhanced satellite image below shows an elongated area of convection oriented northeast-southwest. High cirrus clouds around the periphery of the storm show a good outflow pattern and no significant shearing, both of which are indicators of strengthening.

Another tropical wave is located near Guyana on the north coast of Venezuela (shown below). Some models shows this system strengthening to a tropical depression or tropical storm next week.

Most of the models (shown below) have a general consensus of bringing Tomas northward somewhere between Jamaica and Haiti. The GFDL shows Tomas looping back to the south.

The GFS model valid Friday at 8:00 PM shows Tomas near the Turks and Caicos Islands after being picked up by the East Coast trough.

The longer range GFS valid Tuesday at 7:00 AM show  Tomas still around in the Atlantic and shows him being blocked by an East Coast ridge which will have replaced the trough at that point in time. The GFS also tries to develop a new depression or storm. The next name on the list is Virginie.