Stan storms ashore; Florida getting pounded by a powerful tropical disturbance

Tropical disturbance approaching Florida
The tropical disturbance over the central Bahama Islands is poised to hit Florida Tuesday night and Wednesday with heavy rains and high winds. Although the disturbance is not a threat to develop into a tropical depression today, the impact on Florida will be similar to that of a tropical depression--sustained winds of 30 mph, with gusts to 45 mph along the coast, along with 3 - 6 inches of rain and large battering waves. The disturbance currently has a weak surface circulation center just south of Andros Island. There is no cloud cover on the west side of the circulation center, and shearing winds blowing from the west are keeping all of the convection pushed over to the east side of the disturbance. Contrary to yesterday's computer model projections of much reduced shear affecting the disturbance, the shear over the storm has almost doubled, to 20 - 30 knots. This shear is in part associated with strong upper-level outflow flowing northeastward out of Hurricane Stan. No development of any kind is possible while the shear remains this strong, and I believe that this system will not develop into a tropical depression at all. Instead, the disturbance will interact with an upper-level low pressure system forecast to develop in the Gulf of Mexico on Wednesday, and become a large--and very wet--ordinary low pressure system. This may also turn into hybrid system called a subtropical storm--a system that is similar to a tropical storm, but has its main winds 100 miles or so away from the center. If this happens, the system will be named Subtropical Storm Tammy. Our skill in forecasting these types of hybrid systems is low.

Once in the Gulf of Mexico, the storm is expected to meander for several days, until a cold front pushes into northern Florida Friday and pulls the storm northeastward across Florida and the Carolinas on Thursday through Saturday, dumping very heavy rains across the region. At the same time, another low pressure area may form along the front near the Carolinas and move northeastward across New England. This second low is not expected to be tropical in nature, but will still dump a lot rain and bring high winds to the East Coast.

I speculated about the possibility yesterday of a tropical storm forming near the Carolinas and moving northwards along the coast. This no longer seems likely, due to the high wind shear over the area today.


Figure 1. BAMM model forecast track of Bahamas suspect area. The intensity forecast numbers from the SHIPS model are far too high; this system will be lucky to make it to tropical storm strength (40 mph winds).

Hurricane Stan
Hurricane Stan presents a classic example of why the National Hurricane Center issues hurricane warnings for a much larger area of coast than seems reasonable. Stan made a sudden turn to the southwest last night and speeded up his forward speed from 6 mph to 10 mph, and is now making landfall on the coast of Mexico, a full 24 hours before expected. This change of course and speed were completely unanticipated my any of the forecast models. However, since the NHC was conservative, the area of the coast the eye of Stan is hitting has been under a hurricane warning for a full day, so the sudden turn should not catch the affected area completely unprepared. Why did the models miss this turn? Perhaps because of Stan's interaction with the mountainous terrain nearby, or because of interaction with the developing upper-level low pressure system in the Gulf of Mexico.

Elsewhere in the tropics
Clouds from the tropical disturbance over the Bahamas extend southeastward to Puerto Rico and the Lesser Antilles Islands. The area of disturbed weather near Puerto Rico bears watching, as wind shear values here are 5 - 10 knots, the lowest of anywhere in this disturbance.

The region between Africa and the Lesser Antilles is quiet.

Jeff Masters