Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 1:55 PM GMT on October 03, 2005
The tropics today present a very complex picture, with many potential areas of danger for all residents along the Mexican and U.S. coast. Here's what's happening:
Tropical Storm Stan
Tropical Storm Stan is quickly re-organizing over waters of the southwestern Gulf of Mexico. The chances for continued intensification a are high, as Stan is over 30C waters and is positioned under a large anticyclone that will provide good outflow and wind shear values below 5 knots. Stan will probably be a Category 1 hurricane as it approaches the Mexican coast on Wednesday, and a Category 2 hurricane is not out of the question.
The forecast of a landfall in Mexico between Tampico and Veracruz on Wednesday has increased in confidence since yesterday, but is still uncertain. Stan is being driven westward at 10 mph by a strong ridge of high pressure. This ridge will gradually weaken Tuesday as a weak trough of low pressure swings across the U.S., and Stan will slow down in response. All of the models are now forecasting that the ridge will remain strong enough to carry Stan all the way to the coast. However, there is still a distinct chance that Stan may stall just before the coast, or make landfall, then pop back out over the Gulf of Mexico and re-intensify. Stan may then remain in the Gulf many days, and may eventually move north and threaten the U.S.
Complicating the forecast is the fact that a tropical depression my form tomorrow along the Pacific Mexican coast in the Gulf of Tehuantepec, 100 miles south of the Gulf of Mexico. Storm-storm interactions among two tropical storms are not well understood, and the development of a new tropical depression on the other side of Mexico will make the current forecasts of Stan's motion Wednesday and beyond very dubious. And to complicate matters further, a non-tropical low pressure system is forecast to form over the Gulf of Mexico by Thursday, potentially making a set of three storms that will all interact in unpredictable ways. One positive note about this development is that the upper-level winds associated with this new non-tropical low would bring significant wind shear and weaken Stan--if he is still there. If Stan is not there, at least one model (the UKMET) suggests that this non-tropical low would meander over the Gulf of Mexico for many days, and potentially acquire tropical characteristics and become a tropical storm.
The larger threat to the U.S.?
The greater threat to the U.S. may be the spinning area of intense thunderstorms approaching the central Bahama Islands. This system is an upper-level low pressure system that is interacting with a surface trough of low pressure, and slowly making the transition from a cold-cored non-tropical low to a warm-cored tropical system. While the shear is a high 20 knots over the system today, the shear is expected to decrease to 10 knots tomorrow, accompanied by the formation of an upper-level anticyclone on top. This system has the potential to become a tropical depression tomorrow, and a reconnaissance flight is scheduled for Tuesday at 11 am. Model projections indicate the entire East Coast from Florida to the Carolinas to New England may be at risk from this system. This storm may strike Florida as a tropical depression or weak tropical storm on Wednesday morning. Beaches along central Florida are already suffering erosion from the large pounding waves emanating from this disturbance.
Figure 1. BAMM model forecast track of Bahamas suspect area.
A low pressure system accompanied by a concentrated area of thunderstorms has developed this morning halfway between Africa and the Lesser Antilles Islands, at about 14N 36W. This system has some impressive spiral banding, but is now fighting some significant wind shear, and looks less likely to develop into Tropical Depression 21. It is expected to move northward the next five days over open waters and not threaten any land areas.
Tropical Storm Otis
Tropical Storm Otis has decayed to a 40-mph tropical storm, and is forecast to remain just offshore Baja California and continue to weaken and eventually dissipate two days from now. Otis is not likely to cause any problems for Mexico or Arizona.
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