Not much change to the three Atlantic disturbances

A low pressure system over the southern Gulf of Mexico (90L) continues to be unimpressive. This morning's QuikSCAT pass shows this circulation nicely, with top winds of 30 mph south of the Louisiana coast. Satellite loops show only a small amount of heavy thunderstorm activity, to the northeast of the center. This activity is now visible on New Orleans long range radar. Water vapor satellite loops show a large upper-level low pressure system also covers the entire Gulf of Mexico, with embedded swirls. This upper level low has a cold core and is wrapping plenty of dry air into 90L. These factors, plus the very large size of the surface circulation of 90L, will keep any development of the storm slow. With 90L expected to make landfall Friday near the Texas/Lousiana border, I don't expect 90L will become a tropical storm. However, I do expect it will develop enough heavy thunderstorm activity today to potentially become a tropical depression and cause some isolated flooding problems in southern Louisiana tonight through Friday afternoon. The Hurricane Hunter aircraft scheduled to investigate 90L this afternoon was canceled, and has been rescheduled for Friday afternoon.


Figure 1. Today's line up of tropical disturbances to watch.

Disturbance 92L northeast of the Bahamas
Of greater concern is an area of heavy thunderstorm activity associated with a surface trough of low pressure (92L) near 26N 73W, just northeast of the Bahama Islands. Last night's QuikSCAT pass showed a 200-mile long southeast-northwest oriented zone of converging winds just northeast of the Bahamas, but no surface circulation. Satellite loops show a large area of disorganized thunderstorm activity that is not getting better organized. This disturbance is under about 15-30 knots of wind shear, and development today will be very slow. Wind shear is expected to fall to 10-15 knots beginning Friday afternoon, which may allow 92L to develop into a tropical depression as early as Saturday. The computer models expect a strong ridge of high pressure will force 92L slowly west-southwest over the Bahamas Friday and Saturday, then over the Florida Keys and western Cuba Saturday and Sunday. The storm will probably bring heavy rains to the western Bahamas, South Florida, and the Florida Keys beginning on Saturday. By Monday and Tuesday, the system is expected to be over the Western Caribbean or southern Gulf of Mexico near the Yucatan Peninsula. The GFDL and SHIPS models predict 92L will be a strong tropical storm with 55-65 mph winds on Tuesday. With an upper-level anticyclone with light wind shear expected to set up over the disturbance, the potential exists for a hurricane to form--if 92L can avoid the Yucatan.

The long term fate of the storm is highly uncertain. The GFS model predicts the ridge of high pressure ridge forcing it west will intensify, pushing 92L southwestward into Mexico. There is, however, a strong trough of low pressure expected to swing across the central U.S. next week and become a cut-off low. This system may be able to swing 92L northwards into the U.S. Gulf Coast, as the Canadian model is predicting. The Hurricane Hunters were scheduled to investigate 92L this afternoon, but this flight was canceled and has been rescheduled for Friday afternoon.

Disturbance 91L between Africa and the Lesser Antilles
A tropical wave (91L) near 10N, 45W, midway between Africa and the Lesser Antilles Islands, has become less organized since yesterday. Satellite loops show scattered disorganized heavy thunderstorm activity. The disturbance is headed west-northwest at 10-15 mph.

Wind shear is about 15 knots over the wave, which may allow for some slow development today. However, beginning tonight, wind shear is expect to increase to 20 knots, and will remain 20-35 knots through Monday. This should prevent further development. None of the computer models develop 91L.

Typhoon Krosa takes aim at Taiwan
After a slower than usual September, things have heated up in the Western Pacific this week. Typhoon Krosa, a Category 4 storm with 135 mph winds, is forecast to pass close to the northern tip of Taiwan Saturday, then brush the coast of Mainland China near Shanghai early next week.

I'll have an update Friday morning at the latest.

Jeff Masters