Three Atlantic disturbances to watch

A low pressure system over the southern Gulf of Mexico (90L) has developed a large surface circulation covering most of the Gulf of Mexico, but is not a threat to develop rapidly. This morning's QuikSCAT pass shows this circulation nicely, with top winds of 35 mph to the southeast of the center. Satellite loops show that 90L's heavy thunderstorm activity has decreased since yesterday. Water vapor satellite loops show a large upper-level low pressure system also covers the entire Gulf of Mexico, with two 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. A Hurricane Hunter aircraft is scheduled to investigate the system Thursday afternoon, if necessary. Today's flight was canceled.

The latest computer model runs continue to point to a landfall Thursday night or Friday morning near the Louisiana/Texas border. I don't see 90L becoming a hurricane, and I give equal chances of 90L arriving at the coast as a tropical disturbance, tropical depression, or tropical storm.


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

Disturbance 92L east of the Bahamas
Of greater concern to me is an area of heavy thunderstorm activity associated with a surface trough of low pressure (92L) that has developed just east of the Bahama Islands. This morning's QuikSCAT pass showed an east-west oriented zone of converging winds at 27N between 69W and 72W, 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 10-15 knots of wind shear. Wind shear is expected to remain ten knots or less over 92L for the next five days. The computer models expect 92L will move slowly west-southwest over the Bahamas, then the Florida Straits or Cuba during the next three days. By Saturday, the GFS and ECMWF models predict a tropical depression could form in the Western Caribbean or southern Gulf of Mexico. The NOGAPS and UKMET model forecast that development Monday is more likely. These are the highest heat content waters in the Atlantic, and with a upper-level anticyclone with light wind shear expected to set up over the disturbance, the potential exists for a hurricane to form from 92L next week. The Hurricane Hunters are scheduled to investigate 92L Thursday afternoon.

Disturbance 91L between Africa and the Lesser Antilles
A tropical wave (91L) near 9N, 41W, midway between Africa and the Lesser Antilles Islands, has gotten better organized this morning. This morning's QuikSCAT pass showed a wind shift associated with the wave, but no closed circulation. Satellite loops show a modest area of heavy thunderstorm activity that is not well-organized. The disturbance is headed west at 10-15 mph, and is expected to take a more west-northwesterly track Thursday.

Wind shear is about 10 knots over the wave, and is forecast to remain below 15 knots until Thursday night. This may allow for some slow development. However, beginning Thursday night, wind shear is expect to increase and remain 20-30 knots through Sunday. This should prevent further development.

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

Jeff Masters