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Bahamas tropical disturbance a threat to the Gulf of Mexico

By: Dr. Jeff Masters, 1:04 PM GMT on October 01, 2007

Heavy thunderstorms are firing up over South Florida and the nearby waters, thanks to an upper-level low pressure system interacting with an old front. Long range radar out of Miami shows that this activity is disorganized. This morning's QuikSCAT pass showed some rotation in the surface winds over the Central Bahamas, but with wind shear 20-30 knots over the region, no development is likely today. Most of the computer models forecast that wind shear will fall, and a tropical or subtropical depression will form by Wednesday near the Florida Keys, eastern Gulf of Mexico, or western Cuba. There is a strong upper-level low pressure system just southwest of Florida (Figure 1), and water vapor satellite loops show that this low is pulling plenty of dry, continental air from North America southward over the Gulf of Mexico. The upper low is expected to move southwestward. This is a situation very similar to the one that spawned Subtropical Depression Ten in September, off the Gulf Coast of Florida. In that case, the subtropical depression formed right underneath the upper level low, making for a very slow transition phase to a tropical system. It took two days for Subtropical Depression Ten to become Tropical Depression Ten, and the storm ran out of time to intensify into a tropical storm before moving ashore over the Florida Panhandle. This time, the GFS model is supporting formation of a subtropical depression to the northeast of the upper low. This situation would potentially allow a faster conversion of the subtropical storm to a tropical storm. The UKMET and Canadian model predict that a fully tropical storm will form, instead. I think a subtropical storm is more likely.

Any storm that forms is forecast to move west-northwestward across the Gulf of Mexico, pushed by a strong ridge of high pressure expected to build in. An upper-level anticyclone aloft is expected to develop as well, providing an environment favorable for intensification. However, intensification will be slowed by the presence of all the dry air dragged into the Gulf of Mexico by the upper low, and by the transition of the storm from subtropical to tropical. The models project a landfall in Texas or Louisiana on Friday or Saturday. The Hurricane Hunters are on call to fly into this system Tuesday afternoon, if necessary.

Figure 1. This morning's water vapor satellite image. The counter-clockwise flow of air around the upper southwest of Florida is dragging plenty of dry, continental air (brown colors) from North America over the Gulf of Mexico. Heavy thunderstorm activity (blue and orange colors) is over South Florida.

Karen's remains not expected to develop
Karen's remains continue to generate a large area of disturbed weather a few hundred miles east of the northern Lesser Antilles Islands (Figure 1). This morning's QuikSCAT pass showed some rotation still exists near the surface, with winds up to 35 mph. Wind shear is about 20 knots over Karen's remains, and wind shear is expected to remain too high to allow re-development of Karen over the next several days. None of the reliable computer models resurrect Karen.

Tropical Storm Melissa is dead, torn up by wind shear.

I'll have an update Tuesday, and post my first half of October outlook for hurricane season.

Jeff Masters

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