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Gulf of Mexico disturbance 90L slowly getting organized

By: Dr. Jeff Masters, 2:05 PM GMT on October 02, 2007

Heavy thunderstorms are on the increase over the southeastern Gulf of Mexico, several hundred miles west of Key West, in association with an upper-level low pressure system that is now generating an area of low pressure at the surface. This system is being referred to as "Invest 90L" by NHC. The buoy 262 nm south of Panama City, FL had winds of 36 mph gusting to 45 mph this morning, and there were ship reports this morning of winds of 25-33 knots (29-38 mph) in the eastern Gulf of Mexico. This morning's QuikSCAT pass showed a large area of 30-35 mph winds over the eastern Gulf of Mexico. Satellite loops show a steady increase in heavy thunderstorm activity, but the activity is not well-organized at present.

A surface area of low pressure was over the western Bahamas last night, several hundred miles east of the Gulf of Mexico upper level low. This Bahamas surface low was the initial suspect area we were watching (labeled "Invest 90L" by NHC). This surface low moved westward overnight, and is now underneath the upper low. This is a situation very much like we saw with Tropical Depression Ten last month, which also formed from an upper level low over the Gulf of Mexico. Like TD 10, 90L will probably start off as a subtropical depression. There will be a warm core to the storm at the surface, but the upper low has a cold core aloft. This will make the storm subtropical in nature--a system has to have a warm core all the way from the surface to the upper atmosphere to be truly tropical. 90L will gradually warm up its entire core and become tropical, but this will probably take two days. It took two days for TD 10 to become fully tropical, and it ran out of time to intensify when it moved ashore into the Florida Panhandle as a tropical depression. Until a subtropical system establishes a fully warm core, it cannot undergo rapid intensification. With a landfall expected Thursday or Friday in Louisiana or Texas, this does not give 90L much time to strengthen. There is plenty of warm water in the Gulf of Mexico to aid intensification, but hampering intensification will be dry, continental air from North America that is being pulled southward over the Gulf of Mexico by the counter-clockwise circulation around the upper low. This dry air is readily apparent on water vapor satellite loops (the brown colors). The most likely scenario is that 90L will make landfall as a tropical storm. The Hurricane Hunter mission scheduled for today has been canceled, since 90L is not organized enough to warrant a flight. The mission has been rescheduled for Wednesday afternoon.

I'll have an update by 4 pm EDT this afternoon, and post my first half of October outlook for hurricane season.

Jeff Masters

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