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Dr. Jeff Masters' WunderBlog
Last Updated: 1:53 PM GMT on May 21, 2009

Gulf of Mexico low growing more organized


Posted by: JeffMasters, 1:51 PM GMT on May 21, 2009

The large extratropical storm that has been drenching Florida over the past few days continues to pour rain on the state as it treks slowly west-northwestward over the Gulf of Mexico. Rainfall amounts as high as 23.75 inches have been measured in Northern Florida at the Flagler County Fairgrounds over the past four days, and Daytona Beach has seen over 14 inches, forcing closure of numerous roads. Another 1 - 3 inches are expected in Northeast Florida today from the storm, along with beach erosion, 7 - 10 foot waves, and tides 2 - 3 feet above normal. The bad weather in Northeast Florida should gradually subside Friday and the rains should diminish substantially on Saturday, when the center of the storm is expected to move ashore over Louisiana.


Figure 1. Storm total rainfall estimated from the Melbourne radar.

Latest visible satellite loops show that the circulation of the low has grown more circular and better defined, and heavy thunderstorm activity is starting to build off the Gulf Coast of Florida on the low's east side. These thunderstorms are beginning to form spiral bands characteristic of a developing tropical depression. Water vapor satellite imagery shows that the low is moving into an area of dry continental air, though, and this dry air will slow development. Wind shear is about 20 knots, which is low enough to allow development. While none of the computer models develop the storm, they generally predict the storm should move northwestwards towards Louisiana over the next two days, and the shear is lower to the north. The low is currently over Sea Surface Temperatures (SSTs) of about 27°C in the Loop Current, which is about 1°C (1.8°F) warmer than average. SSTs of at least 26°C are typically needed to support a tropical storm, so I expect the low will continue to develop through Friday. By Friday night, the low will be approaching the Louisiana coast, and SSTs will cool to 26°C. Still, with wind shear expected to drop, this low has a medium (30 - 50%) chance of developing into a subtropical or tropical depression before its expected landfall Saturday over Louisiana. Since there will be strong winds from the west creating wind shear over the storm, plus dry air to the storm's west, most of the heavy rain should be confined to the east of the storm's center, along a stretch of coast from New Orleans to Pensacola. Rainfall totals of 3 - 6 inches can be expected.


Figure 2. Latest satellite image of the Gulf of Mexico storm.

I'll have an update Friday morning, or sooner if NHC declares this storm an area of interest (an "Invest").

Jeff Masters