Florida disturbance 93L getting more organized

By: Dr. Jeff Masters , 6:43 PM GMT on September 18, 2007

An area of disturbed weather (labeled "93L" by NHC this afternoon) has become much more organized this afternoon. Animations of long-range radar out of Miami, Florida and satellite loops show that thunderstorm activity off the Florida coast is steadily increasing. A surface circulation has not yet fully formed, but has almost closed off just south of Grand Bahama Island, about 100 miles east of Fort Lauderdale, Florida. Winds at Settlement Point on Grand Bahama Island were blowing at 9 knots out of the east at 9am this morning, but shifted to northerly and now northeasterly, and have increased to 32 knots (37 mph), with gusts to 34 knots (39 mph, tropical storm force). Winds at Freeport on Grand Bahama Island have increased to 30 mph, with gusts as high as 37 mph.

Water vapor satellite loops show that an upper-level low pressure system has detached from its parent trough over northern Florida. This upper level low is moving southwest, creating more favorable upper-level winds over 93L. Wind shear over 93L has fallen from 30 knots to 20 knots in the past six hours. A subtropical depression could form before it moves over South Florida tonight or Wednesday morning. Since this system is not fully tropical and does not have a warm core, it is very unlikely that it will be able to rapidly intensify today, ala Humberto. Regardless, South Florida and the western Bahamas can expect heavy rains of 3-6 inches from this system. Radar estimated rainfall has been as high as three inches in some spots over the ocean thus far.

Figure 1. Current long range radar out of Miami, Florida.

Most of the computer models predict that a tropical or subtropical depression will form from 93L once it crosses into the Gulf of Mexico. The path such a storm might take is uncertain. A strong ridge of high pressure is setting up over the eastern half of the U.S., and is expected to remain anchored in place for at least ten days. This is the type of steering pattern we experienced during the Hurricane Season of 2005, and favors westward-moving storms. However, this steering pattern will be complicated by the presence of the upper-level low pressure system moving southwest over the Gulf of Mexico. This upper-level low will gradually weaken. Depending on the strength and movement of this low, the counterclockwise flow around the low could steer 93L on a northwesterly path towards Louisiana. This is the solution of the latest (12Z) runs of the UKMET, GFS, and GFDL models. The intensity such a storm may reach is also highly uncertain. The storm is starting off without a warm core, which will hamper intensification. Dry air to the west will also cause it problems. The SHIPS intensity model brings 93L to Category 1 hurricane strength by Saturday, and the GFDL predicts 93L will hit New Orleans as a strong tropical storm Friday night. The NOGAPS model predicts 93L will eventually dissipate over the southwestern Gulf, and the HWRF model does not develop 93L, and takes the disturbance to the central Louisiana coast on Saturday afternoon.

The NOAA jet is scheduled to fly Wednesday evening and collect data to help with the Wednesday evening (00Z) model runs. The first low-level Hurricane Hunter mission is scheduled for Thursday afternoon.

Remains of Ingrid
The remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is still a bit a spin evident on satellite loops and this morning's QuikSCAT pass. Wind shear is a hefty 30 knots, and is expected to gradually decline to 10 knots by Wednesday night. If there's anything left of Ingrid then, we will need to watch this area for regeneration.

Tropical wave in the mid-Atlantic
A tropical wave about 750 miles east of the Lesser Antilles Islands is moving west at 10-15 mph. Thunderstorm activity associated with this wave has diminished substantially since yesterday, and any development should be very slow to occur. Wind shear is about 10 knots, and is expected to remain below 15 knots over the next 2-3 days. None of the computer models develop this wave into a tropical depression.

Typhoon Wipha takes aim at China's most populous city
Typhoon Wipha, a formidable Category 4 storm, is making landfall just south of China's most populous city, Shanghai. About 14.5 million people live in the city. Over 1 million people have been evacuated so far. Wipha must pass over about 50 miles of land before reaching Shanghai, and will probably be a strong tropical storm or Category 1 hurricane when it passes over or just west of the city. Rainfall amounts of up to seven inches in 24 hours are expected along the path of Wipha, which will cause major flooding problems. Fujian province on China's southeastern coast has sent out 1.41 million text messages to warn the public of the upcoming typhoon, the local flood control headquarters said. The Women's World Cup soccer tournament is going on in China, and the U.S. is supposed to play their final group game tomorrow in Shanghai (8pm Tuesday night China time, 8am EST). World Cup organizers are trying to change the game time to get the game in (and then the teams out of Shanghai) before the storm hits.

Yesterday, Wipha briefly intensified into a super typhoon packing 150 mph winds as it brushed Taiwan. One person was killed there. Wipha is a woman's name in the Thai language.

Figure 2. Radar images of Typhoon Wipha as it made landfall just south of Shanghai, China. Image credit: Taiwan Central Weather Bureau.

Jeff Masters

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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