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African wave more organized; Hurricane Flossie still a Cat 4

By: Dr. Jeff Masters, 2:02 PM GMT on August 12, 2007

Thunderstorm activity in association with a surface trough of low pressure over the Western Caribbean remains disorganized. None of the computer models than reliably forecast tropical storm formation are calling for development in this region. Wind shear is about 20 knots, which will keep any development slow.

A strong tropical wave exited the coast of Africa Friday, and is now a 1006 mb low pressure system with heavy thunderstorm activity about 200 miles south of the Cape Verde Islands. NHC is referring to this system as 90L. This morning's QuikSCAT satellite pass at 3:48am EDT found an elongated circulation centered near 12.5N 23W, with top winds of 30 mph. Satellite imagery at the NRL Monterey web site that most people use to track eastern Atlantic storms is not available this weekend. A substitute source of imagery is available at the main Navy weather web site. The thunderstorm activity surrounding 90L is currently not very expansive, thanks in part to relatively cool SSTs of 26-27C and high wind shear of 20 knots. However, the cloud pattern has gotten more organized this morning, with some spiral banding starting to occur. SSTs should increase above 27C by Tuesday, as 90L moves away from the African coast at 15-20 mph. Wind shear should drop to 10-20 knots, and I expect 90L will become a tropical depression on Monday or Tuesday.

What the computer models say
Watching the computer model runs for 90L is not for the faint of heart. All the major models except the NOGAPS develop the system into a tropical storm or hurricane that tracks westward over the Atlantic, reaching the lesser Antilles Islands as early as Thursday night, August 16. There are four possible scenarios to consider:

1) A strong trough of low pressure is forecast to move off the East Coast of the U.S. at that time, and this trough may deflect 90L northwards so that it misses the Lesser Antilles Islands, and then recurves harmlessly out to sea.

2) In keeping with the steering pattern we've observed since late July, the trough is expected to rapidly move onward, allowing a ridge of high pressure to build in. If the trough is not strong enough to recurve 90L out to sea, the storm will be forced to the west once more and eventually hit the East Coast of the U.S. This is the solution of last night's ECMWF model.

3) 90L will be far enough south and next weekend's trough will be weak enough that 90L will plow through the Caribbean, and not be deflected north of the Lesser Antilles Islands. The storm would eventually track into the Gulf of Mexico. This is the solution preferred by this morning's GFS model.

4) 90L will never develop, or will never become more than a weak tropical storm, due to unfavorable wind shear, dry air, or other factors. This is the solution of the NOGAPS model.

Of the four scenarios, I believe #2 or #3 are most likely to occur--90L will develop into a tropical storm or hurricane that will affect the Caribbean and/or U.S. East Coast. Residents throughout the Caribbean and U.S. should anticipate the possibility that 90L may become a hurricane--and possibly a major hurricane--that will not recurve. If you plan on being in the Lesser Antilles Islands Thursday August 16 - Sunday August 19, keep in mind there is a heightened risk of a tropical storm or hurricane during that period. Be prepared to adjust your travel plans.

Figure 1. Latest satellite image of Hurricane Flossie.

Hurricane Flossie
Hurricane Flossie in the Eastern Pacific put on an impressive burst of intensification yesterday that put her at Category 4 strength for a full day. This intensification happened despite the presence of waters that were only 27.5 C, with a very limited total heat content (a relatively shallow layer of warm water, instead of the deep layer usually needs for a hurricane to rapidly intensify). Flossie has likey peaked in intensity, and should decay to Category 1 hurricane status by Tuesday, thanks to the twin influences of cooler SSTs and increased wind shear. Waves along the southeast coast of the Big Island of Hawaii will build to 12 feet by Tuesday morning, and Flossie is expected to make her closest approach to the island Wednesday morning. This morning's model runs have come into better agreement, giving support to the official forecast calling for Flossie's passage 50-100 miles south of the Big Island. Sustained winds of 40 mph currently extend out about 90 miles from the center of Flossie, so the Big Island could experience some damaging winds. Heavy rains and flash flooding are also likely on the Big Island. Right now, it doesn't appear that the Hilo airport on the northeast end of the island will need to close, but the Kona airport on the west side may be forced to close on Wednesday when Flossie passes the island and the winds become more onshore.

On July 21, Tropical Depression Cosme passed 185 miles south of the Big Island, bringing rains of 3-5 inches over a six hour period. Cosme helped with the summer-long drought that has affected the entire island chain, but Hawaii could use another near miss by Flossie to alleviate the moderate to severe drought conditions that persist.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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