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African wave a threat to develop; Hurricane Flossie intensifies to Category 4

By: Dr. Jeff Masters, 3:01 PM GMT on August 11, 2007

Thunderstorm activity in association with a surface trough of low pressure near Jamaica remains disorganized. None of the reliable computer models are calling for a tropical depression to form in this region any more. The four reliable models for forecasting tropical storm formation are the GFS, ECMWF, NOGAPS, and UKMET. The NAM and Canadian GEM models are not reliable for forecasting formation of tropical storms.

A strong tropical wave exited the coast of Africa yesterday, and is now a 1006 mb low pressure system with heavy thunderstorm activity southeast of the Cape Verde Islands. The system is not well-organized at present, and has lost some of its thunderstorm activity since exiting the coast of Africa. This is typical for such systems, which take a day or two to adjust to their new oceanic environment before they build new thunderstorms. Wind shear has dropped from 30 knots early this morning to about 20 knots over the region, and wind shear is forecast to continue to drop as the wave continues westward. There is burst of dry air and African dust emerging from the African coast just north of the Cape Verdes, but this is probably too far away to affect the storm.

This system has the potential to become a tropical depression as early as Monday. Most of the computer models develop it into a tropical depression that moves west to west-northwest over the Atlantic towards the Lesser Antilles Islands, and I think this is a reasonable forecast that has a 60% chance of coming true. We are into mid-August, when these waves traditionally start to develop, and the dry air and dust associated with the Saharan Air Layer (SAL) does not appear to be a major impediment at present. Wind shear should be low enough and water temperatures warm enough. Water temperatures are about 26-27C south of the Cape Verdes Islands, and gradually warm to 27-28C southwest of the islands, so the chances for development increase as the system gets further west.

If you live in the Lesser Antilles Islands, or plan on visiting next week, keep in mind that this forecast puts the Lesser Antilles at increased risk of a seeing a tropical cyclone beginning Friday, August 17. For the period August 17 - August 20, there is at least a 10% chance that a tropical storm or hurricane will affect these islands. Given the high uncertainties that a storm will form, or even make it across the Atlantic without recurving, should it form, one should not be canceling any travel plans at this point. However, you should be giving serious thought to your hurricane plan and what you would do if a hurricane did blow through the islands. Also, keep in minds that several of the computer models develop the next wave moving off the coast of Africa, a week later than the current wave. We are into the active part of hurricane season, and we can expect that one of these waves will develop and threaten the Lesser Antilles Islands--and points beyond--in August.

Figure 1. Latest satellite image of Hurricane Flossie.

Hurricane Flossie
Hurricane Flossie in the Eastern Pacific put on an impressive burst of intensification this morning, and is now a major Category 4 hurricane. Flossie will remain over waters warm enough ( > 80 F) to support a hurricane through the weekend, then hit cooler waters and high wind shear on Monday that should be able to weaken her to a Category 1 hurricane or tropical storm by the time the storm approaches the Hawaiian Islands on Tuesday. Just how strong Flossie is able to stay will influence her track as she approaches Hawaii. A severely weakened Flossie will not be large enough to "feel" the influence of a trough of low pressure expected to path north of the islands on Tuesday. This trough could potentially recurve the storm and bring it over the islands. If Flossie does manage to stay large and strong, the storm is more likely to feel the trough pass perilously close to the islands as a strong tropical storm or Category 1 hurricane. The Hurricane Hunters are scheduled to fly into Flossie beginning Monday at 9am Hawaiian time.

Below are a few wunderphotos of some impressive severe thunderstorms that moved through Ohio and Pennsylvania yesterday. Wow!

Jeff Masters
Tornado Warning
Tornado Warning
Unusual cloud formations in storm front
Storm Clouds
Storm Clouds
Storm Clouds Rollin In

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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