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African wave nears tropical depression status; Flossie powers towards Hawaii

By: Dr. Jeff Masters, 1:31 PM GMT on August 13, 2007

Thunderstorm activity in association with a tropical wave interacting with a upper-level low pressure system over the Western Caribbean remains disorganized. This unsettled weather is expected to move west-northwest into the Gulf of Mexico Tuesday and Wednesday. An upper-level low currently spinning over the Gulf of Mexico is bringing 20-30 knots of shear over the Gulf. This upper-level low is moving steadily westward, and should exit the Gulf by Wednesday, allowing wind shear over the Gulf to drop below 10 knots. I expect a tropical disturbance with heavy rains worthy of being called an "Invest" by NHC will develop in the Gulf on Wednesday. This disturbance may bring heavy rains to Gulf Coast of Mexico and Texas Thursday, as it moves west-northwest. There may be time for the disturbance to grow into a tropical depression before moving inland Thursday. None of the reliable forecast models are predicting a tropical depression will form in the Gulf, though.

What is an "Invest?
When a National Hurricane Center forecaster sees a tropical disturbance that may be a threat to develop into a tropical depression, the forecaster may label the disturbance an "Invest" and give it a tracking identification number. There is no formal definition of what qualifies as an "Invest". Declaring an "Invest" is merely done so that a set of forecasting aids like computer model track forecasts can be generated for the disturbance. The "Invest" is given a number 90-99, followed by a single letter corresponding to the ocean basin--"L" for the Atlantic, or "E" for the Eastern Pacific. Other warning agencies assign "Invests" for the other ocean basins--"W" for the Western Pacific, "A" for the Arabian Sea, etc.

Figure 1. Preliminary model tracks for Invest 90L.

Invest 90L
A strong tropical wave exited the coast of Africa Friday, and is now a 1007 mb low pressure system with heavy thunderstorm activity about 400 miles southwest of the Cape Verde Islands. NHC is referring to this system as 90L. This morning's QuikSCAT satellite pass at 3:21am EDT found a closed circulation centered near 12.5N 29W, with top winds of 40 mph. Satellite imagery thus far has not shown enough organization for a long enough period of time to justify calling 90L a tropical depression, but that should occur later today. The first visible images this morning show some fair spiral banding and a large clump of heavy thunderstorms on the west side of 90L's circulation. If 90L can maintain or improve this appearance for another 12 hours, NHC will likely call it a tropical depression by their 11pm EDT advisory. In fact, some of the products coming out of NHC are already referring to 90L as "Cyclone Four". Wind shear of 20 knots, thanks to strong upper-level winds from the east, are keeping any heavy thunderstorm activity from forming on the east side of 90L's circulation. This shear is expected to decrease to 15 knots by Tuesday morning, which should allow the storm to grow and consolidate. SSTs will gradually warm as the storm heads westward, further aiding the potential for intensification.

What the models say
Several major models--the ECMWF and GFDL--are no longer developing 90L into a tropical storm. The NOGAPS model never has, and continues not to. The GFS, UKMET, and new HWRF model all do develop the system, and bring it to a strong tropical storm or Category 1 hurricane by the time it reaches the Lesser Antilles Islands Thursday night or Friday. Some scenarios to consider:

1) A trough of low pressure is expected to move off the U.S. east Coast Friday, which may be able to deflect 90L northwards enough to pass north of the Lesser Antilles Islands. A high pressure ridge is then expected to build in, forcing 90L more westwards towards the U.S. East Coast. This is the scenario preferred by the Canadian model.

2) 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. This is the solution preferred by the GFS, UKMET, and HWRF models.

3) 90L will never develop, or will never become more than a weak tropical storm, due to unfavorable wind shear, dry air, or other factors.

Of the three scenarios, I believe #2 is most likely to occur--90L will develop into a tropical storm or hurricane that will hit the Lesser Antilles, and pass into the Caribbean. It currently appears unlikely that 90L will recurve harmlessly out to sea, since the storm is too far south and the steering pattern is not generating a sufficiently strong enough trough to recurve it. 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 Category 1 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 has proved resistant to the twin effects of cooler SSTs and increased wind shear, and stubbornly remains a Category 4 hurricane. Wind shear is about 15 knots over Flossie, and should increase to 20 knots by Tuesday. Given that Flossie is so large and well-formed, it will take time for the shear and cooler waters it is now traversing to significantly weaken the storm. Flossie should still be a dangerous Category 2 hurricane Tuesday afternoon, when she make her closest approach to the Big Island. Waves along the southeast coast of the Big Island of Hawaii will build to 12 feet by Tuesday morning, and rains of 10 inches of more could inundate the high mountain flanks of Mauna Loa volcano on the south part of the island. Fortunately, this part of the island is sparsely populated, and flash flooding will probably cause only limited damage. Storm surge will not be a big deal, since islands surrounded by deep water like Hawaii tend to have the storm surge flow around them, instead of up onto shore. The computer model runs continue to show good 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 in addition to heavy rains and flash flooding. The Hurricane Hunters will be flying several missions into Flossie over the next 24 hours.

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.

I'll have an update on things later this afternoon.
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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