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TD 4 forms; Gulf of Mexico distubance a threat to Texas; Flossie powers towards Hawaii

By: Dr. Jeff Masters, 6:22 PM GMT on August 13, 2007

The strong tropical wave that exited the coast of Africa Friday has strengthened into Tropical Depression Four this morning. This morning's QuikSCAT satellite pass at 3:21am EDT found a closed circulation centered near 12.5N 29W, with top winds of 35 mph. The pass captured only half of the circulation of TD 4, so the errors in these wind estimates may be high. Wind shear, thanks to strong upper-level winds from the east, has dropped from 20 knots to 15 knots today. This has allowed heavy thunderstorm activity to begin forming on the east side of TD 4's circulation, where it was previously absent (see NOAA's visible satellite loop.) This shear is expected to gradually decrease to 10 knots by Wednesday, which should allow the storm to grow and consolidate. SSTs will gradually warm as the storm heads westward, further aiding the potential for intensification. The total heat content of the ocean stays relatively low through the next 48 hours (Figure 1), so no rapid intensification is likely until Thursday, when the storm approaches the Lesser Antilles Islands. The 12Z (8am EDT) run of the GFDL model intensifies TD 4 into a Category 2 hurricane five days from now.

Figure 1. Ocean Heat Content (OHC) in kilojoules per square centimeter along the forecast track of TD 4. The left side of the image marks where the Lesser Antilles Islands are. Values of ocean heat content greater than 50 kJ/cm^2 (the yellow regions in the plot above) have been shown to promote greater rates of intensity change for storms in moist air with low wind shear. Image credit: NOAA/CIRA/RAMMB.

What the models say
Several major models--the ECMWF and NOGAPS--do not develop TD 4 at all. The GFS, UKMET, and new HWRF model all develop the system, and bring it to a strong tropical storm or hurricane by the time it reaches the Lesser Antilles Islands Friday. The big question is how strong the trough of low pressure predicted to pass north of TD 4 on Friday will be. If the trough is stronger than currently forecast, it may be able to pull TD 4 far enough north so that it misses the Lesser Antilles. This would be good for the islands, but potentially bad for the U.S. East Coast. The trough will likely not be strong enough to recurve TD 4 harmlessly out to sea, and the storm would then be forced westwards again by the next ridge of high pressure. A landfall along the U.S. East Coast as a hurricane--possibly a major hurricane--could result, unless the next trough of low pressure is strong enough to recurve the storm out to sea. The latest (12Z) run of the UKMET model has TD 4 passing through the very northernmost Lesser Antilles Islands. The 12Z GFDL, GFS, and HWRF have TD 4 jogging just far enough north that it would likely miss the islands. Some of these runs are considerably slower, delaying the possible impact to the islands to Saturday or Sunday. What may happen after 5 days is highly uncertain. Last night's GFS model run had TD 4 eventually making landfall south of Brownsville, Texas. This morning's run had it eventually hitting New England--a difference of about 2000 miles in landfall location!

Gulf of Mexico disturbance a threat to Texas and Mexico
Thunderstorm activity in association with a tropical wave interacting with a upper-level low pressure system over the Western Caribbean has gotten better organized this morning. This disturbance has been labeled "Invest 91L" by NHC this morning, and the preliminary model tracks (Figure 2) show that 91L 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 10-20 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. An Air Force Hurricane Hunter airplane is scheduled to investigate 91L Tuesday afternoon at 1pm EDT. The 12Z (8am EDT) runs of the GFDL and HWRF models are predicting that a tropical depression could form, and there is plenty of time for one to form. The GFDL predicts landfall on Wednesday afternoon near Corpus Christi as a 50 mph tropical storm. The storm won't have enough time over water to become anything stronger than a 50 mph tropical storm, it appears. Residents along Mexican and Texas Gulf Coast should be prepared for heavy rains from 91L as early as Tuesday night.

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 2. Preliminary model tracks for Invest 91.

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. SSTs are about 27 C under the storm, which normally can only support a Category 2 hurricane. Wind shear is about 15 knots over Flossie, and should increase to 20 knots by Tuesday. Given that Flossie is so intense 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 tropical storm force winds of 39 mph or greater currently extend out about 110 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.

Figure 3. Latest satellite image of Hurricane Flossie.

I'll have an update Tuesday morning.

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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