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Afternoon update on Dean, Erin, and Flossie

By: Dr. Jeff Masters, 7:47 PM GMT on August 15, 2007

Tropical Storm Dean continues to strengthen as it continues westwards towards the Lesser Antilles Islands. Wind shear has remained constant at 10 knots today, which has allowed the storm to consolidate its heavy thunderstorm activity, and recent microwave satellite imagery (Figure 1) shows that Dean has made a partial eyewall on the east side. Recent satellite loops show a small Central Dense Overcast (CDO) trying to form over the center. A CDO is a high canopy of cirrus clouds that forms over the center, and is the hallmark of a storm nearing hurricane strength. However, satellite loops also show that the storm is wrapping some dry air into its circulation on the west side, and this will slow down Dean's progress towards becoming a hurricane.

Figure 1. Microwave satellite image of Dean from the AMSU instrument on the polar-orbiting NOAA-18 spacecraft. Photo taken at 12:13pm EDT Wed Aug 15. Think of this image as a weather radar in space. Note the comma-shaped half-eyewall present on the east side of Dean.

Impact on the Lesser Antilles
All of the computer models forecast that Dean will hit the Lesser Antilles Islands. The cone of uncertainty Friday morning, when Dean is expected to pass through the islands, covers a wide stretch of ocean from Grenada to Antigua. We can expect that this uncertainty cone is too wide, since the steering currents are relatively stable right now and Dean is well-formed. This puts the islands of Martinique, Dominica, and Guadaloupe in the bulls-eye. If Dean strengthens to a Category 1 hurricane, one of these islands will suffer considerable damage from hurricane-force winds, and surrounding islands will have minor damage due to flooding rains and tropical storm force winds. I expect Dean will be a Category 2 storm, as forecast by the HWRF model (Figure 2). We can expect two of these islands will have heavy damage. Dean is rather small in size at present, so this wind damage will likely be confined to a relatively small area 30 miles wide. Flooding rains of 5-10 inches can be expected over a wider swath, perhaps 100 miles in diameter. The latest run of the GFDL model unrealistically projects Dean to be a weak tropical storm with a 1004 mb central pressure on Friday morning, when it crosses the Lesser Antilles near Dominica.

The latest (12Z) model runs from this morning don't show much change from last night's. All the models show Dean moving through the Caribbean, passing very near Jamaica on Sunday or Monday, then into the western Caribbean. None of the models show Dean moving northwards into Florida. Landfall in the Yucatan or western Cuba are the preferred solutions.

Figure 2. Forecast pressure 6-hourly rainfall for Dean at 8am EDT Friday, from the 12Z Wednesday run of the HWRF model. HWRF forecasts that Dean will hit Dominica as a Category 2 hurricane (976 mb pressure).

Tropical Storm Erin formed this morning over the Gulf of Mexico. A QuikSCAT satellite pass at 7:38am EDT today showed a weak circulation with winds of 25-35 mph on the northeast side. Since then, the Hurricane Hunters found 50 mph winds at flight level (1500 feet), which corresponds to about 40 mph winds at the surface. Winds at a buoy 70 miles south of Freeport, TX have risen to 21 kt, gusting to 27 kt. Long range radar out of Corpus Christi shows intensifying bands of rain starting to come ashore. Visible satellite loops show the storm is rapidly organizing, and it may have time to reach sustained winds of 55 mph before making landfall over the Texas coast on Thursday. Wind shear over Erin is only 5-10 knots, and an upper-level high pressure system has parked itself directly over the storm. This is an ideal situation for intensification, since the upper-level high provides very favorable outflow at the top of the storm, venting all the air forced up at the center of the storm.

The Big Island says Mahalo to Flossie
"Mahalo" in Hawaiian means "thank you", and residents of the Big Island have plenty to be thankful for today. Tropical Storm Flossie spared the island, as wind shear ripped up the storm just as it was about to make its closest approach to the island. Flossie decayed from a Category 2 hurricane to a tropical storm in just six hours, and her flooding rains never materialized. Radar estimates of rainfall from Flossie over the Big Island (Figure 3) are no more than an inch, and it is unlikely that the island will get as much as four inches from the storm the remainder of today. Some coastal flooding occurred because of waves up to 20 feet, and sustained winds of 40 mph gusting to 48 mph were observed at South Point, the extreme southern tip of the island.

Figure 3. Latest radar-estimated rainfall from Flossie.

I'll have an update Thursday 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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