Hurricane Dean, TD Erin, and Super Typhoon Sepat

Hurricane Dean continues to intensify as it continues westwards towards the central Lesser Antilles Islands. Wind shear has fallen to 5 knots this morning, and recent satellite loops show an eye has appeared. Dean's intensification is being slowed by dry air on it northwest side. This dry air is being wrapped into its circulation, and is keeping the storm relatively cloud-free on its northwest side.

Impact on the Lesser Antilles
Dean will likely pass through the central Lesser Antilles Islands Friday morning as a strong Category 1 or weak weak Category 2 hurricane. Dominica will likely receive the harshest blow, although damage may also be significant on Martinique and Guadaloupe. Heavy wind damage will be the primary threat on these three islands, although torrential rains of 2-7 inches may cause flash flooding problems as well. Storm surge is generally not a problem in the Lesser Antilles, since the surge tends to flow around islands surrounded by deep water. Storm surge values of 2-4 feet are expected with Dean.

Surrounding islands from Grenada to Antigua will experience tropical storm force winds and heavy rains, but Dean's rapid forward speed will keep these rains below four inches. Puerto Rico can expect 1-3 inches of rain from the outer rainbands of Dean, but tropical storm force winds should stay just south of the island. I'll discuss the likely impacts on the rest of the Caribbean this afternoon, in my 4pm EDT update.

What the models say
The latest (00Z or 06Z) model runs from last night and early this morning don't show much change from yesterday's runs. All the models show Dean moving through the Caribbean, passing very near Jamaica and the Cayman Islands on Sunday or Monday, then into the western Caribbean. None of the models show Dean moving northwards into Florida, and I don't see any feature in the steering currents that could potentially lead to a northern excursion by Dean into Puerto Rico, the Bahamas, or the East Coast of Florida. Landfall in the Yucatan are the preferred solutions, followed by an emergence into the Gulf of Mexico, with a second landfall near the Mexico/Texas border. I'd be surprised to see Dean make a turn northwards in the Gulf of Mexico towards Louisiana or points further east, as there are no strong troughs of low pressure coming across the U.S. until late next week.

The GFDL and HWRF models have been mysteriously weakening Dean and keeping the storm weak during its passage over the Lesser Antilles Islands, and through the eastern Caribbean. These models have not been correct, and a continued slow strengthening of the storm as indicated by the SHIPS intensity model seems in order. Dry air will continue to slow the intensification process down until the storm gets in the central Caribbean, at which time Dean should be able to reach Category 3 or 4 status. Once in the western Caribbean, where the ocean heat content is near the record levels observed during 2005 (Figure 1), Dean could reach Category 5 status.


Figure 1. Tropical Cyclone Heat Potential (TCHP) for August 14 2005 (top) and August 14 2007 (bottom). TCHP is a measure of the total heat energy available in the ocean. Record high values of TCHP were observed in 2005, and the TCHP is at similar levels in the Western Caribbean this year. Values of ocean heat content greater than 50 kJ/cm^2 (the light green regions in the plot above) have been shown to promote greater rates of intensity change for storms in moist air with low wind shear. Values greater than 90 kJ/cm^2 can lead to rapid intensification. Image credit: NOAA/AOML.

Erin
Tropical Depression Erin has been downgraded from a tropical storm after making landfall this morning over South Texas. Erin brought a storm surge of up to 2 feet to the coast, and some gusty winds up to 40 mph. Heavy rain will be the main threat from this storm, and rainfall amounts up to 3 inches have already been observed along the coast north of Corpus Christi (Figure 2). Although conditions were favorable for Erin to intensify, it never had time to consolidate its circulation and was not able to take advantage of these favorable conditions. Texas says to Erin, "Mahalo!" (Hawaiian for "thank you").


Figure 2. Current estimated rainfall for Erin from the Corpus Christi radar.

Super Typhoon Sepat
The Western Pacific has its own tropical cyclone drama unfolding, as residents of Taiwan anxiously watch Super Typhoon Sepat. A Super Typhoon is a tropical cyclone in the Western Pacific that has sustained winds of 150 mph or greater, and Sepat's maximum winds of 160 mph also make it a Category 5 storm (winds of 156 mph or greater characterize a Cat 5). Sepat is headed northwest towards Taiwan, and is expected to make landfall Sunday morning local time. There's not much I can see that will weaken Sepat before it makes landfall. Wind shear is 10 knots, and the ocean heat content is an extremely high 100 kilojoules per square centimeter along the forecast track. The typhoon may suffer a drop in intensity down to a Category 4 storm due to an eyewall replacement cycle, but would still strike a devastating blow to Taiwan at that intensity.

I'll have an update by 4pm EDT this afternoon, when data from the first Hurricane Hunter mission into Dean will be available. I will project the likely impact of Dean on the rest of the Caribbean, and discuss the latest model runs.

Jeff Masters