Tropical Storm Dean, Gulf of Mexico development, and Hurricane Flossie

Tropical Storm Dean continues to struggle with wind shear of 15-20 knots this morning, but improved enough in organization since yesterday to be upgraded from a tropical depression. A QuikSCAT satellite pass at 6:17am EDT captured the large circulation nicely, and found top winds of 35-40 mph. Visible satellite loops show a little improvement in organization this morning, with one decent-looking spiral band to the north, and a small amount of upper level outflow to the south and west.


Figure 1. Ocean Heat Content (OHC) in kilojoules per square centimeter along the forecast track of Dean. The left side of the image marks where Puerto Rico is. 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.

Track forecast
Two major models--the ECMWF and NOGAPS--stubbornly refuse to develop Dean at all. These models are in great likelihood wrong. The GFS, UKMET, and new HWRF model all develop Dean into a hurricane that threatens the central and northern Lesser Antilles Islands Friday or Saturday. The big question is how strong the trough of low pressure predicted to pass north of TD 4 on Saturday will be. If the trough is strong enough, it may be able to pull TD 4 far enough north so that it misses the Lesser Antilles. Another big question is, will the trough spawn a cut-off upper-level low pressure system off the Southeast coast of the U.S.? If so, this feature could act to steer TD 4 on a more northerly track early next week, increasing the threat to New England and the mid-Atlantic coast. If not, a high pressure ridge is expected to build in, forcing TD 4 westward into Florida. It's far too early to know which of these scenarios might occur. In any case, there is a significant possibility that TD 4 will hit the U.S. as a hurricane, and possibly a major hurricane. This could happen as early as Tuesday August 21. Of course, Dean could also stay farther south in the Caribbean, as forecast by the UKMET model, and eventually track into the Gulf of Mexico.

Intensity forecast
The 06Z run of new HWRF model indicates that Dean will not start intensifying until Wednesday afternoon. HWRF is then very aggressive intensifying the storm, bringing it to 956 mb (Category 3) on Saturday morning as it passes through the Lesser Antilles, then 920 mb (Category 4 or 5) Sunday morning near Puerto Rico. I'd be surprised to see Dean get that strong that fast, and HWRF is likely overdoing the intensification. The 06Z run of the GFDL model also shows a slow intensification starting Wednesday morning, followed by a steady increase to a Category 3 storm (956 mb) Sunday night as it passes through the northern Lesser Antilles. Given the upper-level high pressure system forecast to develop over Dean beginning Wednesday, combined with steadily increasing Sea Surface Temperatures and total oceanic heat content under the storm (Figure 1), intensification to a major Category 3 hurricane by Sunday is a reasonable forecast.

Gulf of Mexico disturbance a threat to Texas and Mexico
Thunderstorm activity over the Gulf or Mexico near the Yucatan Peninsula has gotten better organized this morning. Visible satellite loops show that a low level circulation has formed just northwest of the Yucatan Peninsula, and heavy thunderstorm activity is on the increase on the northeast side of this circulation. This disturbance has been labeled "Invest 91L" by NHC, and the preliminary model tracks (Figure 2) show that 91L is expected to move west-northwest into the south coast of Texas Wednesday afternoon. Wind shear over the disturbance is only 5-10 knots, and the upper-level low that was keeping high shear over the Gulf has now exited to the west. I expect a tropical depression will form here today, and An Air Force Hurricane Hunter airplane is scheduled to investigate 91L this afternoon at 1pm EDT.

The storm has the potential to organize quickly and become a tropical storm. Last night's run of the GFDL model brought 91L to the south Texas coast as a 55 mph tropical storm by Wednesday afternoon. Heavy rain will be the main threat from this storm.


Figure 2. Model tracks for TD 5.

Hurricane Flossie
Hurricane Flossie in the Eastern Pacific has finally succumbed to the twin effects of cooler SSTs and increased wind shear, and will pass 50-100 miles south of the southern tip of Hawaii today as a much weakened Category 1 or 2 hurricane. Sustained tropical storm force winds of 39 mph or greater currently extend out about 150 miles from the storm's center, so most of the southern part of the Big Island will be at risk of wind damage from Flossie. The biggest problem will be the 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.

Long range radar animations from the Big Island show Flossie's approach nicely.


Figure 3. Latest long-range radar out of the Big Island.

I may make some minor edits to this blog today. The next full update will be late this afternoon or Wednesday morning.

Jeff Masters