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Dean, Erin, and Flossie

By: Dr. Jeff Masters, 2:14 PM GMT on August 15, 2007

Tropical Storm Dean is strengthening as it continues westwards towards the Lesser Antilles Islands. Wind shear has fallen to 10 knots this morning, which has allowed the storm to consolidate its heavy thunderstorm activity into a symmetrical pattern around the center. Estimates of intensity from both traditional satellite images and microwave images have shown a steady intensification this morning, and some well-formed spiral bands are now visible on satellite loops. The major impediment to intensification is the large amount of dry air (Figure 1) to the storm's north, and this will continue to be a problem for it until it reaches a moister environment near the Lesser Antilles Islands. This morning's QuikSCAT satellite pass missed Dean, but last night's pass showed a large and well-formed circulation.

Figure 1. Current water vapor satellite image of Dean, showing very dry air (brown colors) to the north of the storm. Image credit: NOAA.

Track forecast
None of the computer models are forecasting that Dean will miss the Lesser Antilles Islands. The trough of low pressure that will pass north of the islands on Saturday is now expected to be a bit weaker than earlier forecast, which should allow Dean to pass into the Caribbean on a west to west-northwest track. The trough is no longer forecast to spawn an upper-level low pressure system, which means that the danger to the U.S. East Coast north of the Carolinas is minimal. A ridge of high pressure is expected to build in after the first trough passes on Saturday, which should keep Dean on a west to west-northwest path into the middle of next week. The southernmost model solutions (GFDL, Canadian) take Dean into Honduras early next week. The more northerly solutions of the GFS and HWRF take Dean over Jamaica, then into the Gulf of Mexico. No models call for a threat to the east coast of Florida at present, but that could change once we see how strong Saturday's trough of low pressure really will be. The NOAA jet is scheduled to make its first flight Thursday night, and by Friday morning we should have a good set of model runs that will give us a more reliable idea of Dean's likely track. At present, it appears that Dean's main threat to the U.S. will be to the Gulf of Mexico coast.

Intensity forecast
With low shear and warm water ahead of it, Dean's intensification over the next few days will only be limited by the dry air to the north. I expect that this dry air will impede Dean enough so that the storm passes through the Lesser Antilles as a Category 1 hurricane. After that, the environment moistens, shear stays low, and the heat content of the ocean greatly increases. The 06Z run of new HWRF model is again very aggressive intensifying Dean after it crosses into the Caribbean, bringing the storm to 928 mb (Category 4) on Monday morning near Jamaica. The GFDL model is not nearly as aggressive, putting Dean at 964 mb (Category 2) Monday morning. I can't see any reason why Dean wouldn't become a Category 3 or 4 hurricane by the time it reaches the Cuba/Jamaica region, unless it passes very close to the mountainous island of Hispaniola.

Figure 2. Ocean Heat Content (OHC) in kilojoules per square centimeter along the forecast track of Dean. The left side of the image marks where Cuba. 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.

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, where a major blow-up of thunderstorms is visible on the visible satellite loop. 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 only thing holding back Erin from strengthening is its initial disorganization, and the latest satellite loops show that the storm appears to have overcome that problem. Spiral banding is starting to occur, along with good upper level outflow. Erin could grow in strength rapidly. Fortunately, the storm only has about 24 hours over water, so it should not be able to become more than a 55 mph tropical storm. Heavy rains will be the main threat from TD 5. A NOAA Hurricane Hunter aircraft is currently in 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 2. Latest radar-estimated rainfall from Flossie.

I may make some minor edits to this blog today as new information arrives. The next full update will probably be 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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