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Ana, Bill, and Claudette--a rare early season triple threat

By: Dr. Jeff Masters, 8:58 PM GMT on August 16, 2009

After an unusually slow start to the Atlantic hurricane season, the tropics exploded into action this weekend. We have a rare triple threat this afternoon--simultaneous named storms beginning with the letters A, B, and C. The last time this occurred was in the slow-starting 1984 hurricane season, when Tropical Storms Arthur, Bertha, and Cesar were all active on September 1. This afternoon, the Hurricane Hunters confirmed than Tropical Storm Claudette had formed in the Gulf of Mexico. Radar animations out of the Florida Panhandle show a small but well-organized tropical storm with plenty of heavy thunderstorm activity, and improving low-level spiral banding. Claudette has built an eyewall that is 1/3 complete on the east side of the center. Some moderate wind shear from upper-level southwesterly winds is keeping Claudette from forming much heavy thunderstorm activity on its west side. Most of the rain is on the right side of the storm, and regions to the left of where Claudette comes ashore will get only a maximum of 1 - 2 inches of rain. Satellite loops show an area of intense thunderstorms with cold cloud tops continuing to expand near the storm's center.

Figure 1. Current short-range radar out of the Florida Panhandle.

Claudette developed literally overnight, and has enough time over water to be a 60 - 65 mph tropical storm by the time it makes landfall tonight along the Florida Panhandle. Claudette reminds me of 2007's Hurricane Humberto, which became a hurricane just 24 hours after first appearing as a tropical depression. By the time Claudette makes landfall, it will have been in existence just 18 hours since the first advisory. This is not enough time to strengthen into a hurricane. If Claudette had had another twelve hours over the 30°C waters of the Gulf of Mexico, it would likely have been a hurricane. Given that the storm is so small, storm surge flooding should not exceed 3 - 5 feet, and will not be the major hazard from Claudette. Inland flooding from heavy rains of 3 - 6 inches is likely to be the main threat from the storm.

Ana near death
The Hurricane Hunters are in Tropical Storm Ana, and have found that the storm is very disorganized, and may not survive another day. Dry air continues to plague the storm, thanks to the large area of Saharan air the storm is embedded in. Ana's very rapid forward motion of 20 - 25 mph has stretched out the storm's circulation from circular to elliptical. This non-circular flow around the center has made the heavy thunderstorm activity less organized, and it may be difficult for the storm to hold together much longer. The outer rain showers from Ana are now on radar out of Martinique.

Nearly all of the computer models forecast Ana will dissipate over the next two days. If there is anything left of the storm by the time it encounters the rugged terrain of Hispaniola, that should finish Ana off. At this time, it does not appear that Ana will be moist enough to cause a major flooding disaster on Hispaniola.

Tropical Storm Bill continues strengthening
Tropical Storm Bill continues to gather strength in the middle Atlantic Ocean, and appears poised to become a major Cape Verdes-type hurricane later this week. Water vapor satellite loops show that Bill has developed a core region of heavy thunderstorms that should be fairly impervious to the dry Saharan air to its northwest, and the storm should be able to start building an eyewall tonight.

Wind shear is low, 5 - 10 knots, and is forecast to be in the low range for the next three days. With Sea Surface Temperatures at 27.5°C, and ocean heat content sharply increasing 2 - 3 days from now, Bill should be able to intensify to a major Category 3 or 4 hurricane by Wednesday. At that time, some increasing shear may limit further intensification.

The big news is that our most reliable computer model from last year--the ECMWF model--appears to have made the right call yesterday, forecasting that a major trough of low pressure would develop along the U.S. East Coast, turning Bill more to the north. All of the other models have now jumped on the ECMWF bandwagon, forecasting that Bill will pass well north of the Lesser Antilles Islands. The UKMET model, which was resisting forecasting the northward turn, has now joined the other models in its latest 12Z run. The main drama with Bill this week will be to see how close it passes to Bermuda. Several models have Bill passing within 200 miles of Bermuda on Saturday. It is too early to be confident that Bill will miss the U.S. East Coast.

I'll have an update Monday.

Jeff Masters


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