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Slowly Organizing Tropical Storm Erika Prompts Caribbean Watches, Warnings

By: Bob Henson 6:01 AM GMT on August 26, 2015

Tropical Storm Erika is not looking especially fearsome tonight, although it remains a longer-term threat for parts of the southeast U.S. coast and a more immediate concern for the Caribbean. At 11:00 pm EDT Tuesday, the National Hurricane Center placed Erika at 16.0°N, 54.4°W, or about 500 miles east of Antigua. WIth top sustained winds still at minimum tropical-storm strength (40 mph), Erika was moving just north of due west at around 18 mph. That healthy clip has prompted tropical storm warnings for Anguilla, Saba, St. Eustatius, and St. Maarten, with tropical storm watches now in place for many other nearby islands, including Puerto Rico and the U.S. Virgin Islands.

Northerly wind shear on Tuesday pushed Erika’s showers and thunderstorms largely to the south of the partially exposed low-level center of circulation. Over the last several hours, a new cluster of storms has popped up closer to the low-level center, perhaps a sign of better organization to come. Erika has a large circulation, evident in the pockets of convection located far away from its center. One large cluster developed more than 500 miles west of Erika’s core on Tuesday afternoon, far enough west to make it visible on Barbados radar (thanks go to the Weather Channel's Stu Ostro for that tidbit). Together with that wide reach, Erika has a broader pool of relatively moist air to draw from than its compact predecessor, Hurricane Danny. The Sarahan layer of dry, dusty air that enveloped Danny is much less pronounced in the vicinity of Erika.

Figure 1. Infrared image from the GOES-East floater satellite, taken at 0445 GMT (12:45 am EDT) on Wednesday, August 26. Image credit: NOAA/NESDIS.

Erika’s large size will make it slower to organize and intensify than Danny, while also helping to protect the storm from any rapid decay down the line. Track models are fairly consistent in bringing Erika to the Bahamas by this weekend, but there remain big questions in how much and how quickly Erika will intensify during that time. Model guidance has continued to strengthen Erika only very gradually over the next 2 to 4 days. The most reliable longer-range dynamical models have been pessismistic on Erika’s future, with the 1200 GMT Tuesday runs of the GFS and ECMWF (as well as more recent GFS runs) weakening the storm to an open wave by this weekend. Statistical guidance, which tends to perform the best at intensity beyond about 3 days, suggested at 0000 GMT Wednesday that Erika might be only a strong tropical storm by Day 5 (late Sunday, August 30). Meanwhile, the last several runs of the two top high-resolution models (GFDL and HWRF) have sent Erika into fairly rapid intensification mode by days 4 and 5, when the storm should be over the very warm waters of the Bahamas (more than 30°C or 86°F). Before then, Erika will need to barrel through a ribbon of high-level, shear-producing westerly winds, perhaps a key reason why most models are putting little stock in Erika’s shorter-term future. Interaction with Puerto Rico and/or Hispaniola may also be an issue, as Erika is generally predicted to track near or just north of those islands.

Figure 2. The official NHC forecast for Erika as of 11:00 pm EDT on Tuesday, August 25.

Should Erika make it to the Bahamas as a well-organized tropical storm, the picture could change dramatically. This is a very favored spot climatologically for hurricane development, especially in late August and early September. The GFS and ECMWF models from 1200 GMT Tuesday, and the GFS model from 1800 GMT Tuesday, agreed on moving Erika or its remnants toward Florida, as an upper-level ridge builds over and to the northeast of Erika. The 11:00 pm EDT Tuesday forecast from NHC brings Erika to within a half-day of the Florida coast as a Category 1 hurricane by Sunday night, August 30 (see Figure 2). Unlike many systems in recent years, Erika could approach Florida with a weak upper-level trough located well to the west of the state (see Figure 3 below), which would make an immediate recurvature less likely. Putting aside the very valid question of Erika’s durability over the next several days, the overall pattern is the most favorable I’ve seen in a long time for a potential Florida landfall. It has been nearly 10 years since a hurricane has reached Florida’s coastline, the last being Hurricane Wilma (also the nation’s most recent hurricane that was rated Category 3 at landfall on the Saffir-Simpson Hurricane WInd Scale). Since records began in 1851, this is by far the state’s longest “hurricane drought,” beating out the five hurricane-free seasons from 1980 to 1984. If nothing else, any development of Erika would serve to dislodge any misplaced confidence that the state’s hurricane risk has somehow withered over the last decade.

Jeff Masters will have our next complete tropical roundup on Wednesday.

Bob Henson

Figure X Projected steering flow at the 200-millibar level (about 40,000 feet) from the 1800 GMT Tuesday run of the GFS model, valid at 1800 GMT (2:00 pm EDT) on Sunday, August 30. Image taken from our wundermap with the “Model Data” layer turned on.


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