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19 July 2011

Dr. Jeff Masters' WunderBlog

Tropical Storm Bret forms near the Bahamas

Tropical Storm Bret formed last night over the Northwestern Bahama Islands, and is expected to bring heavy rains of 2 - 4 inches to the northernmost Bahama Islands today and Tuesday, as the storm drifts slowly to the north-northeast. Bret's formation date of July 17 is two weeks ahead of the usual formation date for the Atlantic's second storm of the season, which is August 1. The latest data from the Hurricane Hunters, taken between 4 - 5am EDT, showed a 100-mile wide area of tropical storm-force winds in excess of 39 mph affecting only the northernmost Bahama Islands--the Abacos. A personal weather station on Great Abaco Island recorded a wind gust of 48 mph at 12:23 am today. Satellite imagery shows that Bret has not improved in organization this morning, thanks to dry air to the northwest that has been blown into the storm's core by upper-level northwesterly winds. long-range radar out of Melbourne, Florida shows that Bret is currently dumping very little rain over the Bahamas, and one thin rain band from the storm is affecting the Florida East Coast with rainfall amounts less than a quarter inch. Wind shear is a moderate 10 - 15 knots, and sea surface temperatures are 28 - 29°C, which is 2°C above the 26.5°C threshold needed to support a tropical storm.

Figure 1. Morning satellite image of Tropical Storm Bret.

Forecast for Bret
None of the models develop Bret into a hurricane. though the GFDL model has it coming close. Given the current ragged appearance of the storm, plus the forecast by the SHIPS model that wind shear will rise to the high range, 20 - 25 knots, by Tuesday morning, it is unlikely Bret will become a hurricane. NHC is giving Bret a 22% chance of developing into a hurricane by Tuesday.

Steering currents are weak off the coast of Florida, and Bret can be expected to move slowly near 5 mph through Tuesday, before the storm gets caught in a trough of low pressure and lifted northeastwards out to sea. It currently appears that the only land areas that will be affected by Bret will be the northernmost Bahama Islands, today and Tuesday.

None of the reliable models predict tropical cyclone development over the remainder of the Atlantic through July 25.

Figure 2. Visible satellite image of Typhoon Ma-on from NASA's Aqua satellite taken July 18, 2011. Image credit: NASA.

Weakened Typhoon Ma-on headed towards Japan
Typhoon Ma-on has weakened to a large Category 1 storm as it heads towards Japan, and is expected to brush the east coast of the main island of Honshu on Tuesday. The typhoon weakened over the past day, thanks to an eyewall replacement cycle where the inner eyewall collapsed, and a new, larger eyewall formed from an outer spiral band. Ma-on was expected to intensify once this process completed, but the eyewall replacement process significantly disrupted the storm, and it is unlikely Ma-on will be able to recover. This is good news for Japan, since Ma-on is a huge storm with tropical-storm force winds that extend 225 miles north of the center.

Jeff Masters 

- Weather Underground