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Low Pressure Forms East of the Bahamas

A small area of low pressure developed east of the Bahamas on Wednesday afternoon and the National Hurricane Center designated the system as Invest 91L,  At 8:00 p.m. EDT on Wednesday the center of Invest 91L was located at latitude 26.6°N and longitude 67.7°W which put it about 625 miles (1005 km) east of Great Abaco Island in the Bahamas and about 895 miles (1445 km) south-southeast of Charleston, South Carolina.  Invest 91L was moving toward the north at 8 m.p.h. (13 km/h).  The maximum sustained wind speed was he 30 m.p.h. (50 km/h) and there were wind gusts to 40 m.p.h. (65 km/h).  The minimum surface pressure was 1015 mb.

The circulation of Invest 91L is not well organized at the current time.  A small center of circulation was evident on late afternoon visible satellite imagery.  Several thin bands of showers are on the eastern side of the center of circulation.  However, there are very few taller thunderstorms and the circulation probably does not have a warm core at the moment.

Although Invest 91L is over water where the Sea Surface Temperature is between 26°C and 27°C, the atmospheric environment is currently unfavorable for further development.  An upper level trough near Florida is causing strong southwesterly winds to blow over the top of Invest 91L and there is significant vertical wind shear.  The wind shear is quickly dissipating any thunderstorms that start to form.  Invest 91L is forecast to move west-northwest into a more favorable environment in about a day or two.  If the system moves west, it will move into an area where the upper level winds are not as strong.  If the surface circulation is still intact at that time, less vertical wind shear could allow for the system to intensify.  The National Hurricane Center places the probability of development of a tropical cyclone or subtropical cyclone within the next five days at 60%.

Since Invest 91L primarily exists in the lower levels, it will be steered by winds in the lower troposphere.  A surface high pressure system centered off the southeast coast of the U.S. is forecast to move north of Invest 91L and the high will steer the system west-northwest toward the coast of the U.S.  On its anticipated track Invest 91L could move near the southeast coast of the U.S. during the weekend.

If Invest 91L intensifies, the highest probability is that it could become a tropical depression or tropical storm as it approaches the U.S.  It could generate coastal erosion, rip currents and locally heavy rainfall.  The threat for wind damage appears to be minimal at the current time.  People near the coast should monitor trusted sources of weather information.

Joaquin Becomes a Major Hurricane Near the Bahamas

Hurricane Joaquin intensified very rapidly on Wednesday and it reached Major Hurricane intensity.  At 11:00 p.m. EDT on Wednesday night the center of Hurricane Joaquin was located at latitude 23.8°N and longitude 73.1°W which put it about 90 miles (145 km) east of San Salvador and about 750 miles (1215 km) south-southeast of Cape Hatteras, North Carolina.  Joaquin was moving toward the southwest at 6 m.p.h. (10 km/h).  The maximum sustained wind speed was 115 m.p.h. (185 km/h) and there were wind gusts to 140 m.p.h. (220 km/h).  The wind speed made Joaquin a Category 3 hurricane on the Saffir-Simpson Scale and a Major Hurricane.  The minimum surface pressure was 951 mb.  The Hurricane Intensity Index (HII) was 20.6, the Hurricane Size Index (HSI) was 11.6, and the Hurricane Wind Intensity Size Index (HWISI) was 32.2.

A Hurricane Warning has been issued for the Central Bahamas including Cat Island, the Exumas, Long Island, Rum Cay and San Salvador.  A Hurricane Warning has been issued for the Northwestern Bahamas including the Abacos, Berry Islands, Eleuthera, Grand Bahama Island and New Providence.  A Hurricane Watch has been issued for Bimini and Andros Island.  A Tropical Storm Warning has been issued for Andros Island and the Southeastern Bahamas including the Acklins, Crooked Island, Long Cay, the Inaguas, Mayguana and the Ragged Islands.

Joaquin has a well formed structure.  The circulation is fairly symmetrical, although there are more spiral bands of thunderstorms south and east of the center.  The hurricane is over water where the Sea Surface Temperature is near 30°C.  A narrow upper level ridge west of Joaquin is causing light northerly winds over the circulation, but those winds did not inhibit rapid intensification on Wednesday.  The convection generated upper level divergence which pumped out mass and caused the pressure to decrease quickly.  Further intensification is possible during the next 24 hours, although if an eyewall replacement occurs, it would interrupt that process.  An upper level trough over the eastern U.S. will generate more vertical wind shear in a day or two and Joaquin will start to weaken.

A ridge north of Joaquin is blocking the hurricane and forcing it to move toward the southwest.  The ridge will weaken in a day or so and the upper level trough will start to steer Joaquin toward the north.  When the hurricane starts moving northward, it will also begin to move faster.  It could be approaching the Mid-Atlantic Coast by later on Saturday.