The storm was located around 1,515 miles east of the Windward Islands of the Caribbean by 11 a.m. EDT after forming earlier in the morning, the U.S. National Hurricane Center said.
Its winds had reached 40 miles per hour and it was moving west-northwest near 15 mph. The storm was expected to turn to the north before reaching the Lesser Antilles.
While two computer models used to forecast hurricane tracks and intensity indicated Karen could reach hurricane strength with top winds of at least 74 mph, the Hurricane Center said the climate conditions in the path of the storm did not appear to justify those forecasts.
The hurricane center was monitoring at least three other areas where weather disturbances had the potential to develop into tropical cyclones -- one in the Gulf of Mexico, where Mexico and the United States have important oil production facilities, another near the French island of Guadeloupe and the third over Florida and the Bahamas.
The 2007 Atlantic storm season, which lasts through November 30, has generated three hurricanes so far, including two ferocious maximum-strength Category 5 hurricanes.
One of them, Dean, swiped Jamaica and then plowed into Mexico's Yucatan Peninsula. The other, Felix, tore into Central America.
It was the first time since records began in 1851 that two Category 5 hurricanes made landfall in the Atlantic basin in the same year.
Weather experts have forecast an above-average 16 named storms this year. Record-busting 2005 saw 28 storms form, of which 15 strengthened into hurricanes, including Hurricane Katrina, the storm that swamped New Orleans.
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