Tropical Storm Florence gained strength in the open Atlantic early Wednesday and could become a hurricane by the weekend, but forcasters said it was too soon to tell if it would reach the United States.

Florence had sustained winds near 45 mph Wednesday morning, over the 39 mph threshold for a tropical storm. The minimum for a hurricane is 74 mph.

"Several days down the road it could very well strengthen into a hurricane," said National Hurricane Center forecaster Jack Beven. Forecasters have said that could happen as early as Friday.

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At 5 a.m. EDT, the storm was centered 845 miles east of the Northern Leeward Islands, or about 1,985 miles east-southeast of Miami, and was moving west-northwest at about 13 mph.

Tropical storm force winds extended 145 miles from its center.

"Although Florence continues to get better organized, it remains an unusually large Atlantic tropical storm, and large cyclones tend to take longer to develop and intensify than smaller ones do," said hurricane specialist Stacy Stewart.

Florence follows on the heels of Tropical Storm Ernesto, which formed Aug. 25 over the southern Caribbean and was briefly the season's first hurricane before weakening and hitting Florida and North Carolina last week as a tropical storm.

At least nine deaths in the United States were blamed on Ernesto, which also killed two people in Haiti, delayed the launch of the space shuttle Atlantis and blacked out thousands of homes and businesses from North Carolina to New York.

Florence developed in the peak of hurricane season in warm Atlantic waters, the source of energy for storm development this time of year, Willis said.

"It's nothing like we saw last year, but the waters are still warm enough to favor tropical storms and hurricanes and intensification," Willis said.

Last year's Atlantic storm season had a record 28 named storms and 15 hurricanes, including Katrina.

The 2006 Atlantic hurricane season has not been as rough as initially feared. The National Hurricane Center lowered its forecast in August to between 12 and 15 named storms and seven to nine hurricanes.