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(CNN) -- Tropical Storm Bertha barreled toward the northern end of the island of Martinique in the eastern Caribbean on Friday, the National Hurricane Center said.
The storm had maximum sustained winds of 50 mph and was churning through the Atlantic at 24 mph.
Bertha was located about 20 miles northwest of Fort de France, Martinique, the hurricane center's 5 p.m. advisory said.
It formed on Thursday.
Tropical storm warnings were issued for Dominica, Martinique, Puerto Rico, Vieques, Culebra, Martinique, and the U.S. and British Virgin Islands.
Such a warning means tropical storm conditions are expected in the area within the next 24 hours.
A tropical storm watch was issued for the eastern Dominican Republic, the southeastern Bahamas and the Turks and Caicos Islands -- meaning storm conditions are possible within 48 hours, the center said.
Tropical storm conditions were expected in Puerto Rico and the Virgin Islands on Saturday, according to the National Weather Service.
Rainfall amounts from Bertha were projected at 1 to 3 inches, with isolated amounts up to 6 inches across parts of the Leeward Islands, Puerto Rico and the Virgin Islands through Saturday night.
The storm was expected to move back toward the ocean and away from the U.S. mainland in the coming days, according to CNN's Chad Myers. The biggest threat to the United States, he said, would be rip currents up and down the East Coast as large waves come on shore through the weekend.