Watches and warnings were issued Wednesday for Caribbean nations in the projected path of the storm, the National Hurricane Center in Miami said.
Meteorologists expect Isaac to curve northward, toward Florida and the southeastern United States next week, coinciding with the Republican National Convention, said Alex Sosnowski, senior meteorologist with Accuweather.com.
Isaac, which entered the Eastern Caribbean Wednesday with maximum sustained winds of about 45 mph, was becoming more organized and could intensify into a hurricane by Thursday. A hurricane watch was in effect for Puerto Rico, the Dominican Republic, the U.S. and British Virgin Islands and Haiti, the Weather Service said in its 8 p.m. EDT advisory.
A tropical storm watch was posted for the north coast of the Dominican Republic from the Haiti-Dominican Republic border eastward to north of Isla Saona.
The storm was 65 miles southwest of Guadeloupe and 510 miles southeast of San Juan, Puerto Rico, approaching the Leeward Islands as it traveled west at 21 mph.
Isaac was expected to maintain its westerly course for the next couple of days, forecast to pass away from the Leeward Islands Wednesday evening and pass near or south of the Virgin Island and Puerto Rico Thursday, before approaching the Dominican Republic Thursday and Friday.
Isaac could become a hurricane Thursday night or Friday.
Tropical storm-force winds extended outward from Isaac's center up to 45 mph.
Isaac could generate 4- to 8 inches of rain over the northern Windward Islands and Leeward Island, the center said. Puerto Rico and the Virgin Islands could get 1-3 inches of rain with up to 6 inches possible.