SAN JUAN, Puerto Rico (AP) _ Tropical Storm Marilyn strengthened enough Wednesday to worry forecasters that the 65-mph storm could grow into a hurricane by the time it reaches Barbados.

Marilyn, approaching just a week after Hurricane Luis, was the third major storm in a month to threaten the Caribbean.

``Our latest forecast has it becoming a hurricane after the islands, but the pressure is dropping fairly fast, so that's something we've got to think about,'' said Jerry Jarrell, a forecaster for the National Hurricane Center in Miami.

Fed by easterly trade winds, Marilyn formed in the Atlantic as the 13th named storm of the season Tuesday and generated hurricane warnings for islands well south of the destructive path taken by Luis.

A hurricane warning was posted for Barbados, St. Vincent and the Grenadines and St. Lucia, while a hurricane watch was in effect for Dominica. Storm warnings covered Grenada and Tobago, while a storm watch was up for Trinidad. Rainfall of five to eight inches was possible.

Avoiding a repeat of Luis in St. Martin, where more than 200 boats were sunk in Simpson's Bay Lagoon, boat owners scurried to secure vessels or move them to safe harbors.

In Barbados, a hub for relief agencies working in St. Martin and Antigua, residents rushed to grocery and hardware stores to stock up. Authorities opened 75 shelters capable of holding about 8,000 people. Residents boarded up windows and bought supplies, incoming flights were canceled and the government broadcast emergency advisories by radio, said Henderson Seale of the government's Central Emergency Relief Organization.

Similar preparations were reported in Grenada, Dominica and St. Lucia.

At 8 p.m. EDT, Marilyn's center was about 95 miles east of Barbados. The storm was moving west near 15 mph, but a northward push was expected.

``Our forecast has it going through the islands and going into the Caribbean'' early Thursday, Jarrell said.

The possible danger to the United States was uncertain because of the distance.

``As far as the mainland, southeastern United States is concerned, this storm is a little bit too far away for us to know if it is going to threaten,'' said National Hurricane Center forecaster Miles Lawrence.

Luis, the most powerful hurricane so far this season, killed 12 people in St. Martin, Antigua, Puerto Rico, Dominica and Guadeloupe before shooting north deep in the Atlantic to Newfoundland.

Storms are named when steady winds reach 39 mph and become hurricanes if they build to 74 mph. The hurricane season runs from June through November.