SAN JUAN, Puerto Rico – A rapidly strengthening Tropical Storm Omar drenched islands in the southeastern Caribbean on Tuesday, downing trees and blowing off part of a school's roof as it menaced U.S. islands.

Officials in Puerto Rico, already soaked from several days of rain, warned residents to prepare for more and issued a hurricane watch for the U.S. Caribbean territory. Medical authorities appealed for blood donations for possible casualties.

Forecasters said Omar was strengthening rapidly and had maximum sustained winds of 70 mph (110 kph) by Tuesday afternoon. It could become a hurricane later Tuesday or on Wednesday.

Omar's expected path could take it over Puerto Rico overnight Wednesday, or it could drift further east over the nearby U.S. and British Virgin Islands. The governments of Netherland Antilles and Antigua and Barbuda issued hurricane watches.

Several large trees in the Dutch island of Aruba toppled over because the ground was so saturated. The roof over a teachers' lounge and office at an Aruba school flew off. No injuries were reported.

A tropical storm warning was in effect for the British Virgin Islands and the islands of Angilla, St. Kitts, Nevis, St. Maarten and St. Martin.

The U.S. National Hurricane Center in Miami said Omar was expected to plow over the Caribbean islands then head northeast toward the central North Atlantic, well away from the U.S. mainland.

At 5 p.m. EDT (2100 GMT), Omar's center was located about 345 miles (550 kms) south-southwest of San Juan, Puerto Rico, according to the hurricane center. It was moveing east-northeast at near 7 mph (11 kph).

A new tropical depression also formed Tuesday and was moving slowly off the coast of northeastern Honduras. A tropical storm warning was in effect for the coast of the Central American country.