CHARLOTTE AMALIE, U.S. Virgin Islands (AP) -- Omar strengthened into a hurricane late Tuesday and drenched islands in the southeastern Caribbean, downing trees and blowing off a school's roof as it menaced U.S. islands.
Authorities issued a hurricane warning for the U.S. Virgin islands as well as Puerto Rico's Vieques and Culebra islands. Hurricane warnings were also in place for St. Martin, the British Virgin Islands, St. Kitts and Nevis and other islands.
Officials in Puerto Rico, already soaked from several days of rain, warned residents to prepare for a lot more and medical authorities appealed for blood donations for possible casualties.
Omar was a Category 1 hurricane with 75 mph (120 kph) winds, and the U.S. National Hurricane Center in Miami projected it to continue to gain strength.
The center predicted the storm would likely pass by Puerto Rico overnight Wednesday, and possibly deliver a direct blow to the nearby U.S. and British Virgin Islands.
In the U.S. Virgin Islands, business grew steadily at a St. Thomas supermarket Tuesday afternoon as people bought batteries, water and canned goods. Outside, rain fell as Darrell George tugged an overloaded shopping cart topped with a case of large water jugs.
"I try my best to get everything I need and my family needs, so I don't get caught with my pants down," George said. "With what has been going on lately, we can't take anything for granted."
Emergency management director Mark Walters urged islanders to take the warnings seriously.
"This is the time to take those precautions, in terms of getting your family and your personal selves ready for the storm," he said.
Classes and ferry services were canceled in Puerto Rico and the U.S. Virgin Islands.
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.
The hurricane center 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 11 p.m. EDT (0300 GMT) Tuesday, Omar's center was located about 315 miles (505 kilometers) south-southwest of San Juan, Puerto Rico, according to the center. It was moving east-northeast at near 6 mph (9 kph).
A new tropical depression also formed Tuesday and was moving slowly off the coast of northeastern Honduras.