Forecasters said Thursday night that Omar had weakened to a tropical storm with 70 mph winds and was moving into the open Atlantic.

The powerful core of the storm, with the most intense winds, passed Wednesday night and Thursday morning between St. Martin and the U.S. Virgin Islands, said Lixion Avila, a hurricane specialist with the U.S. National Hurricane Center.

"It could have been worse," Avila said. "They were very, very lucky."

It had been as strong as a category 3 hurricane, but a last-minute shift to the east meant that St. Croix, the most populated of the U.S. Virgin Islands, experienced the weaker side of the system. Winds there reached just 48 mph mph, officials said.

The storm knocked down trees, caused some flooding and minor mudslides but there were no immediate reports of deaths or major damage, said Mark Walters, director of the U.S. Virgin Islands disaster management agency.

Click here to track the storm.