Forecasters say Omar has mustered enough energy far out in the Atlantic to become a hurricane again.
The storm was once a fierce Category 3 but dropped to tropical-storm strength by Friday morning.
It had delivered a glancing blow to the U.S. Virgin Islands and dumped rain on Antigua before heading out into the Atlantic, where it was expected to wither away.
However, its maximum winds increased to 75 mph as of 5 p.m. EDT Friday. That's strong enough to return it to hurricane status. Forecasters say the power-up will be brief.
The storm's about 660 miles east of Bermuda. Forecasters say it should gradually weaken as it heads farther out to sea.
Antiguan Prime Minister Baldwin Spencer warned of a produce shortage, saying the farming community "appears to have suffered an extensive loss of crops."
"No one is reported to have perished in this disaster," Spencer said late Thursday, hours after Omar blew past the Lesser Antilles islands as a Category 3 hurricane. "We are, nonetheless, faced with a natural disaster of serious proportions."
The crop damage comes amid spiraling food prices in the Caribbean and around the world. Spencer pledged to monitor the situation and work with farmers to meet their needs.
Omar blew north of the twin-island nation of Antigua and Barbuda early Thursday, dumping more than 5 inches of rain and forcing 75 people to seek refuge in public shelters.
The National Office of Disaster Services said rescue teams evacuated more than 30 people from flooded homes that were submerged under water or had slipped from their foundations.
Omar knocked down trees and caused some flooding and minor mudslides on several Caribbean islands, but all were spared a direct hit by the storm.
On Friday, Tropical Storm Omar was located about 670 miles southeast of Bermuda and posed no threat to land. It was expected to dissipate completely over the next several days, the U.S. National Hurricane Center in Miami said.