MIAMI, Florida (CNN) -- Hurricane Omar weakened Thursday to a Category 1 storm and sped away from the northern Leeward Islands after barreling through them as a major Category 3 storm, forecasters said.
The storm's maximum sustained winds had dropped to about 85 mph, with some higher gusts, the National Hurricane Center said.
Omar was moving northeast at about 23 mph, toward the cooler waters of the north-central Atlantic Ocean and far from Bermuda and land, the hurricane center said.
According to the Saffir-Simpson scale, which rates hurricanes 1 through 5 according to strength, a Category 3 storm has winds from 111 to 130 mph; a Category 1 has winds from 74 to 95 mph. According to the scale, a Category 1 storm usually causes minimal damage.
The storm is expected to drop an additional 1 to 3 inches of rain over portions of the northern and central Lesser Antilles, from Antigua south to Martinique. Isolated amounts of up to 6 inches are possible in those areas, the hurricane center said. iReport.com: St. Kitts reels after Omar
Additional weakening is expected over the next two days, it said.
All warnings and watches throughout the Caribbean are discontinued.
Earlier, the 500,000 barrel-a-day oil refinery Hovensa, on St. Croix, was shutting down all equipment "except those necessary to maintain power supply in the complex," refinery spokesman Alex Moorhead said in a written statement.
The move was to ensure the safety of employees and the operation of the refinery, jointly owned by Hess Corp. and Venezuela's state oil company.
The U.S. Coast Guard closed the Christiansted Harbor, where the refinery is located, on Tuesday. It will remain closed, Moorhead said, until the hurricane passes and all facilities are checked to be in good order. See where Omar is headed »
As of 11 a.m. ET, the center of the storm was 180 miles northeast of the northern Leeward Islands, the center said. It had dumped heavy rain on the U.S. and British Virgin Islands.
The storm's passing could produce large swells along the west- and south-facing coasts of the Lesser Antilles -- the Caribbean islands that stretch from the Virgin Islands southward to the islands off Venezuela's coast. Watch Venezuelans try to save homes, dogs »
The swells could erode beaches and damage coastal structures, the hurricane center said.
The hurricane center reported that storm-surge flooding along the south- and west-facing coasts of the northern Leeward Islands would subside later Thursday morning, but large swells may continue throughout the day.
Omar formed Tuesday in the eastern Caribbean. It is the 15th named storm of the Atlantic hurricane season, which began June 1 and ends November 30.