MIAMI -- Hurricane Isabel weakened slightly Sunday but still was a powerful Category 4 storm as it plowed across the Atlantic Ocean on a course that could slam it into the central East Coast late this week.

"It's looking more and more likely that this is going to be a big event for the eastern United States," National Hurricane Center meteorologist Eric Blake said Sunday.

Computer models predict that weather conditions over the East Coast should prevent Isabel from turning back out to sea and missing land, hurricane specialist Stacy Stewart said.

"Landfall along the U.S Mid-Atlantic coast somewhere between North Carolina and New Jersey between 4 or 5 days is appearing more and more likely," Stewart said. "Little or no significant weakening is expected to occur until after landfall occurs."

Boarding up

In Wilmington, N.C., John Byrnes had already stocked up with 25 sheets of plywood Sunday and enough two-by-fours and screws to barricade the windows at his house, his in-laws' house and their downtown law office.

His household generator was ready and he had an extra tank of propane gas to run appliances.

"We're all pretty much taken care of," Byrnes said. "We're in standby mode."

At midday Sunday, Isabel's maximum sustained wind speed had fallen by 5 mph to 155 mph -- 1 mph below the minimum for Category 5 -- apparently a fluctuation in strength common to major hurricanes, forecasters said. Experts had said it would be extremely unusual for Isabel to maintain Category 5 strength as it moved north over cooler water.

The storm was centered about 370 miles east-northeast of the Turks and Caicos Islands, or about 320 miles north of San Juan, Puerto Rico. Hurricane-force wind of at least 74 mph extended 85 miles out from the center.

It was moving toward the west-northwest at about 12 mph, and was expected to continue on that path into Monday, then turn toward the Carolinas, possibly making landfall Thursday or Friday. Forecasters note that hurricanes can be unpredictable, and long-range forecasts have large possibilities for error.

South Carolina went on an elevated alert status Friday.

In coastal Georgia, the Chatham County Emergency Management Agency urged residents to review their hurricane plans.

"It's still a long ways away, but we have to prepare as if it's coming here," said agency director Phillip Webber.

In Washington, D.C., emergency officials were working on acquiring additional sandbags, and planned to meet with other department and critical services leaders Monday, said Peter LaPorte, director of the Emergency Management Agency.

"We're going to take a proactive approach Monday into Tuesday," LaPorte said. "One, a public education campaign and two, take some steps with some of the government services.

"Then we're going to pray."

The last Category 5 Atlantic hurricane was Mitch in 1998, which killed about 11,000 people in Central America.

The last two Category 5 hurricanes to strike the United States were Andrew in 1992 and Camille in 1969.

The Atlantic hurricane season began June 1 and ends Nov. 30.

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On the Net:

National Hurricane Center: http://www.nhc.noaa.gov