Hurricane Isabel retained its 160 mph winds and its mystery Friday, as forecasters said it still was too early to tell if and where the potentially devastating storm will strike the United States.

Meanwhile, Beaufort County upgraded its warning level in preparation for a potential threat from the massive storm.

William Winn, county emergency management director, said the county elevated its warning level at about 2 p.m. Friday, meaning officials have located all essential personnel and notified other emergency agencies within the county that there is a potential threat.

"It's no more than an alert stage," Winn said.

The S.C. Emergency Management Division also upgraded the state's warning level as a precaution, according to its Web site, but the State Emergency Operations Center has not been activated.

At 11 p.m. EDT, Isabel's maximum wind speeds held steady at 160 mph, making it a Category 5 storm, the strongest. A hurricane hits the top of the scale when its winds reach 156 mph. It was about 320 miles north-northeast of the northern Leeward Islands in the northeastern Caribbean Sea and was moving west at 9 mph.

The long-range forecast placed it roughly 375 miles east of Cape Canaveral early Wednesday, if it makes a predicted turn to the northwest. But that forecast has a possible error of up to 432 miles, and meteorologists said they would know more about the potential direction of the storm late this weekend.

"It is still too early to even speculate which parts, if any, of (the) eastern coast of the United States may get affected by Isabel," said hurricane forecaster Jack Beven.

Wind speeds probably will fluctuate over the next five days, but warm ocean temperatures and other conditions have allowed it to intensify consistently, forecasters said.

Hilton Head Island Town Manager Steve Riley said Friday the town wasn't doing anything extra because of the county's upgrade. He said the town's departments were ready if they needed to prepare to evacuate.

"It's the lowest level of notification," Riley said. "It basically says there's a hurricane out there, and you better check your own preparation plans."

Winn stressed that the warning upgrade doesn't mean any operations will take place, but that county officials are ready in case of an emergency, such as evacuation.

The upgrade should encourage people to review their evacuation plans, he said.

Hilton Head emergency management coordinator Jay Harter said people need to have on hand nonperishable foods such as canned goods, batteries, battery-operated radios, a two-week supply of necessary medications and maps.

Harter also recommended that people review their evacuation routes, so they know where they want to go.

"Have a plan and start putting everything together," he said.

He said people need at least 2 gallons of water per person per day because they will need some for personal hygiene.

People evacuating with animals should make sure they have food, water and any needed medications for them as well, he said. Animals aren't allowed at public shelters, so pet owners need to plan ahead for where they will stay.

The approaching storm has prompted special measures for a Charleston bridge project.

Workers building the $632 million Ravenel Bridge linking Charleston and Mount Pleasant have started securing any material that could float or blow away and moving equipment to high ground.

If Isabel does come calling, all work on the bridge will stop two days before projected landfall. Cranes will be lowered, moved or locked down, said Bobby Clair, the state Department of Transportation engineer overseeing the project.

Florida residents have begun preparing for the worst by stocking up on supplies.

The approaching storm prompted somewhat higher-than-usual sales for items such as plywood, generators, gas cans and chain saws at Home Depot stores in state, said company spokesman Don Harrison. Sales of hurricane supplies at the chain's other East Coast stores were normal, he said.

The last Atlantic hurricane to develop into a Category 5 storm was Mitch in 1998, which moved into the Caribbean Sea and eventually killed about 11,000 and left thousands more missing in Nicaragua, Honduras and Guatemala.

The last two Category 5 hurricanes to strike the U.S. coast were Andrew in 1992 and Camille in 1969.

Andrew -- with a $30 billion damage toll, still the most expensive natural disaster in U.S. history -- tore through South Florida and Louisiana, causing 43 deaths and leaving 180,000 people homeless. Camille killed 143 on the Gulf Coast and 113 in Virginia floods.

Forecasters said Isabel could spawn large ocean swells creating hazardous surf conditions in the Leeward Islands, the Virgin Islands and Puerto Rico over the next few days. At least four cruise ships had changed their planned routes by Friday to avoid getting into Isabel's path.

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