At 5 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 350 miles 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.
"This is about as strong as they get," forecaster Richard Knabb at the National Hurricane Center told CBS Radio News.
The storm's five-day forecast placed it roughly 350 miles east of Vero Beach by early Wednesday, provided it makes a predicted turn to the northwest. Forecasters said they would know more about the potential direction of the storm late this weekend.
"The big question continues to be what will happen beyond the five-day forecast period," National Hurricane Center forecaster Richard Pasch said. "It is still impossible to state with any confidence whether a specific area along the U.S. coast will be impacted by Isabel."
Isabel's wind speeds were forecast to fluctuate over the next five days, but warm ocean temperatures and other conditions have allowed it to consistently intensify, forecasters said.
The last hurricane to develop into a Category 5 in the Atlantic was Mitch, which killed about 11,000 people in Nicaragua, Honduras and Guatemala in 1998.
The last two Category 5 hurricanes to strike the U.S. coast were Andrew in 1992 and Camille in 1969.
Andrew destroyed much of Homestead and southern Miami-Dade County before slamming into Louisiana. Forty-three deaths were blamed on Andrew in the United States. It destroyed 126,000 homes and remains the most expensive natural disaster in U.S. history with a $30 billion damage toll.
Camille hit the Mississippi coast with 200 mph winds and moved north. About 300 people were killed from Mississippi to Virginia.
At 11 a.m., Isabel was about 370 miles northeast of the northern Leeward Islands and was moving west at 9 mph. Forecasters predicted the storm would track just north of due west over the next few days. Hurricane hunter aircraft were to fly through Isabel later Friday.
At least three cruise ships had changed their sailing routes by Friday to avoid Isabel's path.
Forecasters said the storm could spawn large ocean swells creating hazardous surf conditions in the Leeward Islands, the Virgin Islands and Puerto Rico over the next few days.
The Atlantic hurricane season began June 1 and ends Nov. 30.