Florida braced today for the onslaught of its third hurricane in a month, as one of the most powerful storms in recent years, dubbed Ivan, bore down on Jamaica on a path that could bring it over the Florida Keys as soon as Sunday.

With the National Weather Service describing Ivan as "extremely dangerous" and capable of causing "catastrophic damage," a mandatory evacuation order was issued today for "all visitors and non-residents" of the Florida Keys and "all recreational vehicles" there.

A statement issued by the service's National Hurricane Center said a mandatory evacuation may be required for all residents in the Keys and Flamingo, Fla., Friday morning, and that residents who wish to leave today "are encouraged to do so."

Many tourists heeded the warning. Cars laden with summer vacation gear streamed north on the spindly, two-lane road that connects Key West with mainland Florida after the evacuation order was issued. Even many storm-hardened locals in Key West, renowned for their nonchalance about hurricanes and their penchant for all-night parties when big winds are on the way, were making plans to flee.

"The general consensus here is everyone just has a really bad feeling about this one; we are all sick to our stomachs," said Sabrina Johnson, who operates the tourist stalwart World Famous Conch Train. "Most of the people I know didn't evacuate for Charley but are taking this one very seriously." She said many people were stocking up on gasoline out of concern that they might not find any after making the 150-mile road trip up to Homestead.

"There isn't a gas can to be bought in town now," she said.

The holdouts, and there are sure to be some, will not have a government-sanctioned place to go. Monroe County officials said they will not open any shelters in the Keys before the storm.

The mandatory evacuation order affects thousands of visitors to the 120-mile long island chain. People in mobile homes were being asked to leave by 6 p.m. today, and Monroe County authorities were expecting to order all 79,000 residents off the islands on Friday.

The warnings came as Ivan's winds strengthened to 160 mph, making it a Category 5 hurricane, the highest classification on a scale used to estimate potential property damage.

The most powerful hurricane to hit the Caribbean in a decade, Ivan killed at least 20 people when it slammed into the tiny island of Grenada Tuesday, destroying an estimated 90 percent of the island's homes. An old prison was among the buildings destroyed, and all the inmates escaped. Also ruined were the residence of Prime Minister Keith Mitchell, Grenada's emergency operations center and many schools.

"We have got a tremendous hit that we never expected," Mitchell told BBC Radio. "You are talking hundreds of millions of dollars of damage. We have declared the country a national disaster." He said the island state's nutmeg industry, a vital sector of the economy, has apparently suffered extensive damage.

After ravaging Grenada, Ivan moved in the direction of Jamaica, which issued a hurricane warning. Several other governments issued hurricane watches, including those of Haiti, the Dominican Republic, the Cayman Islands and Cuba.

The National Hurricane Center said Ivan was moving west-northwest at 15 mph and would approach Jamaica Friday if it maintains its present track.

"Ivan is an extremely dangerous Category Five hurricane . . . with maximum sustained winds near 160 mph," the center said in an advisory. It said hurricane-force winds extend 60 miles from the storm's eye and tropical storm-force winds reach outward up to 160 miles.

"Storm surge flooding of 3 to 5 feet above normal tide levels, along with large and dangerous battering waves, can be expected near the center of Ivan in the hurricane warning area," it said. It forecast rainfall of five to seven inches, "possibly causing life-threatening flash floods and mud slides," along the path of the hurricane. It said Ivan was expected to fluctuate between a Category 5 and a Category 4 hurricane until reaching Cuba.

On the Saffir-Simpson Hurricane Scale, a Category 5 is defined in part as having winds greater than 155 mph. A Category 4 has winds ranging between 131 and 155 mph.

The approach of Ivan comes on the heels of two hurricanes -- Charley and Frances -- that caused at least 42 deaths in Florida combined and left damage estimated to range between $9 billion and $11 billion for insured property.

Charley struck the southwestern coast of Florida Aug. 13, battering communities with 145-mph winds. Frances made landfall early Sunday on the state's eastern coast with slower winds -- 105 mph -- but with a wider area of coverage and a slower march across the landscape.

Florida Gov. Jeb Bush (R) said that even though it was hard to believe his state could be hit again so soon, it had no choice but to prepare for another hurricane.

"People are at wit's end that have been impacted by this, and the people we're asking to respond have some challenges of their own,'' Bush told reporters. "Maybe someone creative in Hollywood could come up with something like this, but this is past my imagination.''

As Florida prepared for Ivan, the remnants of Frances were still causing damage in other states.

Tens of thousands of people in North Carolina were left without drinking water after storms washed out water lines and sanitation systems in the western part of the state.

In Ohio, Frances was blamed for two deaths, including that of a 9-year-old girl who drowned Wednesday when she tried to cross a rain-swollen stream and was swept off a footbridge, AP reported.

Anxiety about Ivan was widespread in Florida. Shutters and plywood, put up to fend off Hurricane Frances, remained on homes from the Keys to the Orlando area. Long gas lines were already forming early this afternoon in Miami, as residents who have spent weeks listening to reports about gas shortages caused by Frances and Charley stocked up.

Long-range forecasting models did not agree about Ivan's path today, adding to the sense of anxiousness. Some projected the storm to spin toward the state's east coast, others to its southern tip, and still others showed it headed to the west coast, placing it on a track to revisit towns battered by Charley, such as Punta Gorda and Port Charlotte.

"We're waiting on nature to tell us which way to go," said Mike Stone of the Florida division of emergency management.

The numbers were disturbing: Hurricane Ivan has kicked up gusts as strong as 195 mph. Florida's state-of-the-art emergency evacuation center, the sturdiest in the state, is built to withstand 200-mph winds.

The center, which has now been staffed for 28 straight days, already has more than 2,000 state and federal workers spread over the state working on Charley and Frances relief efforts. Thousands of others are on alert.

The state is still grappling with major power outages spawned by Frances. More than 1 million customers were without power this afternoon, with the largest outages in Palm Beach County and communities to the north.

Roig-Franzia reported from Miami Beach. Special correspondent Catharine Skipp in South Miami also contributed.