Hurricane Wilma rapidly strengthened into one of the Americas' most intense storms ever and lashed Caribbean coastlines Wednesday, forcing tourists to flee as it threatened to slam into Cancun and southern Florida.

Wilma briefly grew into a monstrous Category 5 storm before weakening to a Category 4 Wednesday night. The storm forced thousands of people to evacuate low-lying areas in a 600-mile swath covering Cuba, Belize, Honduras, Jamaica, Haiti and the Cayman Islands, officials said.

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At least 13 deaths have been blamed on Wilma this week, including a man who drowned Wednesday while trying to cross a river that overflowed its banks in southern Haiti.

Forecasters said Wilma has the potential to make an extremely damaging impact in a season that has already seen devastation from Hurricanes Katrina and Rita. With its center still over open water, the storm's sustained winds were near 155 mph Wednesday night, down from 175 mph earlier in the day.

The National Hurricane Center Director Max Mayfield (search) said Wilma could reach the Florida Keys Saturday, possibly toward the evening. Visitors were ordered out of the Florida Keys even as schools closed.

The White House, stung by criticism that it had not responded quickly enough to Katrina, promised to stay on top of the situation. "We are closely monitoring what is an extremely dangerous storm," said White House spokesman Scott McClellan. "People should take this hurricane very seriously."

Tourists packed Cancun's airport even though skies were still partly sunny, looking for flights home or to other resorts. MTV (search) postponed its Video Music Awards Latin America ceremony, originally scheduled for Thursday at a seaside park south of the resort town.

Mark Carara cut his family's vacation short by two days and tried to get on a standby flight home to Colorado Springs, Colo. "You hear it was the biggest storm on record, and yeah, that was the clincher right there," he said. "It was time for us to go."

John Hyndman, a 59-year-old electrician from Ottawa said his hotel had asked guests to leave. "I think people are more panicked just about what a hurricane can do," he said. "It can be very scary."

Quintana Roo state, where Cancun is located, announced that hundreds of schools would be closed Thursday and Friday, and many will be prepared to serve as shelters for expected evacuations.

Floridians braced for the storm by boarding up windows and stocking up on supplies, although forecasters at the hurricane center said the forward motion of the storm appeared to be slowing, which could cause it to eventually weaken.

Predictions differed on the hurricane's path and how strong it would be when it reaches U.S. shores. Though some weakening was expected by Thursday, the "potential for large loss of life is with us," said Max Mayfield, director of the U.S. hurricane center.

"This is one of those cases where we have a tremendous amount of uncertainty," said Mayfield. Referring to Wilma's explosive two-day growth from a tropical storm to a Category 5 hurricane, Mayfield said "this is one of the most perplexing storms we have had to deal with" this year.

At 11 p.m. EDT, Wilma was centered about 235 miles southeast of Cozumel (search), Mexico. It was moving west-northwest near 8 mph, with some wobbles, forecasters said. Forecasters warned it could re-intensify Thursday as it turns to the northwest.

Wilma's record-level intensity was measured in its pressure. Confirmed pressure readings early Wednesday dropped to 882 millibars, the lowest minimum pressure ever measured in a hurricane in the Americas, but it later lost power and rose to 892 millibars, according to the hurricane center. Lower pressure translates into higher wind speed.

The strongest Atlantic storm on record, based on pressure readings, had been Hurricane Gilbert in 1988, which registered 888 millibars.

Wilma was on a curving course that would carry it through the narrow channel between Cuba and Mexico on Friday, possibly within a few miles of Cancun and Cozumel. Forecasters warned it could smash into southwestern Florida on Saturday with towering waves, then work its way up the East Coast with devastating effect.

Heavy rain, high winds and rough seas pounded coastal areas of Honduras, knocking out power to some towns, forcing the evacuation of coastal villages and closure of two Caribbean (search) ports.

Four fishermen were reported missing at sea and about 500 U.S. and European tourists were moved to safe locations at hotels on the Bay Islands.

The head of Haiti's civil protection agency, Maria Alta Jean-Baptiste, said a man drowned Wednesday while trying to cross a river that overflowed its banks in the southern town of Les Anglais. She said another man was swept away by the fast-moving current but survived.

The death raised to 12 the number of people killed in rain and landslides since Monday in the island nation. One man also died Sunday in a rain-swollen river.

Cuban authorities suspended classes in the threatened western province of Pinar del Rio and prepared to evacuate tourists from campgrounds and low-lying areas, according to Granma, the Communist Party daily. Heavy rains in the island's eastern province of Granma forced the evacuations of more than 1,000 people.

Forecasters said Wilma was stronger than the Labor Day hurricane that hit the Florida Keys in 1935, the most powerful Atlantic hurricane to make landfall on record.

But disruptive high-altitude winds in the Gulf of Mexico (search) should weaken Wilma before landfall, said Hugh Cobb, a meteorologist at the hurricane center.

Wilma's track could take it near Punta Gorda on Florida's southwestern Gulf Coast and other areas hit by Hurricane Charley, a Category 4 storm, in August 2004. The state has seen seven hurricanes hit or pass close by since then, causing more than $20 billion in damage and killing nearly 150 people.

Forecasters said Wilma should avoid the central Gulf coast ravaged by Katrina and Rita, which killed more than 1,200 people.

Wilma is the record-tying 12th hurricane of the Atlantic season, the same number reached in 1969. Records have been kept since 1851. On Monday, Wilma became the Atlantic hurricane season's 21st named storm, tying the record set in 1933 and exhausting the list of names for this year.

The six-month hurricane season ends Nov. 30. Any new storms would be named with letters from the Greek alphabet, starting with Alpha.