NAPLES, Fla. — In what has become an all-too-familiar drill, Floridians boarded up windows, gassed up their cars and bought storm supplies today. But this time they were looking at the most intense hurricane ever recorded in the Atlantic.

Hurricane Wilma exploded into a Category 5 monster with winds of 160 mph, and forecasters warned it could smash into southwestern Florida on Saturday with towering waves, and then work its way up the East Coast with devastating effect.

"I don't think I want to live in Florida," said Betty Bartelson, a Pennsylvania tourist visiting Marco Island. She planned to flee across the state to Fort Lauderdale.

Like Hurricanes Katrina and Rita earlier this season, Wilma was expected to weaken before coming ashore. But after seeing what those storms did — and after four storms hit Florida in quick succession last year — many people were taking no chances.

Officials began clearing tens of thousands of people out of the low-lying Florida Keys.

"We had well over a 1,000 lives lost in Katrina. If Wilma, you know, comes into the U.S., to the Florida coast as a Category 3 or 4 hurricane, that potential for large loss of life is with us," National Hurricane Center Director Max Mayfield said.

At one point today, Wilma's pressure dropped to 882 millibars, the lowest reading ever measured in an Atlantic basin hurricane. Typically, the lower the pressure, the faster air rushes into a storm.

By midafternoon, Wilma had weakened slightly, with its winds dropping from 175 mph, and its pressure rising to 900 millibars.

The storm's forward motion also appeared to be slowing somewhat, which could weaken the hurricane further and possibly delay Wilma's landfall until Saturday evening, Mayfield said.

Wilma was on a path that could threaten the areas hit last year by Hurricane Charley. Some houses and businesses in the area are still boarded up because of that storm.

The White House promised to stay on top of the situation, hoping to avoid a repeat of the slow initial response to Katrina. The Federal Emergency Management Agency was positioning emergency materials in Jacksonville, Lakeland and Homestead.

Gov. Jeb Bush said the state had ample supplies of food, water and ice ready for hard-hit areas.

Sean Mayo was filling up his sport utility vehicle's 26-gallon tank and a five-gallon gas can in Marco Island. "We don't know if there will be any shortages. I need to make sure I got enough gas to get to Lauderdale and back," he said.

Although Wilma was approaching from the west, forecasters warned that Atlantic Coast cities such as Miami, Fort Lauderdale and West Palm Beach could be hit by winds nearly as strong.

At 5 p.m. EDT, Wilma was centered about 285 miles southeast of Cozumel, Mexico, and about 465 miles south-southwest of Key West. It was moving west-northwest near 7 mph, forecasters said.

Mayfield said the Keys could be hit by storm surge up to 25 feet and battering waves ever higher than that.

"I just don't see how the Florida Keys will get out of this without having a major impact," he said.

Authorities told visitors to leave the Keys today and planned to order residents to get out on Thursday. The Keys were evacuated for Hurricanes Dennis and Rita earlier this year and four times last year.

"It is tough on the nerves," said Leon Dermer, owner of Happy Feather Gift Shop in Key Largo. He said every evacuation costs him about $10,000.

Associated Press writers Mitch Stacy in Sarasota and Mike Schneider in Bradenton contributed to this report.