Nearly half a million people were ordered to evacuate as Hurricane Frances swirled toward Florida on Wednesday just weeks after Charley's devastating visit, threatening to deliver the most powerful one-two punch to hit the state in at least a century.

Those planning to ride out the storm stocked up on canned food, water and generators, while military helicopters and planes were flown out of the area and Cape Canaveral's Kennedy Space Center said it would close on Thursday.

Forecasters said the strengthening Category 4 storm could hit as early as Friday night, less than three weeks after Charley raked Florida's western coast with 145-mph wind, causing billions of dollars in damage and killing 27 people.

"I can't emphasize enough how powerful this is. If there's something out there that's going to weaken it, we haven't seen it," said Max Mayfield, director of the National Hurricane Center.

Gov. Jeb Bush (R) declared a state of emergency, activating the National Guard.

Many homes in southwest Florida still have blue tarps covering holes in their roofs after Charley, and some streets remain full of storm debris that could become wind-blown projectiles.

"We've just gone through 21/2 weeks of torture trying to get our lives back to some sense of order," said Punta Gorda retiree Tom Hamilton.

Evacuation orders were posted for 300,000 residents in coastal areas of Palm Beach County, and 192,500 were told to leave mobile homes and low-lying areas of Brevard and Martin counties. The evacuation orders were set to take effect Thursday afternoon.

Frances was about 700 miles southeast of Florida on Wednesday afternoon, heading northwest on a course that would take it to the central portion of Florida's eastern coast. Residents of Georgia, North Carolina and South Carolina watched the forecast closely in case the hurricane took a sharper turn to the north.

Bush cautioned that "all the science in the world and all the technology in the world isn't going to be able to pinpoint exactly where the storm goes."

Frances would be the fourth storm to affect Charleston, S.C., this summer. Bonnie and Charley arrived within days of each other in August, and Gaston dumped more than 13 inches of rain in some areas when it came ashore Sunday.

With the ground saturated from previous storms, more rain and the slightest wind could cut utility service for thousands, even if they do not take a direct hit from the storm.

"With the ground this wet, trees could easily topple," said Tom Williams, a spokesman for Duke Power in Charlotte.

The storm and the mass evacuations are sure to make a mess of Labor Day travel across the Southeast. Florida may reverse lanes on some highways to handle the evacuation traffic, said Craig Fugate, director of state emergency management.

If Frances hits, it could be the most potent two-hurricane combination to reach a single state in at least a century. Frances is as strong as Charley, but forecasters said it could become a Category 5 with winds of 156 mph or faster.

Hurricane-force winds extended about 80 miles from Frances's center, making it about twice the width of Charley and increasing the possibility for damage.

The last time two major hurricanes hit Florida in rapid succession was 1950. Hurricane Easy struck Tampa around Sept. 4 of that year, and Hurricane King hit Miami six weeks later on Oct. 17. They were Category 3 storms.

Residents planning to stay for Hurricane Frances stock up on supplies at the Home Depot store in Florida City, Fla. Hurricane Andrew damaged the area more than a decade ago.