Miami ? Nearly a half-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 a state in at least a century.

Those planning to ride out the storm snapped up 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 today.

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The National Hurricane Center issued a hurricane watch for about 280 miles of Florida’s Atlantic coast — from Florida City near the state’s southern tip to Flagler Beach about 20 miles north of Daytona Beach.

Forecasters said the still-strengthening Category 4 storm could hit as early as Friday night, just 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,” hurricane center director Max Mayfield said.

Many homes in southwest Florida still have blue tarps patching 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 2 1/2 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 nearly 200,000 were told to leave mobile homes and low-lying areas of Brevard, Martin and Indian River counties, which could be hit by tremendous ocean waves. The evacuation orders were set to take effect this afternoon.

Frances’ center Wednesday night was about 585 miles east-southeast of West Palm Beach and was moving west-northwest at about 14 mph. Residents of Georgia, North Carolina and South Carolina watched the forecast closely in case Frances took a sharper turn to the north.

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

Hurricane-force winds extended about 80 miles from Frances’ 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, smaller than Charley or Frances.

Customers wait in line at a Home Depot on Palm Beach Lakes, Fla., to purchase supplies in preparation for Hurricane Frances. Forecasters said the still-strengthening Category 4 storm could hit as early as Friday night.