MIAMI, Florida (CNN) -- David Rubulcaba loads plywood into his truck in preparation for Hurricane Ike in Miami, Florida, on Friday.

1 of 3 "We're sorry to interrupt their vacations, but we need visitors to leave the Keys to ensure their safety," said Keys Mayor Mario Di Gennaro, who also chairs the islands' Tourism Council. "We do hope they will return and understand our concerns for their well-being."

A couple of hours after the orders were announced, Ike strengthened to an "extremely dangerous" Category 4 storm with winds near 135 mph, said the National Hurricane Center in Miami, Florida.

At 5 p.m. ET Saturday, Ike's center was 90 miles east of Grand Turk Island, moving west-southwest at 15 mph.

Florida emergency management officials began evacuating visitors from Key West and the rest of the Keys on Saturday and planned to help residents leave Sunday.

The threat is more immediate in the Turks and Caicos Islands, where forecasters expect the storm to strike late Saturday or early Sunday.

The British government arranged extra flights to move visitors to Turks and Caicos, a British crown colony, out of harm's way before the Providenciales airport was forced to close about noon. Watch a webcam report from Turks and Caicos »

"The flights look impossible at the moment," Bahamian Patrick Munroe told The Associated Press at the Providenciales airport.

"As I watched the weather forecast, it looks really, really serious, and I think it's going to be devastating," he told AP.

"I don't remember ever seeing a mass exodus like this," Providenciales resident Tracy Paradis told AP. She intended to fly to Seattle, Washington, with her 19-month-old twins and return after the hurricane.

Officials in Monroe County, Florida, which includes the Keys, prepared by closing or planning to close schools and state, officials said in a written statement.

There will be no commercial flights to Key West starting Sunday night, and the airport won't be reopened until the storm passes, Key West Airport Director Peter Horton said. The suspension of flights also applies to the airport at Marathon Key, an hour's drive from Key West, he said.

General aviation flights, including private and charter planes, will be allowed until noon Sunday, he said.

Ike is expected to start turning toward the northwest Monday, a path that would take the storm over the Keys and into the southeast Gulf of Mexico.

Florida Gov. Charlie Crist acknowledged that storm tracks are unpredictable, but he said Ike could be a serious threat by Tuesday. Watch Crist warn about 'a dangerous storm' »

"We continue to watch with much concern the track of Hurricane Ike," Crist said Saturday. "Ike has grown rapidly into a dangerous storm that continues to move ... toward Florida."

Crist declared a state of emergency Friday, allowing officials to get supplies in place, such as drinking water and ready-to-eat meals, near vulnerable areas.

Fran Chipley of Key West, Florida, said she planned to stay put. She manages the front desk at historic Chelsea House, a bed and breakfast on the waterfront.

"Our plans today are business as usual ... then we're going to see what happens tomorrow," she said. "I know there's a mandatory evacuation today, but we are allowing our guests to stay if they choose to, which I think I know they're going to do, because it's beautiful."

Ike "looks like it's going south, and we're hoping for that," Chipley added.

She said she lives in a home that is on higher ground and survived Hurricane Wilma in October 2005. Wilma hit southeast Florida, did extensive damage on the East Coast and killed nearly 20 people in Florida, Mexico and Haiti.

The Caribbean took a beating from Tropical Storm Hanna, and Ike promised to add to the misery.

"What we saw [with Hanna] was pretty bad," Turks and Caicos vacationer Jonathan Cohen, of Queens, New York, told AP. "So for it to be two, three times worse, well, it's time to get out of here."

Forecasters predicted storm-surge flooding of 9 to 12 feet above normal tides and large and dangerous waves when Ike hits. These can be expected near and north of Ike's center.

Rainfall of 4 to 8 inches, and up to 12 inches in some regions, is expected over Turks and Caicos and the southeastern Bahamas.

Forecasters expect Ike to remain strong as it passes over warm water in the Gulf of Mexico. Watch what Ike is doing »

That could mean another major hurricane destined to hit the U.S. Gulf Coast. Tracking maps indicate that Ike could hit anywhere from Texas to Alabama by Thursday