The National Hurricane Center posted a hurricane watch for the southern tip of the Florida Peninsula, including the Keys and the Miami area, as drew closer and threatened to strengthen after traversing Cuba's rough terrain.
The watch — in effect south of Deerfield Beach, near Boca Raton, on the east coast and south of Chokoloskee on the west coast — meant hurricane conditions with sustained wind of at least 74 mph were possible within 36 hours.
The Florida Keys were put under a watch Sunday afternoon.
"We do expect it to reach the Gulf, maybe as a Category 1 hurricane, possibly a Category 2," said John Cangialosi, a meteorologist with U.S. National Hurricane Center in Miami. "It's difficult to say where it will be, but in three days we're projecting it anywhere from the eastern Gulf near the Florida panhandle to the western Bahamas."
Ernesto hit Cuba west of the U.S. naval air base at Guantanamo on Monday after killing one person in Haiti. Cuba ramped up emergency preparations before the fifth named storm of the Atlantic hurricane season moved ashore about 20 miles west of Guantanamo with top sustained winds dropping to near 40 mph. Click here for more on Ernesto's impact on Cuba and Haiti.
At 11 a.m. EDT, Ernesto was about 35 miles west of Guantanamo, moving northwest at 12 mph, and forecasters expected it to strengthen again once it reached the warm waters north of Cuba.
The hurricane center said Ernesto could become a hurricane again after it reaches the warm water north of Cuba. Forecasters said there was a 10 percent chance of hurricane-force wind striking South Florida and a 60 percent chance of tropical storm-force wind.
Experts urged Floridians to have enough supplies and plans in place to get them through 72 hours after a hurricane.
"My suggestion: Take this storm very seriously. A hurricane is a hurricane," Bush said from the state Emergency Operations Center in Tallahassee.
Floridians bought gas, water and other supplies and state officials ramped up emergency plans. The state has been hit by seven hurricanes and brushed by another in the past two years. There were scattered lines formed at gas stations, supermarkets and hardware stores.
Pedro Garcia, 38, and Alex Cabeza, 20, bought four six-gallon gas tanks and other supplies.
"We're getting a little more knowledge that it's gas that's the most important thing," said Cabeza, who worried the price would jump to $5 if a major storm it.
Visitors were ordered out of the Keys, where only one highway is available for evacuation, travel trailers and recreational vehicles were ordered out and residents of mobile homes were urged to evacuate.
Preparations were made to evacuate special-needs residents of the Keys to Miami.
At Cape Canaveral, NASA managers gave up on a Tuesday space shuttle launch and prepared to move Atlantis indoors if the storm continued to threaten.
Some residents of the low-lying Keys island chain flocked to grocery stores and home-supply warehouses to stock up on canned goods, bottled water and other storm essentials.
"Key Westers are used to this," said Jim Bernard, assistant manager at a Home Depot in Key West.
Dan Drum, 56, a contractor who has been living in Key West since 1977, was waiting when Home Depot opened early Monday. He said he was going to spend part of the day securing a property he owns in Sugarloaf Key.
"I just bought $55 worth of screws. That's all they had," he said.
At 8 a.m., the fifth named storm of the hurricane season had a top sustained wind speed of 45 mph, down from 75 mph Sunday. It was centered 20 miles west of Guantanamo, Cuba, and about 515 miles southeast of Key West. It was moving northwest at 12 mph.
"It has a good chance to regain hurricane status," said Max Mayfield, director of the National Hurricane Center.
Ernesto had been the first hurricane of the Atlantic storm season and was 1 mph above the minimum for a hurricane Sunday, but it weakened as it headed toward Cuba.
The storm battered Haiti and the Dominican Republic with heavy rain and wind on Sunday and killed at least one person.
Bush's state of emergency directed counties to activate emergency management offices and activated the National Guard, among other things. The governor canceled a trip to New York on Monday.
Florida's emergency management center in Tallahassee was partially activated Sunday, and several counties were expected to follow suit. People across the state were warned to monitor the storm and check their emergency supplies and disaster plans.
"Ernesto bears watching," said Mike Stone, spokesman for the Florida Division of Emergency Management.
The Associated Press contributed to this report.