Cedar Key, Fla. ? More than 20,000 people along Florida’s Gulf Coast were ordered to clear out Monday as Alberto – the first tropical storm of the 2006 Atlantic hurricane season – unexpectedly picked up steam and threatened to come ashore as a hurricane.

Forecasters posted a hurricane warning for the Gulf Coast and a tropical storm warning from north of Daytona Beach to the Georgia-South Carolina line. Alberto, which could begin battering the Gulf coast early Tuesday, was expected to cross through Florida and into Georgia.

Gov. Jeb Bush signed a declaration of emergency allowing him to call up the National Guard and put laws against price gouging into effect.

“We’re talking about powerful forces of nature,” Bush said. “People need to take this very seriously.”

If Alberto came ashore as a hurricane, it would be the earliest hurricane in 40 years to hit the United States, according to the National Hurricane Center. The earliest on record is Alma, which in 1966 struck the Florida Panhandle on June 9 – the ninth day of the hurricane season.

Alberto started as a tropical depression on Saturday, and forecasters over the weekend were confident it would not become a hurricane. But the storm’s winds accelerated with startling speed from 50 mph to 70 mph in just three hours Monday morning. The minimum for a hurricane is 74 mph.

“We were surprised, but we’ve been surprised before,” said Richard Pasch of the hurricane center. “The center in disorganized storms can re-form and jump.”

Florida homeowners stocked up on chain saws, plywood and other emergency supplies. Employees at a marina in St. Petersburg said they planned to work through the night securing more than 600 boats against the wind and waves.

The Federal Emergency Management Agency said Monday it had evacuation buses and emergency supplies standing by, but state officials in the affected areas had not asked for immediate help.

Forecasters said Alberto would probably become only a weak Category 1 hurricane, meaning winds of 74 mph to 95 mph, because the warm water from which hurricanes draw their strength is not particularly deep in the area.

At 11 p.m. EDT, Alberto was centered about 95 miles south-southeast of Apalachicola and about 105 miles west-southwest of Cedar Key, and was moving northeast near 10 mph, the National Hurricane Center said.