KEY WEST, Fla. — Tropical Storm Fay could strengthen to a hurricane and start pelting parts of the Florida Keys and south Florida as soon as Monday, forecasters said.

The National Hurricane Center in Miami said a hurricane watch was in effect for the Florida Keys from south of Ocean Reef to Key West, and along the mainland from Card Sound Bridge west to Bonita Beach.

The sixth storm of the 2008 Atlantic season picked up some momentum early Sunday morning as it headed toward Cuba, and could be a hurricane by the time it reaches the island's center, forecasters said.

Florida Gov. Charlie Crist had declared a state of emergency Saturday because Fay "threatens the state of Florida with a major disaster," he wrote in an executive order.

Officials in the Keys' Monroe County said they would likely order an evacuation of all visitors on Sunday morning. A decision on whether to require all residents to evacuate was to come, county spokeswoman Becky Herrin said.

Keys emergency officials often take the precaution of ordering early evacuations when a storm threatens, because traffic can back up for miles on the single highway to Florida's mainland.

Besides the threat of damage from high winds, most of the islands sit at sea level and could be flooded by Fay's storm surge. Flooding from the storm on Saturday killed four people in the Dominican Republic and Haiti.

At 11a.m. EDT Sunday, Fay's center was located about 75 miles southwest of Camaguey, Cuba and 350 miles southeast of Key West and moving west-northwest at 13 miles per hour. The storm had maximum sustained winds at 51 mph with some gusting.

WRAL meteorologist Mike Maze said North Carolina can expect rain from the system. The amount will be determined by how the storm progresses in the next few days.

“We do expect rainfall by the end the week from this,” Maze said.

Fay could hit the U.S. as a Category 1 or 2 hurricane, with winds perhaps reaching more than 100 mph (160 kph). A Category 1 has winds from 74-95 mph, and a Category 2 has winds from 96-110 mph.