A hurricane watch was posted for the entire Florida Keys.

"It does look like that there is the potential for it to become a hurricane, near or just before it reaches the Florida Keys," said Daniel Brown, a meteorologist at the National Hurricane Center.

The watch means that hurricane conditions with sustained wind of at least 74 mph are possible by late Monday, according to the Miami-based hurricane center.

Long-term forecasts show the system heading generally toward the west in the Gulf of Mexico toward Texas or Mexico later in the week, but such forecasts are subject to large errors. That means that areas ravaged by Hurricane Katrina could potentially be in the storm's path.

Monroe County emergency management officials told visitors to leave islands extending from the Dry Tortugas west of Key West to the Seven Mile Bridge near Marathon. The islands are connected to each other and the mainland by a single highway.

Hundreds of motorcyclists were in the Keys for an annual event, but most were leaving Sunday, senior emergency management director Billy Wagner said.

County officials also urged residents of mobile homes to prepare for possible evacuation.

Residents of South Florida and central and western Cuba should also closely monitor the system's progress, Brown said.

The 18th depression of the Atlantic hurricane season formed late Saturday east of the Turks and Caicos Islands late Saturday.

At 2 p.m. EDT, the depression had top sustained winds of about 35 mph. It was centered about 385 miles east-southeast of Nassau, Bahamas, and 600 miles east-southeast of Miami. It was moving to the west near 10 mph and was expected to continue that motion for at least 24 hours, taking it over the eastern and central Bahamas.

If its sustained wind speed climbs above 39 mph, the depression would be named Tropical Storm Rita.

Hurricane Dennis brushed by the Keys in July, flooding some Key West streets, toppling trees and knocking out power. Dennis then struck a direct blow on the Florida Panhandle.

Katrina and Dennis were among six hurricanes to hit Florida in the last 13 months. Last year's four hurricanes killed dozens of people and caused $19 billion in insured losses in Florida alone.

Farther out in the Atlantic, Tropical Storm Philippe formed late Saturday well east of the Lesser Antilles. At 11 a.m., Philippe had maximum sustained wind near 50 mph, up 10 mph from Saturday. It was centered about 425 miles east-southeast of the Leeward Islands and was moving to the north-northwest near 7 mph.