Thousands of tourists jammed the highways Sunday after they were told to evacuate the lower Florida Keys because Tropical Storm Rita (search) developed over the Bahamas and moved toward the vulnerable, low-lying island chain.

A hurricane watch was posted for the entire Florida Keys, which means hurricane conditions of winds of at least 74 mph are possible by late Monday.

In Tallahassee, Gov. Jeb Bush (search) declared a state of emergency for Florida, which gives the state authority to oversee evacuations and activate the National Guard, among other powers.

"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 (search).

Click here to track Tropical Storm Rita.

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 (search) could potentially be in the storm's path.

"Once it reaches the Gulf, really everybody should pay attention at that point," Brown said.

Rita is the 17th named storm of the Atlantic hurricane season. That makes this season the fourth busiest since record keeping began in 1851 — 21 tropical storms formed in 1933, 19 developed in 1995 and 1887 and 18 formed in 1969, according to the hurricane center.

At 5 p.m. EDT, Rita had top sustained winds of about 40 mph, just above the 39 mph threshold for a tropical storm. It was centered about 355 miles east-southeast of Nassau, Bahamas, and about 600 miles east-southeast of Key West. It was moving to the west near 10 mph.

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.

Michael Knowles, general manager of the Hampton Inn Resort in Key West, told his guests to leave Sunday but said he wasn't too worried about the latest weather system because it wasn't as strong as Katrina.

"These are like rehearsal drills for us," Knowles said. "I'm originally from the Bahamas so this is like my 25th hurricane."

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

Katrina hit the Miami-Fort Lauderdale area last month as a Category 1 storm, killing 14 people and flooding some suburban and farming areas for days. The hurricane only caused minor damage in the Keys before destroying parts of Louisiana and Mississippi as a much stronger Category 4.

Hurricane Dennis (search) 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.

Peak storm activity typically occurs from the end of August through mid-September. The season began June 1 and ends Nov. 30.

Farther out in the Atlantic, Tropical Storm Philippe (search) formed late Saturday well east of the Lesser Antilles. At 5 p.m., Philippe had maximum sustained winds near 50 mph, up 10 mph from Saturday. It was centered about 400 miles east of the Leeward Islands and was moving to the north-northwest near 8 mph.

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

The previous storm, Ophelia, was headed to the north Atlantic Sunday after striking North Carolina and brushing Massachusetts and Nova Scotia.