KEY WEST, Fla. — Florida Keys officials closed schools, opened shelters, and urged visitors to leave as Tropical Storm Fay threatened to strengthen into a hurricane yesterday, but residents and tourists seemed in no hurry to evacuate.

Traffic remained light leaving Key West and the Lower Keys yesterday afternoon as the sky darkened with storm clouds and the National Weather Service issued watches and warnings.

“We’ve seen worse than this in Omaha,” Diego Sainz, who was visiting from Nebraska with his wife and friends, said. They had intended to leave yesterday but couldn’t get a flight out.

Authorities said traffic was becoming heavy in the Upper Keys, where the 110-mile, mostly two-lane highway that runs through the island chain meets the mainland. The Florida Highway Patrol sent in extra troopers to assist and tolls were suspended on parts of the northbound turnpike.

Fay could start pelting parts of the Keys and South Florida late today or early tomorrow as a strong tropical storm or minimal hurricane. Aside from wind damage, most of the islands sit at sea level and could face some limited flooding from Fay’s storm surge.

Officials in the Keys and elsewhere planned to open shelters and encouraged or ordered people who live in low-lying areas and on boats to evacuate. Schools in the Keys will be closed today and tomorrow.

Keys officials earlier yesterday had issued a mandatory evacuation order for visitors and asked those who had not yet arrived to postpone their trips. Officials said hotels and businesses won’t be forced to remove visitors, but they should use common sense.

Fay, the sixth storm of the 2008 Atlantic season, picked up some momentum yesterday afternoon as it headed toward Cuba, and could be a hurricane by the time it reaches the island’s center, forecasters said. Fay has already killed at least five people after battering Haiti and the Dominican Republic with weekend torrential rains and floods.

At 5 p.m. EDT yesterday, Fay’s center was located about 270 miles south-southeast of Key West and moving west-northwest near 15 mph. The storm had maximum sustained winds near 50 mph with some gusting.

Forecasters yesterday afternoon shifted its track a little more westward, but the Keys could still be affected. Fay was still forecast to move up the western coast of Florida, but could stay over open water longer, a hurricane support meteorologist, Corey Walton, said. Fay likely won’t traverse as much of the Florida peninsula as initially thought, but the state will be affected by its winds.