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

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

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 Monday or early Tuesday 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 Monday and Tuesday.

Keys officials earlier Sunday 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 Sunday 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.

Some Key West businesses began putting up hurricane shutters Sunday, but tourists and residents still strolled lazily through town, where the weather alternated from sunny to occasional downpours with light wind gusts. By Sunday evening, it still looked like a normal summer day in the Keys.

In the Tampa Bay area, residents bought plywood, water, extra batteries, generators, and candles. Home Depot Manager Tony Quillen said his Pinellas Park store was sold out of water by 9 a.m., two hours after opening, but he expected another supply in the afternoon.

“People are playing in their head, considering what happened the last time,” Quillen said, referring to hurricanes including Charley in 2004, a Category 4 storm.

Key West was last seriously affected by a hurricane in 2005, when Category 3 Wilma sped past. The town escaped widespread wind damage, but a storm surge flooded hundreds of homes and some businesses. The deadliest storm to hit the island was a Category 4 hurricane in 1919 that killed up to 900 people, many of them offshore on ships that sank.

The Category 5 Labor Day Hurricane of 1935 passed over the middle Keys, killing more than 400 people, more than half of them World War I veterans living in rehabilitation camps.