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 Sunday, but residents and some tourists seemed in no hurry to evacuate.

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

Authorities said traffic was heavier 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 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.