The storm, still south of Cuba on Wednesday evening, could veer from Florida to the Yucatán peninsula in Mexico, but its current path is aimed at southern Louisiana and Mississippi, which bore the brunt of Hurricane Katrina. Because any evacuation would have to be declared well in advance, officials warned residents to stay alert.

“It’s still too early to tell exactly what it will do,” Jerry Sneed, the city’s director of emergency preparedness, said at a news briefing in City Hall. “Way too early to give a good timeline.”

Gustav was downgraded to a tropical storm after grazing Haiti as a Category 1 hurricane with winds of 90 miles per hour. The Associated Press reported that 15 people were killed on Haiti’s southern peninsula, while 8 died in a landslide in the neighboring Dominican Republic.

The storm was thought likely to intensify in the warm Gulf waters, with forecasters saying it could be as close as 300 miles from the mouth of the Mississippi River on Sunday afternoon as a Category 3 hurricane, with winds of 120 m.p.h.

At that strength officials here would order a mandatory evacuation of the city 60 hours before the predicted landfall. The highways would become one-way exit routes for people with cars. Those without cars would be transported by bus or train, officials said, unlike in the period before Hurricane Katrina, when the plan was to shelter those residents in the Superdome. That arena and the Convention Center, which became symbols of human misery after Katrina, will not be used as hurricane shelters.