Mayor C. Ray Nagin said Monday that residents should be ready to evacuate if Tropical Storm Wilma strengthens and moves closer. He cautioned that levees would not protect the city from much more than a weak hurricane.

The storm's outer edge was nearing the Cayman Islands on Monday, but forecasters said it could pose a threat to the southern United States this weekend.

Nagin said pumping stations were operating at about 60 percent capacity and levees had been rebuilt to at least a 10-foot barrier level. He said that could provide protection only from a Category 1 or Category 2 storm.

"The people that are moving back to New Orleans should be very mobile," Nagin said. "At a moment's notice, people should be ready to evacuate."

Nagin said that buses were staged should an evacuation be necessary, and that authorities would help ensure that people were not trapped if Wilma posed a threat.

Wilma became the Atlantic hurricane season's 21st named storm before dawn, tying the record set in 1933.

It could strengthen into the year's 12th hurricane by Tuesday. That many hurricanes formed in 1969, the most since record keeping began in 1851.

"I think the message is that the season is certainly not over. People in the Gulf Coast are going to have to watch Wilma," said Max Mayfield, director of the National Hurricane Center in Miami.

Computer models show the storm heading for Mexico's Yucatan Peninsula by Friday and then turning sharply in the Gulf of Mexico toward Florida over the weekend. Those models have large margins of error this far in advance, however, and so anywhere in the Gulf could be in danger of being hit.

"There's no scenario now that takes it toward Louisiana or Mississippi, but that could change," Mayfield said.

A man walks through floodwaters in Kingston, Jamaica, after Tropical Storm Wilma hit -- the season's 21st named storm.