The storm's outer edge was near the Cayman Islands, but forecasts said it could pose a threat to the U.S. Gulf Coast this weekend.
"The people that are moving back to New Orleans should be very mobile. That's why we have not encouraged the repopulation of children nor senior citizens that are not very active," Mayor Ray Nagin said. "We're going to continue to monitor the storm and at a moment's notice people should be ready to evacuate."
After Hurricane Katrina devastated the city Aug. 29 and Hurricane Rita reflooded it weeks later, Nagin said levees had been rebuilt to at least a 10-foot barrier level. He said that could only provide protection from a hurricane with sustained winds up to 110 mph.
Nagin said about 150,000 to 200,000 people have been in New Orleans during the day, while only 60,000 to 75,000 people were believed to remain overnight.
The city had nearly half a million residents before Katrina left tens of thousands of homes uninhabitable and ruined much of the community's infrastructure.
Nagin said buses were prepared to evacuate residents if necessary.
CBS News correspondent Trish Regan reports that Wilma is expected to be a big hurricane — most likely a category 3 or higher — but early forecasts indicate
"We think high pressure will build in the east," said CBS News hurricane expert Brian Norcross. "That will push the hurricane up to the Gulf of Mexico … it's most likely to be a Florida hurricane."
Wilma is the 21st named storm of the season, tying the record for the most storms in an Atlantic season. The only other time that many storms formed since record keeping began 154 years ago was in 1933.
"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 National Hurricane Center Director Max Mayfield. The Atlantic hurricane season runs from June 1 to Nov. 30.
At 8 p.m. EDT, Wilma had sustained winds of about 50 mph, up 10 mph from earlier in the day. The storm was nearly stationary, centered about 265 miles south-southeast of Grand Cayman, but was expected to turn west.
A hurricane watch was issued for the Cayman Islands, meaning hurricane conditions could be felt there within 36 hours. The storm was expected to bring 2 to 6 inches of rain in the Caymans, Cuba, Haiti, Honduras and Jamaica, with as much as 12 inches possible in some areas.
"We're on alert but we're not panicking," said Tootie Eldemire, owner of the Eldemire Guest House on Grand Cayman.