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.
Since 1995, the Atlantic has been in a period of higher hurricane activity. Scientists say the cause of the increase is a rise in ocean temperatures and a decrease in the amount of disruptive vertical wind shear that rips hurricanes apart. Some researchers argue that global warming fueled by man's generation of greenhouse gases is the culprit.
Forecasters say the busy seasons are part of a natural cycle.
"Between the 70s and 80s, there was relatively light activity, and now in the 90s and 2000s, we've seen this increased cycle, so we're in it for the next 10-15 years at least," meteorologist Ron Goodman told CBS Radio News.
It's also difficult to know whether the Atlantic was even busier at any time before record keeping began in 1851. And satellites have only been tracking tropical weather since the 1960s, so some storms that just stayed at sea before then could have escaped notice.
The six-month hurricane season ends Nov. 30. Wilma is the last on the list of storm names for 2005; there are 21 names on the yearly list because the letters q, u, x, y and z are skipped.
"If we happen to have to go to another storm, we go to the Greek alphabet, so the next storm would be Alpha, and then Beta and Gamma," said CBS News meteorologist George Cullen.
That has never happened in roughly 60 years of regularly named Atlantic storms.
A tracking map on the hurricane center's wall already had Alpha, Beta, Gamma and Delta listed just in case they form.
Forecasters say they wouldn't be surprised if another storm formed this year, even though the official forecast only called for 21 named storms this year. Wind shear typically increases and sea temperatures usually fall toward the end of October in the Atlantic, hurricane specialist Richard Knabb said.
"But the western Caribbean is an especially favorable location for late season development because the water remains quite warm and the vertical shear often is not that strong down there, that far south. So conditions are still favorable down there sometimes in late October and into November," he said.