The National Hurricane Center upgraded Hurricane Rita Tuesday afternoon to a Category 2 storm as it was moving south of the Florida Keys and projected that it could intensify to a Category 4 after it heads into the Gulf of Mexico toward the region devastated by Katrina.

At 5 p.m., the center said that Rita was about 50 miles south-southwest of Key West, moving west at about 15 mph and had sustained winds of nearly 100 mph. The core of the storm, forecasters said, will probably miss the Keys and pass over the Florida straits, between the Keys and Cuba, but strong winds are still expected to affect portions of the chain of islands. Isolated tornado warnings were issued.

Once Rita is beyond the Keys, the National Weather Service said the vulnerable areas would be the southern shores of Mississippi and Louisiana and the southeast coast of Texas, including Houston and Galveston.

Ed Rappaport, a spokesman for the National Weather Service, said in an interview on CNN that there was less than a 5 percent chance that Rita would hit New Orleans, but the area could get heavy rainfall that might impact the flooding still left from Katrina and the levees that were weakened in that storm. Rappaport stressed, however, that forecasters have difficulty with predictions about hurricanes beyond several days and that projections made Tuesday could be significantly altered as the storm moves on. A Category 4 storm has sustained winds of up to 131 mph.

New Orleans Mayor Ray Nagins urged anyone still left in the city to get out before Rita hit the area. He estimated that 500 to 600 people are still there and said two buses had left the Convention Center Tuesday with evacuees.

"The protection [from the levees] is very tenuous at best," said Dave Wurtzel, the Army Corps official responsible for repairing the 17th Street Canal levee, whose huge breach during Katrina three weeks ago caused the worst of the floods that wrecked the city, the Associated Press reported.

President Bush, on a tour of Gulf Coast region Tuesday, said, "We're watching very closely, of course, its track. All up and down the coastline people are now preparing for what is anticipated to be yet another significant storm."

In Texas, Gov. Rick Perry (R) recalled all emergency personnel helping with recovery from Hurricane Katrina to prepare for Rita, including almost 1,200 Texas National Guard members, wire services reported.

"The time is now to begin mobilizing our resources and implementing our plan to ensure an orderly response before Texas is hit," Perry said.

About 1,100 evacuees from Katrina who were still living in Houston's two largest shelters, Reliant Arena and the George R. Brown Convention Center, were being sent to Fort Chaffee, Ark., because Houston officials said the shelters might not hold up in a major hurricane, the AP reported.

"A lot of people didn't want to go," said Wayne Sylvester, who was wearing a T-shirt that proclaimed: "I Survived Katrina." "It looks like the storm is following me."

Arkansas Gov. Mike Huckabee (R) said emergency officials told him to be prepared to take in evacuees from Texas because of Rita.

In Louisiana, military officials prepared to move ships anchored near New Orleans for relief purposes out to sea to avoid damage to them when Rita arrives, possibly Thursday or Friday. The Pentagon said that the USS Tortuga, the USNS Comfort and the USS Iwo Jima would move eastward to avoid Rita.

In Key West, the streets of this city were largely deserted in the afternoon and power was out, except for those businesses and homes with generators. One bar was open and serving patrons, and several hotels had remained open for local residents. Tourists were ordered evacuated over the weekend. Hurricane gusts occasionally swept across the island.

The hurricane center predicted that the storm could bring 6 to 8 inches of rain and a 4- to 6-foot storm surge, along with "dangerous battering waves" in the Keys. On the mainland, it forecast flooding of 2 to 4 feet along the far southeastern Florida coast.

Parts of U.S. 1, the major highway through the Keys, has been closed by flooding and debris.

Rita began the morning as a tropical storm, but weather service officials upgraded it to a hurricane mid-morning.

While the Keys have been closed to tourists, there were reports that about 80,000 residents that had remained in the area to stay put in the hopes of riding out Rita's fury safely.

"If you've not left the Keys already," Gov. Jeb Bush (R) said, "don't leave now," as winds and rain were already picking up to dangerous levels, with branches and some debris blowing across highways.

"This is a very serious storm that is about to hit our state," he said, noting that Rita's storm bands extend about 120 miles outward from the storm's center.

Bush said shelters had been set up across the affected area and that at least three hospitals have been evacuated. He said he had mobilized 2,400 National Guardsmen with another 2,500 placed on alert.

Search and rescue units, he said, are also being mobilized by the state.

Miami-Dade Mayor Carlos Alvarez also cautioned southern Florida not to dismiss the power of the coming storm, which sent intermittent squalls over the Miami area as the main core neared the Keys.

The federal government, stung by criticism of its slow reaction to Katrina, moved quickly to set up a military staging area at Homestead Air Reserve Base south of Miami for distribution of food, water and ice, the Pentagon said. The Pentagon also said it planned to reposition some of its ships and helicopters with the intent of "following behind" Rita in order to support possible relief efforts.

Fred Barbash reported from Washington.