Tropical Storm Rita headed toward southern Florida last night on its way to the Gulf of Mexico, forecast to strengthen into a hurricane and to possibly reach the coasts of Texas and Louisiana by the weekend.

Rita is expected to pass through the Florida Keys today, before heading into the Gulf’s warm waters. It is forecast to reach Texas by the weekend, sweeping past the region devastated by Hurricane Katrina last month. The storm prompted the New Orleans mayor, Ray Nagin, to suspend plans for residents to re-enter the city and Galveston, Texas, to call for a voluntary evacuation.

“Our guidance points to a large, powerful hurricane in the Gulf of Mexico in the next few days,” said a National Hurricane Center meteorologist, Eric Blake. “We’re forecasting a Category 3 hurricane, but Category 4 is not out of the question.”

The hurricane center’s five-day projection has the center of the storm hitting Texas. Rita may, however, strike “anywhere from the Texas through Louisiana coast,” said another National Hurricane Center meteorologist, Michelle Mainelli.

“It’s still developing and its path could change,” Ms. Mainelli said.

New Orleans’s levees are weak and can’t handle more than 9 inches or a 3-foot storm surge, Mr. Nagin said at a press conference. He told residents of the Algiers neighborhood, who were allowed back for the first time today, to prepare to leave as early as tomorrow.

Meanwhile, President Bush warned yesterday that Mr. Nagin needed to heed the concerns of Coast Guard Vice Admiral Thad Allen about Rita and about fouled water supplies and general contamination in the city before allowing some 180,000 people back into New Orleans.

The city – once home to a half a million people – was flooded after Katrina’s storm surge overwhelmed the system of levees and pumps that held out the waters of Lake Pontchartrain and the Mississippi River.

Galveston, about 50 miles southeast of Houston on the Gulf Coast, will begin voluntary evacuations today at 2 p.m., unless the forecast changes.

“Rita could hit Galveston,” the city manager, Steve LeBlanc, said at a news conference. “Even though it’s expected to be Category 3, we’re planning for a Category 4. We’re going to move as many people off this island as we can.”

A fleet of 88 buses will start transporting residents who don’t have a way to leave the island, available beginning at 10 a.m. tomorrow. The buses should be able to remove 2,300 people before the storm forces a suspension of service, expected on Friday, Mr. LeBlanc said.

No other areas of Texas have called for an evacuation yet, said William Ayres, a spokesman for the Division of Emergency Management.

Katrina hit Florida on August 25 as a Category 1 hurricane. The storm passed into the Gulf and strengthened to a Category 5, with winds of as high as 175 mph over open water. It slammed ashore in Louisiana’s Plaquemines Parish with 140 mph winds, a Category 4 storm.

When Katrina moved into the Gulf after crossing Florida, the storm was expected to track northward and make landfall again in the state’s Panhandle.

Rita’s center was about 130 miles south-southeast of Nassau, in the central Bahamas, as of 5 p.m., according to the hurricane center. Rita is moving west-northwest at 14 mph and expected to gain power as the storm moves west, Mr. Blake said.

The current path puts the storm on course to threaten Texas’s oil facilities, including the refineries of Exxon Mobil Corp. and Royal Dutch Shell Plc.

Corpus Christi, which is about 125 miles from the border with Mexico, is home to two refineries operated by San Antonio-based Valero Energy and one each by Citgo Petroleum and Koch Industries.

Rita isn’t likely to hit eastern Louisiana near New Orleans, said David Tolleris, a meteorologist with Windsong Forecast, which provides weather forecasts to oil companies, energy traders, brokers, and utilities.

The storm probably will head west across “super, super warm” areas of the Gulf and hit Texas south of Corpus Christi, possibly as a Category 4 or 5 hurricane, he said.

Governor Bush of Florida declared a state of emergency yesterday for some areas of the state, said Mike Stone, a Florida Emergency Management Division spokesman. All 60,000 residents of the Keys have been ordered to evacuate, said Kristy Campbell of the emergency management division.

Rita might dump as much as 10 inches of rain. A hurricane warning, indicating such conditions are expected within a day, is in effect for the very southern Florida Peninsula, including parts of Miami-Dade County, as well as for the Florida Keys from Ocean Reef to the Dry Tortugas. A warning is also in effect for several Cuban provinces and parts of the northwest Bahamas.