GALVESTON, Texas Hurricane Rita expanded into a mammoth Category 5 storm and lumbered slowly Wednesday toward Texas, fortified by fierce 175-mph winds. Coastal highways clogged with cars and buses loaded with evacuees as more than 1 million residents were ordered to leave beach cities and low-lying sections of Houston.

The storm is "now the third most intense hurricane in the Atlantic Basin on record," the National Hurricane Center said Wednesday night. Early today, forecasters said the storm"s center was 540 miles east-southeast of Galveston and about 645 miles east-southeast of Corpus Christi, Texas, moving west at 9 mph toward the western Gulf Coast.

Drawing explosive energy from the warm surface water of the Gulf of Mexico, the storm quickly grew during the day from a potent Category 3 storm, with 130-mph gusts, into a maelstrom rarely recorded in modern annals of American weather. The storm was expected to make landfall Saturday, but the first probes of raking winds could be expected as early as Friday, meteorologists said.

Only three recorded Category 5 hurricanes have ever struck the continental U.S., and National Weather Service experts said that under current climate conditions, the storm could gather even more strength over the gulf. Hurricane Katrina was a Category 4 storm when it rampaged through the Gulf Coast on Aug. 29, causing extensive flooding that killed at least 1,036 and left hundreds missing.

"At this point, Rita has become a potentially very devastating Category 5 hurricane," said Chris Landsea, a National Hurricane Center meteorologist. "Fortunately it"s over the open ocean of the Gulf of Mexico, and will be over the open ocean for the next day. But we expect it to gradually make a turn to the north."

Frightened by Katrina"s example and stern warnings from authorities that Rita could land anywhere along the Texas-Louisiana coast, hundreds of thousands of residents streamed north from Galveston and surrounding coastal counties.

As 1.3 million of the region"s 5.2 million residents were ordered to uproot, Houston"s hospitals and nursing homes emptied and schools closed. Outbound highway traffic was snarled, and long lines formed at gas station pumps. Late Wednesday, the coastal city of Corpus Christi ordered its 277,000 residents to join Galveston"s nearly 60,000 residents in a mandatory evacuation.

President Bush declared federal emergencies in Texas and Louisiana as thousands of National Guard troops began moving out of harm"s way, preparing to wait in safe staging areas before redeploying to new disaster zones.

"We hope and pray that Hurricane Rita will not be a devastating storm, but we"ve got to be ready for the worst," Bush said as federal and military emergency officials marshaled resources for the new storm, pressing to improve on their hesitant preparations and sluggish response to Hurricane Katrina three weeks ago.

In Galveston"s port and elsewhere along the coast, oil refineries began shutting down in advance of Rita"s high winds and waves. In the gulf, operations ceased on oil rigs as crews were flown out by helicopter. At least 16 refineries dot the Texas coastline from Corpus Christi to Port Arthur, composing more than one-quarter of the nation"s capacity to make fuel.

Wholesale prices rose in energy markets as traders worried about new disruptions to oil and natural gas production after the devastation caused by Katrina. The cost of crude oil for delivery in October rose 60 cents to $66.80 on the New York Mercantile Exchange, and gasoline futures jumped 8 cents to $2.053 a gallon.

National Weather Service warnings that gulf tides would rise as much as 4 feet along the Mississippi and Louisiana coasts hastened preparations in the Katrina disaster zones, where thousands remained homeless and military teams still searched for corpses.

"It"s getting to be a large-sized hurricane," Landsea said. "If the storm hits Texas, some of the rain and the squall conditions will affect other areas as well and probably Louisiana."

In New Orleans, Army Corps of Engineers crews laid heavy steel barriers over two levees breached three weeks ago by a torrent of water from Lake Pontchartrain that flooded 80% of the city. Most of New Orleans was dry Wednesday, but after two brief days of revival, the city was fast becoming a ghost metropolis again, as residents evacuated in buses and troops prepared to move to safer areas to the north.

Louisiana officials said an estimated 9,700 residents of Cameron Parish on the border with Texas were told to leave their homes. At least 2,662 people housed in shelters since Katrina struck were relocated to facilities farther north in the state, and 5,054 more were scheduled to be moved, probably today.

In Houston, 50 miles north of the storm"s possible landfall point, NASA closed its Johnson Space Center and evacuated its staff, transferring control over the International Space Station and two space-bound astronauts to Russian partners.

Houston Mayor Bill White told hundreds of thousands of residents who lived near bayous and along low-lying flood plains to seek safety in shelters or move to higher ground. He also ordered mobile home park residents to evacuate, worried that the structures were too flimsy to withstand Rita"s punishing winds.

"We are asking all residents of Houston and the greater Houston area who are in the storm surge area to begin making their evacuation plans," White said.

Acknowledging that chartered buses might not be able to take everyone in need, White called on residents to act as good Samaritans and take in neighbors who had no means to evacuate.

"There will not be enough government vehicles to evacuate everyone," he said. "Citizens are the first line of defense."