Ike was steering almost directly for Houston, where gleaming skyscrapers, the nation's biggest refinery and NASA's Johnson Space Center lie in areas vulnerable to wind and floodwaters. Forecasters said the storm was likely to come ashore as a Category 3, with winds up to 130 mph.
But the storm was so big, it could inflict a punishing blow even in those areas that do not get a direct hit. Forecasters warned that because of Ike's size and the state's shallow coastal waters, it could produce a surge, or wall of water, 20 feet high, and waves of perhaps 50 feet. It could also dump 10 inches or more of rain.
"It's a big storm," Texas Gov. Rick Perry said. "I cannot overemphasize the danger that is facing us. It's going to do some substantial damage. It's going to knock out power. It's going to cause massive flooding."
Perhaps the sternest warning came from the National Weather Service for residents along a Gulf-facing stretch of Galveston Island and neighboring Bolivar Peninsula, which are both under mandatory evacuation orders. People ignoring the orders in single-family one- or two-story homes "will face certain death," read the statement Thursday from the local weather forecast office.
Hurricane warnings were in effect over a 400-mile stretch of coastline from south of Corpus Christi to Morgan City, La. Tropical storm warnings extended south almost to the Mexican border and east to the Mississippi-Alabama line, including New Orleans.
In Surfside Beach, a coastal community about 40 miles south of Galveston, the police chief was so worried that the entire force planned to ride out the storm inland.
"I don't have a crystal ball, but if I did, I think it would tell me a sad story. And that story would be that were faced with devastation of a catastrophic range," said Chief Randy Smith. "I think we're going to see a storm like most of us haven't seen."
Most of the evacuations were limited to sections of Harris County outside Houston, as well as nearby bayous and Galveston Bay. But the 2 million residents of the city itself and 1 million in other areas of the county were asked to remain at home.
"We are still saying: Please shelter in place, or to use the Texas expression, hunker down," said Harris County Judge Ed Emmett, the county's chief administrator. "For the vast majority of people who live in our area, stay where you are. The winds will blow and they'll howl and we'll get a lot of rain, but if you lose power and need to leave, you can do that later."
Authorities hoped to avoid the panic of three years ago, when evacuations ordered in advance of Hurricane Rita sent millions scurrying in fright and caused a monumental traffic jam so big that cars ran out of gas or overheated. Ultimately, the evacuation proved deadlier than the storm itself. A total of 110 people died during the exodus, including 23 nursing home patients whose bus burst into flames while stuck in traffic.
This time, traffic was bumper-to-bumper on the freeway leading away from Galveston immediately after the evacuation order, but by late afternoon, many evacuees had made it past Houston, to the north. And just in time: Waves were already inundating the beach on one end of Galveston Island.
Some gas stations began running out of fuel, but fuel trucks were called in to replenish them.
Houston Mayor Bill White said one of the lessons of the Rita mess was that too many people fled who didn't need to. Instead, he asked residents to protect their homes.
"Think how your barbecue could become a flying object," he said.
At 11 p.m. EDT, the storm was centered about 340 miles southeast of Galveston, moving to the west-northwest at 12 mph. Top sustained winds were 100 mph.
NASA closed the Johnson Space Center, including Mission Control, and set up temporary quarters Thursday near Austin and Huntsville, Ala., to watch over the international space station until the storm threat passes. Most NASA aircraft at Ellington Field, just north of Johnson, have been flown to a facility in El Paso.
The oil and gas industry was closely watching the storm because it was headed straight for the nation's biggest complex of refineries and petrochemical plants. The upper Texas coast accounts for one-fifth of U.S. refining capacity.
Wholesale gasoline prices spiked 30 percent Thursday, or nearly $1 a gallon, out of fear of what Ike might do. That means motorists can expect higher prices at the pump, though how much higher depends largely on how long refineries are shuttered after the storm.
Exxon Mobil Corp., Valero Energy Corp., ConocoPhillips and Marathon Oil Co. began halting operations as Ike closed in. Dow Chemical Co. started closing up its enormous Freeport complex, home to 75 plants producing some 27 billion pounds of chemical products each year.
BASF, the world's largest chemical company with 14 manufacturing sites in the Gulf Coast region, also began shutting down some operations. Spokesman Daniel Pepitone said each site has a hurricane plan that outlines detailed steps for securing plants, and precautions such as tying down hoses and taking down scaffolding began days ago.
Industry officials said their refineries and chemical plants are designed to withstand high winds. But power outages could still knock them out of service.
Ike would be the first major hurricane to hit a U.S. metropolitan area since Katrina devastated New Orleans three years ago. For Houston, it would be the first major hurricane since Alicia in August 1983 came ashore on Galveston Island, killing 21 people and causing $2 billion in damage.
Ike is huge, taking up nearly 40 percent of the Gulf. The National Hurricane Center said tropical storm-force winds of at least 39 mph extended across more than 550 miles, and hurricane-force winds of at least 74 mph stretched for 230 miles. A typical storm has tropical storm-force winds stretching only 300 miles.
Because of its great size, storm surge and gigantic waves are the biggest risk, said Hugh Willoughby, former director of the federal government's hurricane research division. The larger the storm, the longer it hits and the higher waves can build.
And because the water is so shallow along the Texas coast, the waves pile up, creating a big storm surge, he said.
"We're not talking about gently rising water," Harris County's Emmett said. "We're talking about a surge that will come into your homes."
Authorities put the frail and poor on buses headed for shelters. And thousands of Texas prison inmates were also moved out of the storm's path.
Officials worried that after Labor Day's Hurricane Gustav proved to be a dud in Texas, people wouldn't take the warnings seriously. "The most important message I can send is do not take this storm lightly," Homeland Security Secretary Michael Chertoff said. "Do not look back at Gustav and say, `Well, that turned out to be not as bad as some people feared, therefore, I'm going to gamble with this storm."'
Some stayed put anyway.
Johnny Tyson, 33, his girlfriend, Martha Jones, 38, and her three children planned to ignore the order to leave. Tyson, loading into his truck plywood he bought at a Home Depot in Beaumont, complained that officials waited too long to call for an evacuation.
"We left for Gustav and we didn't have to leave," Jones said. "They cut all the roads and bottleneck everybody into one road and make traffic worse." He added: "Everybody and their momma is trying to leave right now."
Associated Press Writers Monica Rhor in Houston, Juan A. Lozano in Beaumont, Kelley Shannon in Austin and Seth Borenstein in Washington contributed to this story.