Such a storm could cause a storm surge of 18 feet in Matagorda Bay and four to eight feet in Galveston Bay, emergency officials warned. The surge in Galveston Bay could push floodwaters into Houston, damaging areas that include the nation's biggest refinery and NASA's Johnson Space Center.
Four counties south and east of Houston announced mandatory or voluntary evacuations, and authorities began moving weak and chronically ill patients by bus to San Antonio, about 190 miles from Houston. A mandatory evacuation was also ordered for the west end of Galveston Island. No immediate evacuations were ordered in Harris County, which includes Houston.
Johnny Greer, a 54-year-old retired plant operator at Dow Chemical Corp., boarded up his house a mile from the Gulf of Mexico in Brazoria County and planned to hit the road.
"Gas and stuff is high. But you can't look at all that," he said. "I think my life is more valuable than high gas prices." About 1 million people live in the coastal counties between Corpus Christi and Galveston. An additional 4 million live in the Houston area, to the north.
The oil and gas industry watched the storm closely, fearing damage to the very heart of its operations.
Texas is home to 26 refineries that account for one-fourth of U.S. refining capacity, and most are clustered along the Gulf Coast in such places as Houston, Port Arthur and Corpus Christi. Exxon Mobil Corp.'s plant in Baytown, outside Houston, is the nation's largest refinery. Dow Chemical has a huge operation just north of Corpus Christi.
Refineries are built to withstand high winds, but flooding can disrupt operations and - as happened in Louisiana after Hurricane Gustav - power outages can shut down equipment for days or weeks. An extended shutdown could lead to higher gasoline prices.
As always, some hardened old-timers decided to ride it out. Fourth-generation fisherman James Driggers, 47, planned to spend the storm aboard his 80-foot boat docked in Freeport.
"We like to stay close to our paycheck," he said.
At 5 p.m. EDT, Ike was a Category 2 storm with winds near 100 mph. It was about 720 miles east of Brownsville, Texas, and was moving northwest at 8 mph, after ravaging homes in Cuba and killing at least 80 people in the Caribbean.
No matter where Ike hits, its effects are likely to be felt for hundreds of miles, said Mark Sloan, emergency management coordinator for Harris County, which includes Houston.
"It's a very large storm," Sloan said. "The bands will be over 200 miles out from the center of storm, so we have to be aware of its size as it grows over the next 24 to 48 hours and what impacts it will have on Friday, Saturday and Sunday."
Isaias Campos, 27, boarded up the church he attends in Freeport. He said he was grateful the church planned to evacuate much of the congregation to Houston by bus.
"If it wasn't for the church, it would be difficult for many of our members to leave," Campos said.
Associated Press writers Melinda Deslatte in Baton Rouge, John Porretto in Houston, Juan A. Lozano in Brazoria, Michelle Roberts in San Antonio and Christopher Sherman in Corpus Christi contributed to this report.