HOUSTON (AP) -- Gleaming skyscrapers, the nation's biggest refinery and NASA's Johnson Space Center lie in areas that could be vulnerable to wind and damaging floodwaters if Hurricane Ike crashes ashore as a major hurricane.

Forecasters expect the storm to make landfall this weekend somewhere between Corpus Christi and Houston, creating the potential for heavy punishment for Houston even if it's not hit directly.

Some forecasts say Ike could strengthen to a fearsome Category 4 hurricane with winds of at least 131 mph over the Gulf of Mexico, and emergency officials warned it could drive a storm surge as high as 18 feet.

If current projections of the storm's path hold up, the area surrounding Houston - home to about 4 million people - would be lashed by the eastern or "dirty" side of the storm, said meteorologist Jeff Masters, co-founder of San Francisco-based Weather Underground. This stronger side of the storm often packs heavy rains, walloping storm surge and tornadoes.

"I expect a lot of damage in Houston from this storm," said Masters, adding that Ike could cause a "huge storm surge" affecting at least 100 miles of the Texas coast.

Houston officials were expecting some flooding, the question is how much.

Patrick Trahan, spokesman for the city of Houston, told The Associated Press early Thursday that "based on the current forecast (we) would expect to see some flooding based solely on the surge in some low-lying areas."

Four counties south and east of Houston have announced mandatory or voluntary evacuations, and authorities began moving weak and chronically ill patients by bus to San Antonio, about 190 miles from Houston. About 1 million people live in the coastal counties between Corpus Christi and Galveston.

But no immediate evacuations were ordered in Harris County, which includes Houston.

The Galveston-Houston area could be on the edge of hurricane-force wind gusts, even if the storm makes land 100 miles to the southwest as some forecasts say is likely, said forecaster Joe Bartosik. Storm surges in the Houston-Galveston area could reach 10 to 14 feet in a Category 3 storm, and as much as 20 feet for a Category 4, said Bartosik, senior meteorologist with WeatherBug, a private weather company with 1,500 weather stations along the Gulf Coast.

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.

Diana Rangel, a lifelong resident of Freeport, which is under a mandatory evacuation order in Brazoria County, said it is better that she, her family and other residents on the Texas coast, leave.

"We don't want to get stuck out here (in flood waters)," she said Wednesday as she filled her car with gasoline at a convenience store in Freeport overrun with other vehicles waiting in line to fill up.

The oil and gas industry also 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.

At 5 a.m. EDT, Ike was a Category 2 storm with winds near 100 mph. It was about 620 miles east of Brownsville, Texas, and was moving west-northwest near 9 mph, after ravaging homes in Cuba and killing dozens of people in the Caribbean.