A mandatory evacuation for residents in surrounding low-lying areas of Houston will begin Thursday afternoon, officials said.
"We strongly urge you to evacuate before tomorrow," said Judge Ed Emmett, Harris County's chief executive officer, at a joint news conference Thursday with Houston Mayor Bill White.
Seven other counties have begun partial or full evacuations.
Forecasters said the storm could slam into the Texas coast -- south of Galveston -- as a powerful Category 4 storm late Friday or early Saturday. At 8 a.m. ET Thursday, Ike was heading toward the northwestern Gulf of Mexico, the National Hurricane Center said.
Ike was moving west-northwest near 10 mph; that motion should continue over the central and western Gulf of Mexico on Thursday and Friday, the hurricane center said
At 8 a.m., Ike had top sustained winds near 100 mph, and was about 575 miles east of Brownsville, Texas, and 270 miles south-southeast of the mouth of the Mississippi River, the National Weather Service said.
Hurricane-force winds extended outward up to 115 miles from the center, the hurricane center said, and tropical storm-force winds extended outward up to 255 miles.