(CNN) -- Residents living in single-family homes in some parts of coastal Texas face "certain death" if they do not heed orders to evacuate ahead of Hurricane Ike's arrival, the National Weather Service said Thursday night.
The unusually strong wording came in a weather advisory regarding storm surge along the shoreline of Galveston Bay, which could see maximum water levels of 15 to 22 feet, the agency said.
"All neighborhoods ... and possibly entire coastal communities ... will be inundated during the period of peak storm tide," the advisory said. "Persons not heeding evacuation orders in single-family one- or two-story homes will face certain death."
The maximum water level forecasts in nearby areas, including the shoreline of Matagorda Bay and the Gulf-facing coastline from Sargent to High Island, ranged from 5 to 8 feet. But authorities warned that tide levels could begin rising Friday morning along the upper Texas coast and along the shorelines of the bays.
The advisory comes on the heels of similarly urgent messages earlier Thursday from federal authorities, who warned of a "massive storm" that could affect roughly 40 percent of the U.S. Gulf Coast.
"Do not take this storm lightly," Michael Chertoff, secretary of the U.S. Department of Homeland Security, said Thursday afternoon. "This is not a storm to gamble with. It is large; it is powerful; it carries a lot of water."
Chertoff and representatives from the Federal Emergency Management Agency said their efforts were focused on evacuations as Ike, a Category 2 storm about 700 miles across, headed toward the northwestern Gulf of Mexico with top sustained winds near 100 mph.
Chertoff also urged people not to succumb to "hurricane fatigue," referring to concerns that authorities were overestimating Ike's potential impact.
"Unless you're fatigued with living, I suggest you want to take seriously a storm of this size and scale," he said Thursday. "This is not a game of chicken with Mother Nature."
At 5 p.m. Thursday, the National Hurricane Center said a hurricane warning was in effect between Morgan City, Louisiana, and Baffin Bay, Texas. A warning means hurricane conditions are likely within 24 hours. Watch CNN meteorologists track Hurricane Ike »
A tropical storm warning also is in effect from east of Morgan City to the Mississippi-Alabama border, including New Orleans and Lake Pontchartrain.
Roughly 3.5 million people live in the hurricane's potential impact zone, FEMA Administrator David Paulison said Thursday.
"This hurricane is Mother Nature's weapon of mass destruction," Paulison said after noting the anniversary of the September 11, 2001, terrorist attacks.
On Thursday, Ike was moving slowly through the central Gulf of Mexico. The National Hurricane Center said hurricane conditions may occur on the coast between Morgan City, Louisiana, and Baffin Bay, Texas, by late Friday, with possible landfall near Galveston Island as early as Saturday morning. Track the storm »
As of 5 p.m. ET Thursday, Ike's forecast track was through Galveston and the Houston metro area as a Category 2 or Category 3 hurricane with maximum sustained winds of 120 mph.
The hurricane center said hurricane-force winds extended outward up to 115 miles from Ike's center, and tropical storm-force winds extend outward up to 275 miles.
At 8 p.m. ET, the center of Hurricane Ike was about 475 miles east-southeast of Corpus Christi, Texas, and about 370 miles east-southeast of Galveston, Texas.
Forecasters predict coastal flooding of up to 20 feet above normal tide levels and battering waves near the center of where Ike makes landfall.
In Galveston, which sits on a coastal island, Mayor Lyda Ann Thomas told the island's 60,000 people that they should leave.
Mandatory evacuations were announced for low-lying coastal areas northeast and southwest of Galveston, in Chambers, Matagorda and Brazoria counties. Elsewhere in the state, voluntary evacuations were issued in San Patricio and Victoria counties and parts of Jackson County, according to the governor's office.
Some Brazoria County residents said they didn't want to leave but realized it was in their best interest to do so.
"You don't have a choice when you have kids," Deborah Davis of Freeport told CNN affiliate KPRC-TV in Houston.
Farther inland, about 100,000 residents in low-lying areas surrounding Houston began evacuating Thursday afternoon as Ike headed for the Texas coast, officials said. Watch Gov. Rick Perry warn residents of Ike's potential »
But the remaining 4 million residents were told they could stay home, even as government offices and schools prepared to close Friday in Houston in anticipation of the hurricane.
"We are only evacuating areas subject to a storm surge," said Harris County Judge Ed Emmett, the county's chief executive officer. "Yes, we know you will lose electricity. But you're not in danger of losing your life, so stay put."
Ships in port were told to leave, said Port of Houston spokeswoman Linda Whitlock. The area's two major airports, George Bush Intercontinental and William P. Hobby, also halted all commercial flights.
Forecasters said the storm stood a 41 percent change of slamming into the Texas coast late Friday or early Saturday as a Category 3 storm, with winds between 111 mph and 130 mph.
There was a 25 percent possibility Ike could be a Category 2, with wind speeds between 96 mph and 110 mph, at landfall, forecasters at the National Hurricane Center said.
More than 1,300 inmates from the Texas Correctional Institutions Division's Stevenson Unit in Cuero were being evacuated to facilities in Beeville and Kenedy, Perry's office said, and 597 were transferred from the substance abuse Glossbrenner Unit in San Diego, in south Texas, to Dilley.
Evacuations appeared to have saved lives in Cuba when Ike slammed into the island. Four deaths were reported from the storm, according to the Cuban government. The Cuban Civil Defense brought buses or trucks to take people to shelters. Watch Cubans wade through the streets »
The United States, which provided $100,000 in emergency aid to communist-run Cuba through private aid agencies after Hurricane Gustav hit the island August 30, said it was considering additional emergency aid for Cuba because of Ike.
