Houston/Washington - National Weather Service (NWS) forecasters issued ominous and unusually blunt warnings about the storm surge heading for the Texas coast Friday ahead of Hurricane Ike.

The local statement issued by NWS for Houston and nearby Galveston, Texas, with Ike threatening a direct hit, warned of storm surge on the Galveston Bay shoreline of more than 6 metres with "life threatening inundation likely!"

Waves atop the storm surge could push water several metres higher still.

Galveston Island, a barrier island on the Gulf of Mexico, and several coastal counties were completing mandatory evacuations overnight Thursday, with voluntary evacuations urged far inland.

Registering Category 2 late Thursday and expected to reach Category 3 strength as a major hurricane before landfall, Ike's rotating storm system was covering much of the warm Gulf of Mexico, siphoning moisture and energy into the atmosphere.

The vast breadth of the hurricane, rushing inland through the shallow seas off Texas, threatened unusually severe surging seas and waves.

For Galveston Bay, doom was literally in the forecast.

"All neighborhoods and possibly entire coastal communities will be inundated during the period of peak storm tide," the NWS statement said. "Persons not heeding evacuation orders in single-family, one- or two-storey homes will face certain death."

The weather service predicted that average dwellings on the coast would be destroyed in large numbers, with "widespread and devastating personal property damage" even away from the shoreline.

Cars not driven out in the evacuation would "likely be swept away," while local roads could be washed away by hours of battering wave action while the hurricane passes over the island.

More than 1 kilometre inland, the storm surge could approach 3 metres.

Buildings that survive the initial landfall could still face "massive destruction" from the pounding of elevated waves.

"Damage from beach erosion could take years to repair," the weather service said.

The eye of the storm is expected to approach landfall around midnight Friday (0500 GMT Saturday) in Texas.

While waves posed the greatest threat, hurricane-force winds extend across a diameter of more than 360 kilometres. Houston's skyscrapers were expected to lose windows, with roofs and trees in danger across the region.

Winds and high water from Hurricane Ike were expected to begin lashing the coast before noon (1700 GMT) Monday, intensifying throughout the evening.

Forecasters said that destruction from wind "will include the majority of mobile homes being severely damaged. Those that survive will be uninhabitable until repaired."

As many as one in four gabled roofs will fail in the storm, NWS said.

Several centimetres of rain were predicted, with 15 to 25 centimetres in some locations by Saturday.

Authorities were already watching for Ike to spawn tornadoes on the Gulf by early Friday.