Hurricane Ike's gargantuan width — not its strength — will likely push an extra large storm surge inland in a region already prone to it.
Ike's giant girth means more water piling up on Texas and Louisiana coastal areas for a longer time, topped with bigger waves. So storm surge — the prime killer in hurricanes — will be far worse than a typical storm of Ike's strength, the National Hurricane Center said.
And because coastal waters in Texas and Louisiana are so shallow, storm surge is usually larger there than in other regions, according to storm experts. A 1900 hurricane following a similar track to Ike inundated Galveston Island, killing more than 6,000 people — America's deadliest storm.
"It's a good recipe for surge," said Benton McGee, supervisory hydrologist at the U.S. Geological Survey's storm surge center in Ruston, La.
The National Hurricane Center is forecasting a 20-foot surge — a rapid rising of water inundating areas and moving inland — for a large swath of Texas and the Louisiana coasts. Above that, the center predicts "large and dangerous battering waves." Waves could be 50 feet tall, said hurricane center spokesman and meteorologist Dennis Feltgen.
Big waves should break offshore
Some computer models have waves topping out at 70 feet, but the waves usually break well before hitting shore, so the maximum usually doesn't get quite that high.
"It's going to do tremendous damage over a large area even if its doesn't strengthen anymore," predicted former hurricane center director Max Mayfield.
That's directly due to Ike's size. Experts are trying to figure out when they've seen a storm this wide. Ike's tropical storm force winds stretch for 510 miles, and weather radar from Galveston to Key West can see its outer bands. That's about 70 percent larger than an average hurricane.
"Because of the very large expanse of hurricane force winds, Ike will create a storm surge well in excess of what would normally be associated with a storm of its intensity," the National Hurricane Center warned.
Areas that have a hurricane warning can expect storm surges up to 20 feet. Areas with a tropical storm warning can expect five to seven feet of storm surge, Feltgen said.
The size and relatively slow speed means more water keeps building, pushing inland for hours after Ike hits the coast, McGee said.
Texas, La. worst areas for surge
Geography doesn't help either. Experts say the Texas coast ranks second, behind Louisiana, as the worst region for storm surge in the United States. That's because the water there is shallower than in most other regions. The energy from a hurricane needs a way to escape. Deeper water can absorb more of it, dissipating the surge, but along the Texas coastline, the water has nowhere to go but up on shore, McGee said. Think of the Gulf of Mexico as a shallow bathtub with a big-time disturbance in it, Mayfield said.
Storm surges reached 16 feet during 2005's Hurricane Rita, which hit just east of Galveston, McGee said. Because the worst surge is always just east of the eye of the hurricane, the Galveston-Houston area was spared the worst of the damage.
Houston is buffered by Galveston Island — which sits in the way of the surge — and the bay system, but still is likely to get a rush of high water as the bay, rivers and canals fill up, McGee said. And water that rushes into Galveston Bay may not be able to get out after the storm, he said.
The U.S. Geological Survey on Thursday sent five teams to the Texas and Louisiana coast installing 80 storm surge devices to measure the flood to come, McGee said.