Source: USA Today research by Jack Williams

How Hurricanes Create Killer Surges

Storm surge is a hurricane's biggest killer Over the years, "storm surge" flooding has accounted for more hurricane deaths than winds. Howling winds around the hurricane's eye push water along, tending to pile it up. In the deep ocean, this dome of water sinks and harmlessly flows away. But as a storm nears land, the rising sea floor blocks the building water pile's escape and it comes ashore as deadly storm surge. An intense hurricane can send a dome of water more than 18 feet deep ashore as the storm hits land.

MEOWs Help Gauge Surge Danger

When a hurricane threatens any place along the U.S. coast, a "MEOW" from a computer helps officials decide who should evacuate. "MEOW" is the "Maximum Envelope of Water" likely to be pushed ashore by a particular hurricane.

Local MEOWs are available for every place along the Gulf and Atlantic coasts from Brownsville, Texas, to the New Hampshire-Maine border. The final set of figures, for the Maine Coast, are expected to be finished next year.

"In the early days of the studies I gave presentations to local groups that wanted to understand the hazards," says Brian Jarvinen of the National Weather Service. "By the time I got to what a Category 4 storm could do, I could sense a feeling that people were awestruck by high the floods could be."

A computer program called "SLOSH" for "Sea, Lake, and Overland Surge from Hurricanes" produces the figures used to make maps showing what kind of flooding to expect from just about any hurricane.

Storm surge, a dome of water pushed ashore by a hurricane, causes the flooding. Over the years, storm surge floods have killed more people than hurricane winds. The worst surge recorded in the U.S. this century was the 24-foot-high dome of water that Hurricane Camille pushed into Pass Christian, Miss., on Aug. 17, 1969. At least three feet of surge hit places as far as 125 miles east and 31 miles to the west of Pass Christian. It destroyed or seriously damaged more than 18,000 homes and 700 businesses.

When a hurricane hits land, the dome of water built up around its eye comes ashore as storm surge, highest on the right side of the storm's eye in the Northern Hemisphere. Storm strength, how fast it's moving, the direction it comes from, the shape and depth of the ocean's floor, and the shape and height of the shore all help determine how high the surge will be and which places will be flooded.

For a particular storm strength, surge will be highest where the ocean is shallow offshore. The danger is greatest where large numbers of people live near shallow oceans. This is why Jarvinen says forecasters worry most about a strong hurricane hitting New Orleans-or Florida's Southwest Coast between Tampa Bay and Everglades National Park.

"New Orleans has been improving the levees," Jarvinen says. "We've run the model for that basin three times as they've improved the levee system. Still, a strong Category 3, or a 4 or 5 could cause serious flooding in New Orleans.

"In southwest Florida there is a large population right along the coast and the high ground is far inland," Jarvinen says. "A Category 1 storm coming from the Gulf would be fine. A category 2 would present problems., a 3 could be a disaster."

On the Atlantic Coast, Jarvinen says the area north and south of Savannah, Ga., is dangerous because the water is shallow offshore.

But the New York City area worries experts such as Jarvinen the most. Here the north-south running coast starts to run west-east, creating a "corner" that helps push the water of a surge higher. Jarvinen says a computer simulation using 1989's Hurricane Hugo shows what could happen.

In the simulation, the computer had Hugo turning north instead of moving ashore just north of Charleston, S.C. It weakened to a Category 3 storm, but picked up forward speed before coming ashore near Atlantic City, N.J. From here, its eye followed New Jersey's Garden State Parkway north to around Newark, N.J. This path would bring the highest surge into New York Harbor.

The computer simulation showed that such a storm would push more than 10 feet of water over John F. Kennedy Airport and Battery Park on the tip of Manhattan. It would be a major disaster for New York City.

Jarvinen notes that no strong hurricane has ever followed such a course. But, strong storms have followed courses on both sides of such a path. "It's not of matter of will it happen," he says. "It's a matter of when it will happen. It could be 500 years from now. It could be next week."