Storm surge SOURCE: NATIONAL WEATHER SERVICE This graphic shows what parts of this region will be under water with increasingly higher storm surges. The areas in red would be under water when a storm surge reaches four to six feet above sea level. While the National Weather Service map doesn’t show all of Mustang Island, a spokesman for the agency said the entire island would be submerged with an eight-foot surge. The National Weather Service has a message for Texas coastal residents who consider only a hurricane’s wind speed category when deciding whether to evacuate.

That message is: Don’t.

Storm surge is the most deadly part of a hurricane, and it can be extremely high even when a hurricane’s wind speed isn’t particularly severe. Nine out of 10 hurricane related deaths occur due to storm surge, said Scott Cordero, meteorologist in charge of the National Weather Service office in Corpus Christi.

National Weather Service officials are pushing that message hard in the wake of Hurricane Ike. That storm, which devastated the Galveston area Sept. 13, was the third-most destructive hurricane in United States history, largely because of a storm surge of 16 to 20 feet and not so much because of its 110 mph wind speed, Cordero said.

Ike’s wind speed made it only a Category 2 hurricane. At the top of the scale, Category 5 hurricanes have wind speeds of more than 156 mph.

The causes of tidal surge are complicated and not directly associated with a storm’s wind speed, Cordero said.

Storm surges are influenced by factors including bathymetry (the contour of the sea bottom), shallowness of coastal waters, the size of the storm, forward speed, pressure, track and winds, Cordero said.

Fetch is another important factor, Cordero said. A greater fetch is produced by a bigger storm moving over a longer period of time from east to west, he said.

As an illustration of fetch, Cordero said a person moving his hand along the surface of water in a bathtub would create a small wake, while a person using his entire arm in a vessel the size of a hot tub would produce a bigger one.

Ike created a large storm surge partly because it was an immense storm, at 100 miles wide.

Even though it struck far north of Port Aransas, Ike caused a five-foot storm surge to strike Mustang Island, said Michael Kovacs, city manager of Port Aransas. The flooding reduced State Hwy. 361 on the island to one lane.

While the National Hurricane Center will continue to assign Saffir-Simpson Scale category numbers to show a hurricane’s wind speeds, local offices of the National Weather Service will provide notices of expected storm surge heights for specific areas within the Coastal Bend as a hurricane approaches, said Dennis Feltgen, a spokesman for the National Hurricane Center in Miami.

Computer models have made those kinds of predictions much more accurate and detailed in recent years, and they’re going to be getting even better in the next few years, Cordero said.

Lying on a barrier island that isn’t much above sea level, Port Aransas is highly vulnerable to storm surges.

The area from Rockport to Padre Island is “one of the most susceptible areas in South Texas,” Cordero said.

If a large hurricane with a big storm surge seems on track to hit the Port Aransas area, there likely will be a phased evacuation plan that could start 48 hours, or even further out, from the storm’s landfall, Kovacs said.

As evacuations proceed, detailed surge information will be provided to cities from the National Weather Service.

“At that point, it is absolutely critical that people listen to our statements and storm surge messages,” Kovacs said. “We will use the local media and all our emergency notification tools.”

(The South Jetty’s Web site, www. portasouthjetty.com, provides frequently updated advisories when a tropical storm or hurricane is threatening the Coastal Bend. And the city recently updated its web site, www.cityofportaransas. org, with new hurricane preparedness information.)

If a 12-foot storm surge is coming, it could have wave action of another six to 12 feet on top of that, Kovacs said.

“Folks who have determined to stay, against our recommendations, will have to re-evaluate their decision and act quickly in order to survive,” Kovacs said.

Storm tides from an approaching hurricane could begin to hit beaches and lowlying roads up to 24 hours from a storm’s landfall. At six feet of surge, the portion of State Hwy. 361 that runs down Mustang and Padre islands will have water over it, Kovacs said.

“Motorists could drive through water thinking that there is a road, (and then) discover it’s been washed out, “ he said. “At six feet, you’re cut off from hospitals, EMS service, and HALO (medical helicopter) can’t fly in high winds.”

At that point, city public safety workers are likely wrapping up final activities and will leave the island shortly thereafter if more storm surge is predicted to arrive, Kovacs said.

When tropical storm force winds hit the island, the ferries likely will no longer run, and instead will be on their way to mooring areas to ride out hurricane-force winds, Kovacs said.

Depending on the storm, ferries may leave for their moorings earlier, if directed by officials with the Texas Department of Transportation, which runs the ferry system.

“It is important for citizens not to wait or try to guess when the last ferries will leave, but plan for an evacuation and leave early enough to beat the traffic,” Kovacs said.