It’s the storm surge.
“People underestimate the danger of the storm surge,” says John Metz, the warning coordination meteorologist at the National Weather Service office here. “They may not even know what it is.”
Time for physics 101.
A hurricane is an intense low pressure system. One of the measures of a storm’s intensity — among wind speed and its diameter — is the atmospheric pressure in the eye, or center, of the storm.
Usually, the lower the pressure, the more severe the storm.
Atmospheric pressure is the weight of the atmosphere against the surface of the earth.
(Pay attention: there could be a test before the November end of the hurricane season.)
What all this means is that when a hurricane is at sea, there is less pressure against the level of the ocean. If you could view the level of the sea across a hurricane 200 miles in diameter, you would find the sea level is higher inside the eye.
Sometimes, the difference is as much as 30 or more feet.
When a hurricane makes landfall, that 30-foot rise in the water comes ashore as a 30-foot wave.
That is the storm surge.
Combine the surge with a high tide and water blown ashore by hurricane-force winds, and it results in a wall of water coming ashore that has an unnerving resemblance to a tsunami.
Its effect can be devastating.
“Nine out of 10 people who die in a hurricane are killed by the storm surge,” Metz says.
To counter this deadly statistic, the Weather Service Office here, in conjunction with the National Hurricane Center in Miami, is honing its skills at better forecasting the storm surge of a hurricane if it makes landfall along the South Texas coast.
The program is called “SLOSH” — an acronym that stands for Sea, Lakes, Ocean, Surge and Hurricane.
“What we are trying to do is to create a total surge model,” Metz told a gathering of reporters Wednesday afternoon, June 22, “which will incorporate river flow and wave height.”
That data is used to generate forecast maps of what areas will be affected by a storm surge and the expected height of the water.
Bee County is too far inland to be affected by a storm’s surge, but the northern part of Refugio County is under threat. The average elevation in the Bayside area, Austwell and Tivoli is four to six feet.
Computer-generated forecast maps — which always assume a storm makes landfall during a two-foot tide — eerily shows a surge halting at the barrier surrounding the cooling pond of the South Texas project, a nuclear power plant that helps supply power to South Texas.
A comparison with the tsunami-damaged Fukushima nuclear power plant north of Sendai, Japan, may be unfair, but compelling.
The Weather Service will issue storm surge predictions starting 48 hours before the expected landfall of an approaching hurricane.
These are pre-generated maps, based on previous storms with similar intensities. Meteorologists also will issue worst-case scenarios.
Once a hurricane is 36 hours away, the Corpus Christi Weather Service Office will begin issuing what it terms “probabilistic” surge forecasts based on the latest known conditions, such as the size of the storm, its intensity and how fast it is approaching.
Those forecasts will be updated every six hours.
The current forecasts are based on elevation surveys. Next year, the areas along the coast susceptible to storm surge flooding are expected to be re-surveyed with higher resolution, or accuracy.
The push for greater storm surge accuracy is but one facet of a continuing effort by the National Hurricane Center to offer better hurricane forecasts.
In the last five years, its accuracy in predicting the path the eye of a hurricane will follow has made significant gains, thanks to refined computer programs, or models. Forecasts showing the expecting position for five days ahead are standard. Center scientists are routinely generating seven-day forecasts but only on an experimental basis and only for in-house use.
Yet, Metz notes, even the most accurate forecast about where a hurricane will make landfall is off by 50 miles.
But countering the gains in accuracy about a hurricane’s track is the center’s ability to forecast a storm’s intensity.
Despite the use of satellites, hurricane-hunter aircraft and — for the first time this year — the use of a NASA drone aircraft, “there has been no improvement in intensity forecasting in the last 20 years,” Metz says. “We’re 10 to 20 years from solving the intensity puzzle.”
What stubbornly defies increased forecasting accuracy is public apathy. Sixty-three hurricanes have made landfall on a Texas coast since 1851. That’s an average of one every seven-and-one-half years.
But the last major hurricane — category 3 or above — to hit Texas was Hurricane Celia in the 1970s.
“There are a couple of generations living in South Texas who have never experienced a hurricane,” Metz says.
The easiest and quickest way to access any of the forecasts issued by the Weather Service is online. During the August-September height of the hurricane season, the National Hurricane Center’s website has measured as many at 16 million hits a day.
This year, the center is predicting an above-average season, calling for 11 named storms, of which six will become hurricanes, two of them developing into major systems with a strength of category 3 or more.
Five of those hurricanes are predicted to make landfall.
With a storm surge.
Bill Clough is a reporter at the Bee-Picayune and can be reached at 358-2550, ext. 122, or at beepic@mySouTex.com.