25 March 2010

Forecasters Improving at Tornado Prognostication

Fort Worth Star-Telegram

For all their fury, thunderstorms are fragile things, dependent on a delicate balance of wind, water and heat.

Too much wind at a certain altitude can scatter storm clouds before they form. A storm's own rain can smother the columns of rising air that give the storm its strength.

Even rarer are storms that spin off a tornado. And it's rarer still for a tornado to hit a populated area and kill people.

It's happened only once in Fort Worth -- a decade ago. The storm killed two people, ripped houses from their foundations and pummeled several downtown high-rises.

The chance to spot a tornado -- and sound the warning early enough to save lives -- has spawned a corps of volunteer storm chasers who work with the National Weather Service every spring. And they're getting better at what they do.

Advances in radar and computer mapping have helped forecasters get a better idea of how severe storms work. For instance, forecasters originally thought that the storm that struck Fort Worth on March 28, 2000, was going to hit close to North Loop 820. They quickly adjusted their warning, Fox said. But today, the weather service would have a more accurate picture of the storm before it spawned a tornado.

Still, the weather service relied on a corps of more than 3,000 volunteer storm spotters.

"Even with all those technological advances, the electronic tools still don't show us all that's going on with a storm," said Gary Woodall, chief of the weather service's Phoenix bureau.

The idea of using volunteers started in Dallas-Fort Worth, the biggest metropolitan area in Tornado Alley, which stretches from Texas to Iowa, said Martin Lisius, chairman of the Texas Severe Storm Association and a storm tracker since 1987. During a typical storm, 20 to 40 volunteers take to the streets, communicating with ham radios. They provide crucial on-the-ground information to the weather service.

"You don't see them; they're sort of like unsung heroes, but they're out there," Lisius said.

About 450 storm spotters, Red Cross volunteers, students and other enthusiasts attended a storm association conference Saturday in Colleyville.

Most of the conference dealt with technical issues: wind shear, cold pools, rear flank downdrafts.

What most worries the spotters is the theoretical big one -- a severe tornado hitting a populated area.

Three years ago, an F-5 tornado, the top category on the Fujita scale, struck Greensburg, Kan., laying waste to 99.8 square miles. Eleven people were killed and more than 60 injured. By comparison, the 2000 Fort Worth twister had a footprint of 0.5 square mile.

But Greensburg is a small town. If the same storm struck, say, the Arlington entertainment district on a busy night, it could be devastating. Scott Rae, a planner with the North Central Texas Council of Governments, overlaid a map of the Metroplex on maps of the Greensburg storm to estimate that such a storm would affect 265,000 people in 111,000 Fort Worth apartments and houses.

Lisius projects that if it hit on a Friday night in April -- with crowds arriving for Good Friday church services and a Texas Rangers game -- hundreds or thousands of people could die. The roads would be too jammed for people to escape, and things would only get worse if drivers tried to take shelter under freeway overpasses, which is common during storms.

"A lot of them are not going to be able to get out of the path of the storm," Lisius said.

The upside is that the warning system is getting better. In 1986, 30 to 40 percent of the weather service's tornado warnings had no lead time, said Mark Fox, the warning coordination meteorologist at the service's Fort Worth office. By 2009, 80 percent of the warnings had lead time -- usually 10 to 12 minutes.

"Our average last year was about 12 minutes," he said.

Fox says the lead time could eventually reach 30 minutes. At that point, he said, the problem would become one of education: "Making sure people know what to do and making sure they believe the warning when they hear it."

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