Tornado damage in Plains states rises, but no upsurge in severe twisters seen

Economic development, population growth raise cost

more storms counted

May 05, 1999|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

The devastating outbreak of tornadoes that raked Oklahoma and Kansas on Monday night is expected to rank among the worst in history.

One monstrous Oklahoma twister -- among the 76 in one unofficial count -- was up to a mile wide. It churned across some of the state's most densely populated communities for four hours.

But despite dramatic TV images, meteorologists say there is no evidence that such deadly outbreaks are becoming more severe or more frequent as a result of global warming or some other factor unique to our times.

"There is no existing basis for saying we have an increasing trend of severe tornadoes," said Robert Livezey, of the National Centers for Environmental Prediction.

What has changed is what people have put in the tornadoes' path.

Inflation and development across "tornado alley" -- a wide swath from western Texas to Iowa -- have begun to produce outbreaks with damage above $1 billion. In January, 106 tornadoes killed 17 people over three days in Arkansas and Tennessee, causing $1.7 billion in damage.

Population increases, the ubiquity of video cameras and the interest of news media have also placed more observers in the field, given us higher counts and a more acute awareness of the storms and their impact.

"If you look at the number of tornadoes counted by month, or by year from 1950, there has been a gradual increase. But there are also more people paying attention," said Joseph S. D'Aleo, chief meteorologist at Weather Services International, a private weather services firm in Billerica, Mass.

In earlier times, he said, "a tornado crossed the prairie and nobody heard about it." Today, even if no one is around, tornadoes are spotted on the new Doppler weather radar systems that blanket the country.

Tornado counts may also be skewed by new counting methods. A twister used to be counted each time someone saw it touch the ground. Today they're counted each time they enter a new county, triggering a new warning.

"If it goes through three counties, with three warnings, it might be counted as three tornadoes," D'Aleo said.

Despite rising populations, however, tornado-related deaths are declining. Thanks to radio, TV, public education campaigns and technologies such as Doppler radar, "people are better warned, and also better prepared. That, no doubt, has saved a lot of lives," he said.

The nation's deadliest tornado outbreak was on Feb. 19, 1884. Despite the sparser populations of the day, it killed more than 800 people in Mississippi, Alabama, North and South Carolina, Tennessee, Kentucky and Indiana.

The weather conditions that spawned Monday's outbreak were typical of the spring, D'Aleo said.

"Spring is the time of year when you still have relatively cold water off the West Coast and snow in the mountains of the West. So the jet stream, and the storms that track along the jet stream, are still fairly strong," he said.

At the same time, strong spring sunshine is heating the moist air over the Gulf of Mexico.

As the storms track across the Great Plains, they draw cold air down from the northwest, and warm, moist air up from the southeast. The warm air surges aloft, destabilizing the atmosphere. Severe thunderstorms form and start the air spinning.

America's midsection is especially well-situated for tornadoes. From 1950 to 1995, Oklahoma and Kansas ranked second and third after Texas in tornado "traffic." Oklahoma saw an average of 52. Kansas saw 47. Annual damage averaged $23 million in Oklahoma, and $26 million in Kansas.

Monday's outbreak was intensified by a very strong storm system and very cold air moving out of the southwest. That made the atmosphere especially unstable, D'Aleo said. "It was also a very slow-moving system, which just makes it worse. More typically in the spring they're real fast-moving."

This is also a La Nina year. Forecasters have learned that La Nina -- an unusual cooling of surface waters in the eastern tropical Pacific -- can enhance the normal springtime patterns that lead to tornadoes. But they aren't sure how decisive a factor it is.

"We cannot say that La Nina contributed to the formation of these particular tornadoes," Livezey said. "Not only is La Nina just one of the factors that can affect the development of storms, but these storms can occur without a La Nina."

On the other hand, said D'Aleo, "If we look back at La Nina years, they tend to be more active. All the monthly [tornado] records we hold, from January to April, are all in La Nina years."

The biggest outbreak ever recorded was in April 1974, a La Nina year. Officials recorded 148 tornadoes in 24 hours. "They had tornadoes in 11 states that day, and even in Canada. There were 309 lives lost, and 5,500 injured," D'Aleo said. The town of Xenia, Ohio, was almost completely wiped out that day.

Pub Date: 5/05/99

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