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Sunday, February 19, 2012

Weather pattern could fuel more Ala. tornadoes

HUNTSVILLE, Ala. (AP) – This isn't a tornado warning, nor is the siren about to go screaming across the Tennessee Valley.

A tornado roars through Tuscaloosa, Ala., in April 2011. Dusty Compton, AP

A tornado roars through Tuscaloosa, Ala., in April 2011.

Dusty Compton, AP

A tornado roars through Tuscaloosa, Ala., in April 2011.

But the potential and the indicators are in place to make the upcoming spring tornado season a rocky one to ride out.

This follows a storm season in 2011 that saw several killer tornadoes lash Alabama.

John Christy, the state climatologist and director of the Earth System Science Center at the University of Alabama in Huntsville, cited the presence of the La Niña weather pattern as a cause for tornado apprehension this spring.

"In a La Niña type year, we tend to have more of these types of experiences with the trailing cold front creating the opportunity for those specific ingredients to provide the high contrast between cold and warm," Christy said.

"We are still in La Niña pattern. The long-range forecast for the spring is for warmer than usual. So that sets up an opportunity for a contrast along the way for these ingredients to come together."

If it happens, Kevin Knupp will be ready. A professor of atmospheric sciences at UAH who studies tornadoes, Knupp is working on research about the influence gravity waves have on tornadoes.

Knupp compared the gravity waves to the way water moves across a lake. As the gravity waves encounter a potential tornado, it can enhance the likelihood of a twister developing, according to Knupp's theory.

"When the waves intersect the storm, oftentimes - not always but most of the time - there is a response from the storm," Knupp said. "If there is an existing circulation, it intensifies that existing circulation there and if there is no tornado genesis, the formation of a tornado can occur at that time."

Watching gravity waves -- which is easier in a humid climate like Alabama -- can aid forecasters in identifying tornadoes. Knupp said the National Weather Service in Huntsville was already using gravity waves among its tools for distinguishing tornadoes.

"We can be more specific on when and where it might form," said Knupp, who is also studying the impact topography has on the intensity of tornadoes. "We want to do that because the false alarm rate on tornado warnings has been so high in this area."

So what's on the horizon for the Tennessee Valley as prime tornado season approaches in March and April?

"From a historical standpoint, Alabama gets about 60 tornadoes a year," Christy said. "Most of them come in the spring. So by any standard measure, you should be ready for something to happen this spring.

"With the way the global atmosphere circulation is set up (with the La Niña pattern), there is a bit more chance that number will be higher than average this spring."

The La Niña pattern, Christy and Knupp said, brings cold air from the Pacific Ocean into close proximity with warm air from the Gulf of Mexico. Christy described it as an "opportunity" for the two air masses to "collide" over Alabama and that's when the atmosphere becomes unstable.

But Christy downplayed any seeming increase in tornadoes in recent years.

"We still have just as many tornadoes as we've ever had," he said. "We just put more stuff in their way to hit. So the damage from tornadoes will just continue to rise."

Of course, weather experts are loathe to look farther into the future than absolutely necessary. But history, in a sense, can be a crystal ball with a peek at what's to come.

"If it happened before, it'll happen again and probably worse," Christy said. "That's my general rule of climate."

Said Knupp, "I remember that word for word and that's what I tell my students."

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