Why So Many Tornadoes?

Mar 5, 2012 5:54pm

A “just right” mixture of energy and churning winds have already made 2012 an especially active—and deadly—year for tornadoes.

In January, there were 95 twisters reported in the U.S., far above the 1991-2010 average of 35, according to the National Weather Service Storm Prediction Center.

Last week alone, 440 tornado warnings were issued, and storms killed 39 people in five states.

The early outbreak of tornadoes appears to be the kind of weather event that only comes along once every 10 or 20 years,  meteorologists say, helped by a perfect recipe of atmospheric conditions.

“The ingredients for making a thunderstorm are warm, moist air, at low levels in the atmosphere, and dry, relatively cool air above that,” said Harold Brooks, a research meteorologist with the National Severe Storms Laboratory in Norman, Okla. “That provides the basic energy for a thunderstorm.”

The second major factor driving a tornado spike is known as “wind shear,” gusts of wind that change speed and direction at different altitudes in the atmosphere, causing rotation within a storm. More rotation equals more tornadoes.

“That’s the big thing,” said Brooks. “The tornadoes that tend to form in environments that have high wind shear tend to be stronger than those that form with less wind shear.”

Does the tornado activity so far this year foreshadow a busy tornado season?

Not necessarily, says Brooks, who notes that current tools are not yet good enough for forecasters to make seasonal tornado predictions, as they can for hurricanes.

“There really is nothing we can say about a month or two months from now,” he said.

Researchers including Purdue University professor Jeff Trapp are trying to change that. Using advanced computer modeling, his team is working to make better long-range predictions about future tornado seasons.

“We’re trying to say something about the potential for tornadoes in a given year or season in specific regions,” Trapp said, noting that it may be several years before those predictions are put to practical use.

Trapp is also investigating whether the climate-warming greenhouse gasses that humans are pumping into the atmosphere are making tornadoes worse.

“Our biggest question is: how has, or will human activity affect tornadoes?” he said.

Trapp says to answer that question, more research is needed using computer models that can work at a finer scale than those available now.

However, he points out that by providing more heat and moisture, a warmer climate may cause more severe weather, such as thunderstorms. But a warmer climate could actually decrease the amount of wind shear and result in fewer tornadoes.

“That’s the thing that we haven’t been able to unravel yet,” said Trapp.

For now, scientists say that while they are confident that climate change is already contributing to more heat waves and more intense rain and snowfall, the scientific jury is still out on tornadoes.

The National Center for Atmospheric Research in Boulder Colorado recently ranked how strongly different types of extreme weather could be linked to climate change.  Tornadoes ranked at the “less confidence” end of the scale.

“Although the number of observed U.S. tornadoes has more than doubled since the 1950s, as more spotters and chasers watch the skies, there has been no significant trend in the strongest twisters,” the researchers said.

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