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Weather variations lead to increase in tornadoes

This article first appeared in the St. Louis Beacon, June 3, 2011 - More than 500 tornadoes were reported in the United States at this time last year. That number has more than doubled in 2011 as nearly 1,300 twisters tore through the Midwest between March and May, leaving a path of destruction and more than 500 people dead. Adding to the unusually active season, at least two tornadoes struck western and central Massachusetts Wednesday, leaving four people dead so far.

Climate scientists expect the heavy tornado season to end in June.

"Some of the outbreaks this season are well beyond anything we were expecting to see," said Harold Brooks, meteorologist with the National Severe Storms Laboratory at the University of Oklahoma. "The biggest question that remains is why the death toll was so high this year."

At least 134 people lost their lives as a result of the massive tornado that struck Joplin, Mo., on May 22. Studies have shown that advanced warnings of five to 15 minutes can reduce fatalities by more than 40 percent. Residents in Joplin were given a 24-minute warning. Even so, more than 900 people were reported injured by the storm system.

"It was a well forecasted event, but this was a one-in-20-years event and it was just extremely bad luck for Joplin," Brooks said.

Brooks, who studies historical weather patterns, said no evidence suggests that tornadoes are becoming more frequent in the long term. Brooks said that each month and each tornado season have its variations. This variation is evident with April and May. May saw 202 tornadoes, one of the slowest months on record. By comparison, April was one of the most active months on record with more than 875 tornadoes.

Tornado Season Is Coming To An End

Timothy Eichler, an assistant professor in the Department of Earth and Atmospheric Sciences at Saint Louis University, studies weather patterns in the Midwest and says this season is a perfect example of weather pattern variation.

"It has been difficult to understand this season," Eichler said. "It has been one of the most active we've seen in terms of activity and we didn't predict that. We had baseball-sized hail in St. Louis last week and that is not a normal occurrence."

Eichler said that from a climate point of view, we should be coming out of the heavy tornado season in June. Scientists are unsure why the conditions that cause tornadoes have been more frequent this year. Brooks and Eichler say it has to do with multiple factors.

An increase in moisture this season caused more thunderstorms creating the conditions necessary for tornadoes. A strong jet stream and fast flowing air currents over the south have also been cited as contributors to increased storm activity. Eichler said that climate change could not be linked as a cause until more information is gathered.

Tornadoes can form in any situation that produces severe weather such as increased moisture and a strong jet stream, but less than 1 percent of all thunderstorms produce tornadoes.

"It is hard to say when a tornado is going to form, many storms have the potential for tornadoes, but only a small amount of storms result in a touchdown," Brooks said. "We have a lot of forecasting capabilities, but we are still not good at forecasting how the season will turn out from year to year. We will just have to wait and see how this season will end," Brooks said.

Fast Facts on Tornadoes

  • Less than 1 percent of all thunderstorms produce tornadoes.
  • Because of the tremendous pressure differences associated with a strong tornado, maximum winds can sometimes approach 300 miles per hour.
  • Pressures within some tornadoes have been estimated to be as much as 10 percent lower than immediately outside the storm.
  • Tornadoes have been reported in every state in the US and also in every season.
  • The chances that a tornado is an F5, the highest classification for a tornado on the F-scale, is less than 0.1%.
  • Tornadoes are most likely to occur between 3 and 9 p.m. but have been known to occur at all hours of the day or night.

Source: National Weather Service

Jonathan Ernst, a student at St. Louis University, is a summer intern at the Beacon.

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