Tornadoes occurring earlier in Tornado Alley

About 1,300 tornadoes hit the United States every year, killing an average of sixty people, according to the National Weather Service’s Storm Prediction Center. This year, the majority of the 309 tornadoes that have hit the United States occurred in May and the deadliest storms were in April, according to the Storm Prediction Center.

“From a public safety perspective, if this trend (of an earlier tornado season) is indeed occurring, then people need to begin preparing for severe weather earlier in the year,” said Greg Carbin, the warning coordination meteorologist at the Storm Prediction Center in Norman, Oklahoma, who was not involved in the new study.

The new research analyzed National Weather Service tornado data for Tornado Alley from 1954 to 2009. The authors broke the data into ten-year time frames and analyzed how the dates of peak tornado activity changed over time.

The analysis showed the date of peak tornado activity in the region moved earlier at a rate of 1.55 days per decade over the time period studied. In the heart of Tornado Alley, an area with the highest density of tornadoes, peak activity shifted by seven days: from 26 May in the 1950s to 19 May in the early 2000s.

Although there is a consistent movement in the region toward earlier tornado activity, it is difficult to pinpoint a cause, said Paul Stoy, assistant professor in the Department of Land Resources and Environmental Sciences at MSU and co-author of the new study. Records of tornado activity in the United States only date back to the 1950s, making it difficult to study changing trends in tornado activity. Furthermore, tornadoes can be influenced by many regional factors, including topography of the land and areas where cooler air meets warm, subtropical air, making it difficult to attribute the shift in the tornado season to any one factor, he said.

Carbin, of the Storm Prediction Center, said a warmer climate might play a role. “If winters are not as cold, or if spring times are warmer, the location of the jet stream is most likely displaced north of where it has been in the past,” he said. This would cause tornado activity to shift earlier in the year, like what is seen in the new study, Carbin said.

The study has revealed a connection between one global climate pattern and tornado activity, specifically in the state of Oklahoma. When El Niño conditions occur between January and April, peak tornado activity in Oklahoma occurs earlier in the spring, the researchers report. El Niño, an oscillation of the ocean-atmosphere system that is associated with warm ocean waters in the Pacific Ocean, changes the air surface pressure and atmospheric circulation.

“The relationship we do see in Oklahoma is a light but significant connection to El Niño,” Stoy said. “This makes one suspect that if global climate change is changing these larger circulations, then there is a connection between a global [variability] and tornado activity.”

— Read more in John A. Long and Paul C. Stoy, “Peak tornado activity is occurring earlier in the heart of ‘Tornado Alley’,” Geophysical Research Letters (10 September 2014) (DOI: 10.1002/2014GL061385)