US tornado frequency shifting eastward from Great Plains

A new study finds that over the past four decades, tornado frequency has increased over a large swath of the Midwest and Southeast and decreased in portions of the central and southern Great Plains, a region traditionally associated with Tornado Alley.

The study, by meteorology professor Victor Gensini of Northern Illinois University and Harold Brooks of NOAA's National Severe Storms Laboratory in Norman, Okla., found significant decreasing trends in frequencies of both tornado reports and tornado environments over portions of Texas, Oklahoma and northeast Colorado.

Tornado Alley remains the top zone for tornadoes in the United States, but other areas, including the so-called Dixie Alley that includes much of the lower Mississippi Valley region, are catching up.

The researchers identified significant increasing trends of tornado reports and tornado environments in portions of Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee and Kentucky.

"Regions in the Southeast and Midwest are closing the gap when it comes to the number of tornado reports," said Gensini, who led the study published Oct. 17 in the Nature partner journal, Climate and Atmospheric Science.

"It's not that Texas and Oklahoma do not get tornadoes," Gensini said. "They're still the number one location in terms of tornado frequency, but the trend in many locations is down over the past 40 years."

The study examined tornado frequency trends in fine-scale resolution using two separate approaches, Gensini said.

The researchers tracked the number of tornado reports from 1979 to 2017, while also investigating regional trends in the daily frequency of tornado-environment formation over the same time period, using an index known as the Significant Tornado Parameter (STP). Frequently used for predicting severe weather, the index captures the coexistence of atmospheric ingredients favorable for producing tornadoes.

Both the number of actual tornado reports and the historical STP analysis showed the eastward uptick in tornado frequency.

"One could argue that because a region's population has increased, more tornadoes are sighted and reported," Gensini said. "But we also identified this eastward trend when using the STP index, which looks at the frequency of tornado environments and has nothing to do with people. This increases our confidence in the reporting trend that we're seeing."

https://www.sciencedaily.com/releases/2018/10/181017172846.htm