Tornado frequency has increased over a large swath of the Midwest and Southeast over the past four decades. At the same time, it has decreased in portions of the central and southern Great Plains, a region long known as Tornado Alley.

The emerging trend could help guide building code updates, assist in identifying potentially impacted community assets and bolster efforts to enhance risk awareness and make emergency preparations.

Tornado Alley remains the top U.S. location for tornadoes. Source: NOAAThe study, by meteorology professor Victor Gensini of Northern Illinois University and Harold Brooks of NOAA's National Severe Storms Laboratory in Norman, Oklahoma, 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, the researchers said. 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.

Both the number of actual tornado reports and the historical STP analysis showed the eastward uptick in tornado frequency. Source: NIUThe trend is important for understanding the potential for future tornado exposure, damage and casualties. The researchers say that severe thunderstorms accompanied by tornadoes, hail and damaging winds cause an average of $5.4 billion of damage each year across the United States, and events with $10 billion or more in damages are no longer uncommon.

Previous research identified the Southeast as particularly vulnerable to tornadoes. Because of factors such as longer and larger tornado paths, expanding population density, mobile-home density and higher nighttime tornado probabilities, most tornado fatalities occur in the Southeast, particularly the mid-South region.

The study examined tornado frequency trends using two separate approaches. First, the researchers tracked the number of tornado reports from 1979 to 2017. Second, they investigated regional trends in the daily frequency of tornado-environment formation over the same time period. They did that work using an index known as the Significant Tornado Parameter (STP). 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.