A shift and an uptick in tornadoes

U.S. tornado hot spots are shifting from the Plains to Midwest and Southeast, a study finds.

Tornado frequency has increased across the eastern third of the country, and especially across the mid-South, according to a new study in the journal Climate and Atmospheric Science.

While tornadoes have increased in the East, there has been a notable decrease in twister activity across a large chunk of the southern plains of Texas and Oklahoma as well as the high plains of Colorado. These focal zones are among a broad downward trend across the Plains — historically known as tornado alley.

These are problematic changes, given the tendency for the shift to place increased tornado activity over a particularly vulnerable region, the study says. It found the starkest increase in tornado frequency across a region including Alabama, Arkansas, Mississippi, Missouri, Illinois, Indiana, Kentucky and Tennessee.

This was already the deadliest terrain for tornadoes in the country on average, given multiple factors, such as greater population density than areas farther west. There are also more trees and less visibility, more weak dwellings and a tendency for the development of fast-moving storms that spawn nighttime tornadoes, which are the deadliest.

The authors also point to increased variability in the year-to-year number of tornadoes.

The 2010s have demonstrated this: 2010 was active, then 2011 was the year of the “super outbreak,” among many tornado events. Since then, a majority of tornado years have been relatively quiet. That’s the theme of 2018, as it threatens to become the first in modern history with no recorded tornadoes rated EF4 or higher.

The study was authored by Victor Gensini and Harold Brooks, authoritative figures on the subject of severe weather and tornadoes. Gensini is a professor of atmospheric sciences at Northern Illinois University, and Brooks is a scientist at the federal government’s National Severe Storms Laboratory in Oklahoma.

For a number of years, ideas about shifting tornado frequency — a nudging to the east of the traditional hottest spots in the Plains — have floated around, but little hard data has supported them.

“(We) believe these trends in tornado environments are significant and have not been documented with this level of detail by previous research,” Gensini and Brooks write in the study.

The authors used a metric known as the Significant Tornado Parameter, or STP, to approximate tornado activity over the years in their study (1979-2017). It is a complicated index of twister ingredients that measures the potential potency of a tornado environment.

Gensini told The Associated Press that while “we don’t know” the cause of the shifts seen, “(this) is super consistent with climate change.”