How is climate change affecting tornadoes?

Weather

AUSTIN (KXAN) — One of the most widely-recognized weather phenomenon, tornadoes are a force to be reckoned with. But is climate change making them stronger and more frequent?

On average, the United States sees 1,200 tornadoes per year, the most of any country in the world. For this reason, the United States is home to Tornado Alley, which stretches across the High Plains from Texas to the Dakotas.

So many tornadoes occur in Tornado Alley because of its unique geography that allows all of the meteorological ingredients for tornadoes to come together. Warm, moist air from the Gulf of Mexico with hot, dry air from the Desert Southwest and cold air aloft from the Rockies all come together to create an unstable atmosphere, perfect for tornado development.

From the 1950s to the 1990s, the number of observed tornadoes doubled, but that wasn’t due to climate change — it was the result of the increased numbers of storm spotters. But most of these higher numbers were actually weaker tornadoes because the bigger and more violent ones were already pretty hard to miss.

Tornadoes are very sensitive to their local environment, so it’s difficult to link them and their parent thunderstorms to global climate trends and differentiate their normal behavior from the not-so-normal. Scientists have noticed a few signals in tornado seasons over recent decades; some may be the result of temporal variability, and some could be related to climate change.

Fortunately, global warming isn’t making violent tornadoes more common, but it is responsible for shifts in the variability, seasonality, and location of tornado threats.

Tornado seasons are getting more variable

According to 2014 study, while the annual average number of tornadoes has been relatively constant, the monthly variability of EF-1+ tornadoes has increased over the last four to five decades. It also noted a growing occurrence of record-busy and record-calm months.

Tornado outbreaks are getting larger and more frequent

With the increased variability, that means more tornadoes are occurring in outbreaks (a period of one to several days with at least six closely-spaced EF-1+ tornadoes), a 2016 study found. The study also found that the outbreaks themselves are also occurring more frequently.

Tornado Alley is shifting eastward

A 2018 study found that EF-1+ tornadoes have increased in frequency over the past 40 years from Louisiana to Missouri eastward, south of the Ohio River and west of the Appalachians. This region, often referred to as Dixie Alley, has seen many of the deadliest and most destructive tornadoes of the 21st century, including the catastrophic 2011 Super Outbreak.

The study also found that the weather conditions conducive to tornadoes displayed a similar eastward shift.

For us, the shift means that tornado-favorable conditions are less frequent over Texas and traditional Tornado Alley; however, the annual average tornado count remains higher in these states than those further east. Like the other trends, the eastward shift hasn’t been conclusively linked to a particular aspect of climate change yet.

The number of annual days on which weather conditions were favorable for tornadoes decreased across southern parts of traditional Tornado Alley from 1979 to 2020, while increasing from the Mississippi Valley across much of the Southeast (Courtesy of Victor Gensini, Northern Illinois University.)

Tornado vulnerability is on the rise

The Southeast is home to more tornado-vulnerable homes than anywhere else, and a tornado shelter can be miles away. Compared to other locations, tornadoes in the Southeast US are also more likely to strike at night, when people are asleep and tornadoes are nearly impossible to see.

Population increases are another vulnerability, even without considering climate-related changes in tornado behavior. While there’s a low-probability risk of a strike, locations such as New York City are at a surprisingly high tornado risk due to the extreme impacts a tornado could inflict on its dense, urban population.

The tornado outlook for later this century

Only a few studies have tried to investigate how tornadic thunderstorms might behave in a warmer climate, but there have been differing results. So far, results are tending to confirm previous research that severe weather will become more frequent overall.

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