When it comes to tornado outbreaks, this year is starting to crank up. Deadly tornadoes struck Louisiana on Wednesday and the atmosphere remains active into next week with more severe weather likely for Texas, Oklahoma, Arkansas, and Louisiana.
As climate change warms the atmosphere, it's still unclear how that works into the equation of tornadogenesis (or the beginning of tornadoes). Heat is energy and it would be easy to draw a conclusion that tornado outbreak numbers would go up. But the research is inconclusive on how climate change is affecting wind shear — the necessary spin that generates tornadoes. How tornadoes react to a changing climate continues to be vigorously researched close to home at Northern Illinois University and some trends are surfacing.
There has been a subtle but detectable increase in tornado risk over the past few decades. And while "Tornado Alley" remains ground zero for touch downs, there are more outbreaks east of the Mississippi River. For the sake of research, an outbreak is determined to be six tornadoes occurring in six hours.
While there are connections to climate variability and things like La Nina and El Nino, these overall trends are consistent with an eastward shift in the drier climate zone of the western U.S. Climate change projections indicate that severe storm environments will become more common in the eastern U.S.
In addition to the changing geography and number of tornadoes, there is a shift in the time of year they occur. On average, tornadoes are starting about a week earlier in the year in the "Tornado Alley" region from Nebraska to Texas, with summer tornadoes are declining nationwide. But in the colder months between November and February, tornado frequency has increased, especially in the Southeast. More troubling, nighttime tornadoes, which are more than twice as likely to cause fatalities, are more common during these colder months of the year. Given the complex, forested terrain and high density of mobile homes in the Southeast, this region is especially vulnerable to these overnight storms.
Tornado outbreaks follow the definition set by Tippett et al (2016) of six or more tornadoes in six consecutive hours nationwide. Thanks to the Michael Tippett and Chiara Lepore for providing SPC tornado count data. Tornado climatology is based on Gensini and Brooks (2018). Special thanks to Harold Brooks and Victor Gensini for their guidance with this Climate Matters.