Swarming tornadoes

January 4, 2021

The violent winds of tornadoes are one of the most dangerous natural hazards on Earth. While relatively rare in any one location they often occur in swarms. I’ve been working with graduate student Zoe Schroder to answer the question of whether or not climate change is making tornado swarms worse, and if so, how.

Tornadoes can occur any where in the United States but they are most common east of the Rocky Mountains (see Figure above). They form from giant thunderstorms primarily during the months of March through May. These giant thunderstorms are capable of producing very destructive long lasting tornadoes. These long-lasting thunderstorms are different than the summertime thunderstorms here in Tallahassee, which usually rain themselves out in less than an hour or so.

Swarms of tornadoes are also most likely during the spring when warm, moist air originating over the Gulf of Mexico collides with the still cool air across the Rockies. This occurs over a large swath of real estate from Kansas to Alabama and sets the stage for giant thunderstorms. The juxtaposition of two air masses with different temperature and humidity characteristics produces an environment that is unstable in the presence of wind shear. The largest tornado swarm in recent memory occurred on April 27, 2011. It produced 173 tornadoes that killed 316 people and led to millions of dollars in property damage across a wide swath of the Southeast. Our research shows that tornado swarms are responsible for a large percentage of all tornado-related fatalities. Our statistical models show that tornado swarms have about 10% more tornadoes and 50% more casualties when the instability increases by a 1000 J/kg and when the shear increases by 10 m/s.

Our models quantify the influence the environmental factors have on the probability of casualties (deaths and injuries) as modulated by the number of people in harms way. The probability of a large number of casualties increases with increasing wind shear and increasing instability and those increases depending on population. They also quantify the decline in the number of casualties per year and indicate that swarms have a larger impacts in the Southeast than elsewhere after controlling for population and geographic area.

While we know the ingredients needed for tornado swarms, we are still trying to determine how these ingredients might be different as the planet continues to warm. Future work will quantify changes in tornado swarm environments using climate change variables such as global sea surface temperatures and arctic sea ice. Through these models we should be able to better understand how climate change is influencing the risk of life and property from killer tornadoes.