The death toll from Friday night’s destructive tornado outbreak in six states continues to rise. Fighting back tears, Kentucky Gov. Andy Beshear said Monday that there are now 64 confirmed deaths in his state alone with at least 100 more people who are unaccounted for.
While tornadoes can happen in any month of the year, an outbreak this strong in winter is a fairly freak occurrence. The season is usually missing one of the key ingredients: warm, moist air from the Gulf of Mexico that usually helps fuel outbreaks in late spring and early fall. But a tap of hot air from the Gulf intersected with a powerful storm system screaming out of the West on Friday to unleash chaos over an area spanning from Arkansas to Illinois.
Heat is one of the hallmarks of climate change. Sussing out changes in tornado behaviour driven by climate change, though, is still a relatively young area of research. Nevertheless, there are some unsettling signs that outbreaks like this one could become more common as the planet heats up.
There are a few atmospheric ingredients needed for tornadoes to form. The main thing is a clash of two opposing air masses. The U.S. is particularly well-situated for those clashes in spring and fall, with the aforementioned warm air in the Gulf and the jet stream that can usher in storms out of the Rockies. When the storms heading from west to east hit the warm air coming from south to north, it creates enough energy in the atmosphere to spin up twisters.
Winter tornadoes are relatively rarer because the warm air part of the equation is missing. Data covering 1991 to 2015 shows that the country sees just 27 tornadoes in an average December. In comparison, May sees 269 twisters on average.
Climate change, though, is changing the equation a bit by adding more heat in all seasons. And for winter, late fall, and early spring — basically, the tornado offseason — that heat is upping the odds of storms forming.
“In terms of projections under a warmed climate, there is ample evidence that increases to instability and warm moist air near the surface is fuelling increases to severe storm likelihood on the margins of the season, in the fall, winter and early spring,” John Allen, a tornado researcher at Western Michigan University, wrote in an email.
More challenging is figuring how tornado behaviour itself could change. Previous research has identified that clusters of tornadoes are happening more frequently, including work by Florida State researcher Jim Elsner. He said in an email that Friday’s severe weather “counts as a cluster.”
“We’ve looked at clustering in a few different ways, but usually 10 tornadoes with the same synoptic setting are considered a cluster,” he said. “It occurred in a year of relatively few tornadoes, so it fits the pattern of more tornadoes occurring in clusters.”
Elsner pointed to other recent research, which shows that winter tornadoes — while still a small portion of overall twisters — are becoming more common as are outbreaks with more coming all at once. That includes a significant uptick in storms in the Midwest and Southeast, where this cluster did the majority of its damage.
La Niña, a natural climate pattern, also almost certainly played a role in creating the conditions favourable for Friday’s deadly outbreak. That natural phenomenon is characterised by cooler-than-normal waters in the eastern tropical Pacific, which affects weather far from the region.
“During La Niña there tends to be more tornadoes across the Midsouth due to the positioning of the jetstream and to the warmth across the Southeast,” Elsner said. “La Niña does not cause a tornado outbreak, but it does increase the probability of one occurring. Folks tend to be comfortable with this idea but less so with the idea that although climate change does not cause a tornado outbreak, it might make them more severe (more and/or more intense).”
Ultimately, though, figuring out the exact influence of natural and unnatural factors in driving Friday’s damage is still an area of very active research. Allen said until recently, tornado modelers were limited by computing power to study the large-scale influence of climate change on small-scale weather like tornadoes, which occur over relatively small areas and for short periods of time. Heat waves and hurricanes — two longer-lasting and more widespread forms of extreme weather — have proven relatively easier to capture. But new methods have unlocked a better understanding of what’s happening — and what’s to come.
“This work,” Allen wrote, pointing to a study he co-authored that was published just last month, “also tells us that in terms of temperature — the response in fall, winter and spring is more sensitive — a 14-25% increase in severe thunderstorm environment frequency relative to the present occurs per 1 degree increase in global average temperature.”