How a warming climate may make winter tornadoes stronger

NEW ORLEANS — Warmer winters could make twisters more powerful.

Though tornadoes can occur in any season, the United States logs the greatest number of powerful twisters in the warmer months from March to July. Devastating winter tornadoes like the one that killed at least 88 people across Kentucky and four other states beginning on December 10 are less common. 

But climate change could increase tornado intensity in cooler months by many orders of magnitude beyond what was previously expected, researchers report December 13 in a poster at the American Geophysical Union’s fall meeting.

Tornadoes typically form during thunderstorms when warm, humid airstreams get trapped beneath cooler, drier winds. As the fast-moving air currents move past each other, they create rotating vortices that can transform into vertical, spinning twisters (SN: 12/14/18). Many tornadoes are short-lived, sometimes lasting mere minutes and with a width of only 100 yards, says Jeff Trapp, an atmospheric scientist at the University of Illinois at Urbana-Champaign.

Over the last 20 years, tornado patterns have shifted so that these severe weather events occur later in the season and across a broader range in the United States than before, Trapp says (SN: 10/18/18). But scientists have struggled to pin down a direct link between the twister changes and human-caused climate change.

Unlike hurricanes and other severe storm systems, tornadoes happen at such a small scale that most global climate simulations don’t include the storms, says Kevin Reed, an atmospheric scientist at Stony Brook University in New York who was not involved in the new research.

To see how climate change may affect tornadoes, Trapp and colleagues started with atmospheric measurements of two historical tornadoes and simulated how those storm systems might play out in a warmer future.

The first historical tornado took place in the cool season on February 10, 2013, near Hattiesburg, Miss., and the second occurred in the warm season on May 20, 2013, in Moore, Okla. The researchers used a global warming simulation to predict how the twisters’ wind speeds, width and intensity could change in a series of alternative climate scenarios.

Both twisters would likely become more intense in futures affected by climate change, the team found. But the simulated winter storm was more than eightfold as powerful as its historical counterpart, in part due to a predicted 15 percent increase in wind speeds. Climate change is expected to increase the availability of warm, humid air systems during cooler months, providing an important ingredient for violent tempests.

“This is exactly what we saw on Friday night,” Trapp says. The unseasonably warm weather in the Midwest on the evening of December 10 and in the early morning of December 11 probably contributed to the devastation of the tornado that traveled hundreds of miles from Arkansas to Kentucky, he speculates.

Simulating how historical tornados could intensify in future climate scenarios is a “clever way” to address the knowledge gap around the effects of climate change on these severe weather systems, says Daniel Chavas, an atmospheric scientist at Purdue University in West Lafayette, Ind., who was not involved in the new research.

But Chavas notes that this research is only one piece of a larger puzzle as researchers investigate how tornados might impact communities in the future.

One drawback of this type of simulation is it often requires direct measurements from a historical event, Reed says. That limits its prediction power to re-creating documented tornadoes rather than broadly forecasting shifts in large-scale weather systems.

Though the team based its predictions on only two previous tornados, Trapp says he hopes that adding more historical twisters to the analysis could provide more data for policy makers as well as residents of communities that may have to bear the force of intensifying tornadoes.