2019 has been an exceptionally active year for tornadoes. An article in The New York Times claims that 500 of them have been reported in a recent 30-day stretch, which is an anomaly.
The Tornado Belts
It may be taken for granted that the Midwest is generally prone to tornado activity, but fewer people know exactly which states are the most susceptible to it and why. “Tornado Alley is actually a large region of the United States that covers the states of Texas, Oklahoma, Kansas, Nebraska, North and South Dakota, Eastern Colorado, Iowa, Missouri, Illinois, and Indiana,” said Professor Eric R. Snodgrass, Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign. Another region with higher-than-average tornado statistics is called Dixie Alley. “[Dixie Alley] includes Arkansas, Louisiana, Mississippi, Alabama, Georgia, and Tennessee,” Professor Snodgrass said.
So why are these areas so tornado-prone? The Rocky Mountains hold the first part of the answer, acting as a natural wall that hinders the wind’s natural jet stream. Professor Snodgrass said that as the air comes down off the mountains, new low-pressure systems are formed in the Great Plains. Next, he said, the massive influx of warm, humid air into the central United States is lifted high into the air by unstable fronts, and they result in what Professor Snodgrass called “ideal for supercell development.” Supercell thunderstorms, the oversized anvil-shaped storms seen on the horizon, are parent storms for most tornadoes. Between the low-pressure fronts east of the Rockies, the moist air flowing into the Midwest, and the warm and cold fronts lifting the humid air up, they form a recipe for tornado-causing supercells.
Amateur weather enthusiasts and professional meteorologists alike will notice that reports of tornadoes spike between April and June, just as they have this year. Is it just the warm weather that puts tornado season in the spring? Not exactly.
“The real key for the high frequency of tornadoes during these months is in the atmosphere destabilization that occurs each spring and early summer,” Professor Snodgrass said. “Winter effectively cools the upper layers of the atmosphere, and they stay cool for some time after the spring warmth returns near the surface. Each time the ground warms up on south winds and strong solar heating, the thermodynamic profile of the atmosphere becomes very unstable.” He explained that as the year goes by, the consistent warm weather warms the upper layers of the atmosphere and leads to less atmospheric instability, which in turn leads to lessened tornado activity.
Even with early-warning systems for severe weather preparedness available to meteorologists through the use of satellites, Doppler weather equipment, and other technology, tornadoes still pose a dangerous threat to populations in affected weather-system areas. Professor Snodgrass recommends taking all precautions advised by your local or national weather service in the event of a tornado.
Eric R. Snodgrass contributed to this article. Professor Snodgrass is the Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign, where he also received his master’s degree. Previously, he earned his bachelor’s degree in Geography from Western Illinois University.