About 1,200 tornadoes hit the United States in an average year. They are prevalent in the United States, more so than anywhere else in the world, because its geography creates the perfect conditions, especially in spring and summer. Westerly winds from the Pacific Ocean lose their moisture as they push over the Rocky Mountains, becoming strong, dry, and cool as they move farther east. Similar winds can descend from Canada. Meanwhile, low, warm, moist air is flowing north from the Gulf of Mexico. The flat terrain along these paths allows the winds to move relatively uninterrupted, at contrasting elevations, until they meet. The angles at which they collide tend to create unstable air and wind shear, two important factors that favor the formation of tornadoes. Although somewhat similar air masses clash in other locations, such as Uruguay and Bangladesh, the forces are much stronger over the United States. Canada ranks second in the world with 100 tornadoes per year.
Although tornadoes to land in many places in the eastern half of the country, from the 1950s through the 1990s, they struck most often in Tornado Alley, an oval area centered in northeast Texas and south-central Oklahoma. More recently, this concentration has shifted 400 to 500 miles east. Over the past decade, tornadoes have become common in eastern Missouri and Arkansas, western Tennessee and Kentucky, and northern Mississippi and Alabama, a new region of concentrated storms.
Tornado activity in early 2023 epitomized the trend. A violent tornado with wind speeds of 170 miles per hour hit Rolling Fork, Mississippi, on March 24, killing at least 26 people. A week later, storms in New Tornado Alley killed more than 30 people, and another cluster on April 4 damaged more than 80 structures in Bollinger County, Missouri. These events happened just before the high season in April and May.
Data collected over the past two years show that in addition to solitary storms, major tornado outbreaks– multiple tornadoes spawned by a single weather system – are moving even more definitely eastward. The swarms are also clustering in a narrower geographic area than in the old Tornado Alley. And outbreaks can become fiercer and more frequent. “It looks like we have fewer days in the United States with a single tornado and more days when there are multiple tornadoes,” says Naresh Devineni, an associate professor at the City University of New York who co-led a geographic analysis of 2021. large outbreaks of tornadoes.
Why is this change happening now? Most often, tornadoes are created by a supercell – a strong thunderstorm with a rotating updraft. Supercells tend to form when warm, humid, low-altitude air interacts with cool, dry high-altitude air, and climate change generates warmer, more humid air. Tornadoes are also more likely to develop when the local atmosphere is unstable, “and warming increases instability,” says Zuohao Cao, a tornado expert at Environment and Climate Change Canada, who co-led a recent study on places to touch storms. Climate change is also warming the Gulf of Mexico, which can send generous amounts of water vapor into the southeastern United States.
Research suggests that the so-called dry line is also moving east. The imaginary line runs north from the US-Mexico border to Canada, separating the wetter eastern United States from the drier western United States (to the east, thirsty crops such as maize predominate; in the west drought-resistant wheat prevails.) The line, which for centuries fell roughly along the 100th meridian, has moved east about 140 miles since the late 1800s. The dry line “may be a boundary for convection – the rising of warm air and the sinking of cooler air that can fuel storms,” wrote Ernest Agee, professor emeritus of science atmospheric studies at Purdue University, the conversation in 2022.
Climate change may also extend the typical tornado season. Milder winters mean unstable air masses that can create supercells could become more likely in March or even earlier in the southeastern United States.
Tornado Alley moving east is more than a weather oddity. The change is serious: Tornado shelters are common in Texas and Oklahoma, but less so elsewhere. The southeast is more densely populated and mobile homes, which do not withstand windstorms well, are much more common. Tornadoes in the southeast too happen at night more often than they do further west, in part because the winds can bring lots of moisture from the gulf after dark. Studies show that tornadoes that strike at night are 2.5 times more likely to cause fatalities.
Local and state governments in the new target center region may want to improve community shelters and warning systemsstrengthening building codes, better equipping emergency responders, and educating residents on what to do — and what not to do — if a tornado is headed their way.