About 1,200 tornadoes strike the US during an average year. They are common in the US – much more so than anywhere else in the world – because the geography creates the perfect conditions, especially in the spring and summer. Western winds from the Pacific Ocean drop their moisture as they push up over the Rocky Mountains, becoming high, dry and cool as they move further east. Similar winds can come from Canada. Meanwhile, low, warm, moist air flows north from the Gulf of Mexico. The flat terrain along these trails allows the winds to move relatively uninterrupted, at contrasting heights, until they meet. The angles at which they collide tend to create unstable air and wind shear, two big factors that favor tornado formation. Although somewhat similar air masses collide in other places, such as in Uruguay and Bangladesh, the armed forces are much more powerful than the US. Canada ranks second worldwide with 100 twisters per year.
Although tornadoes touch down in many places in the eastern half of the country, from the 1950s to the 1990s they hit most often in Tornado Alley, an oval region in northeastern Texas and south-central Oklahoma. More recently, that focus has shifted 400 to 500 miles to the east. Over the past decade or so, tornadoes have become prevalent 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 winds of 170 miles per hour struck Rolling Fork, Miss., on March 24, killing at least 26 people. A week later, storms in the new tornado lane killed more than 30 people, and another group damaged more than 80 buildings in Bollinger County, Missouri, on April 4. Those events took place leading up to the high season in April and May.
Data collected over the past two years shows that in addition to solo storms, major tornado outbreaks — multiple tornadoes spawned by a single weather system — are shifting even more definitively eastward. The swarms also gather in a tighter geographic area than the old Tornado Alley. And outbreaks can become more intense and frequent. “We seem to have fewer days in the U.S. with just one tornado and more days with multiple tornadoes,” said Naresh Devineni, an associate professor at the City University of New York who co-led a 2021 geographic analysis of major tornado outbreaks.
Why is this shift happening now? Usually, tornadoes are caused by a supercell – a strong thunderstorm with a rotating updraft. Supercells tend to form when warm, moist air at a low level interacts with cool, dry air at a higher level, and climate change creates warmer, moister air. Tornadoes are also more likely to develop if the local atmosphere is unstable, “and warming increases instability,” said Zuohao Cao, a tornado expert with Environment and Climate Change Canada who co-led a recent study of storm landing sites. Climate change is also warming the Gulf of Mexico, which could send large amounts of water vapor into the southeastern US
Research shows that the so-called dry line is, too shift eastward. The imaginary line runs north from the US-Mexico border all the way to Canada, separating the wetter eastern US from the drier western US (thirsty crops such as corn predominate in the east; drought-tolerant wheat predominates in the west). , which has descended approximately along the 100th meridian for centuries, has moved eastward by 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 colder air that can fuel storms,” Ernest Agee, professor emeritus of atmospheric sciences at Purdue University, wrote in the conversation in 2022.
Climate change can also extend the typical tornado season. Milder winters mean the unstable air masses that supercells can create become more likely in March or even earlier in the southeastern US.
Tornado Alley moving east is more than a meteorological curiosity. The shift is serious: Tornado shelters are common in Texas and Oklahoma, but less so elsewhere. The Southeast is more densely populated and mobile homes, which fare poorly in storms, are much more common. Tornadoes in the Southeast are also more likely to occur at night than further west, in part because winds can pull enough 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 bulls-eye region may want to improve community shelters and warning systems, strengthen building codes, better equip emergency responders, and educate residents about what to do and what not to do if a tornado is headed their way.