Tornadoes these days tend to travel in groups, often more than a dozen in the same area on the same day. On the worst days, hundreds can form at once.
More than a dozen tornadoes were reported across the Great Plains and Midwest on Monday and Tuesday, according to the National Oceanic and Atmospheric Administration's Storm Prediction Center. On April's most active day two weeks ago, 105 tornadoes were reported.
Such outbreaks have happened before, but they have become more common in recent decades.
The total number of tornadoes that occur each year in the United States has remained relatively stable for the past few decades, but they are now occurring more frequently and more frequently on fewer days throughout the year.
From the 1950s to the 1970s, on average, about 69 percent of tornadoes in the United States occurred on days with fewer than 10 tornadoes, and about 11 percent occurred on days with 20 or more tornadoes. According to a 2019 study, these percentages have changed significantly in recent decades. Researchers found that since 2000, on average, only about 49 percent of tornadoes occurred on days with low crowds, and about 29 percent occurred on days with 20 or more tornadoes.
“Right now, when tornadoes occur, they often occur in large outbreak environments,” said Tyler Frick, assistant professor of geography at the University of Louisiana at Monroe and one of the study's authors.
Although the timing of this trend coincides with rising global temperatures, scientists are hesitant to definitively attribute tornado clustering to anthropogenic climate change.
“The link between climate change and tornadoes remains fairly tenuous,” Fricker said. “This is a really open and difficult question for us.” One problem, he says, is that tornadoes are too small on a global scale, and the global mathematics scientists use to study climate change. It is too short to appear in the model.
However, researchers can examine patterns in the historical record. Zoe Schroeder, an assistant professor of meteorology at Embry-Riddle Aeronautical University, said heavy days become busier as tornadoes occur in greater concentrations. “When outbreaks like this occur, they are often larger, which means more tornadoes,” she says.
Scientists can also study how atmospheric conditions that produce tornadoes are changing.
Jana Houser, an associate professor of meteorology at The Ohio State University, said there are two main factors contributing to tornadoes. The first is atmospheric instability caused by warm, moist air near the ground meeting cold, dry air above. The second is vertical wind shear, or changes in wind speed and direction at different altitudes.
As the climate changes, there may be fewer days when we have access to both of these foods. “But when conditions are favorable, it's almost supercharged,” Dr. Hauser said, noting that there will be more tornadoes on fewer days.
Tornadoes are also spreading farther east in the Great Plains states, which run south to north from Texas to the Dakotas, an area that people have historically considered “tornado alley.” (Central Canada also experiences tornadoes, but there are fewer of them than in the United States.)
In the past few weeks, tornadoes have struck parts of the Midwest, Appalachia, and the Southeast, as well as the Great Plains.
Spring is typically the time of year when tornadoes occur the most. “It's not at all unusual that we're having a very active season right now,” Dr. Schroder said.
There have been a total of 639 tornadoes in the United States so far in 2024. That's slightly more than average for this time of year, but far fewer than 2011, which already had 1,287 tornadoes as of May 7, the worst year on record.
However, Hauser emphasized that although there are seasonal and geographic patterns, tornadoes “can and do occur anywhere” in the United States.