These days, tornadoes tend to move in groups, with more than a dozen often forming in the same area on the same day. On the worst days, hundreds can form simultaneously.
More than a dozen tornadoes were reported across the Great Plains and Midwest on Monday and Tuesday of this week, according to the Storm Prediction Center operated by the National Oceanic and Atmospheric Administration. Two weeks ago, on April's most active day, 105 tornadoes were reported.
Outbreaks like this have always happened, but have become more common in recent decades.
The total number of tornadoes that occur in the United States each year has remained relatively constant over the past several decades, but they now occur more intensively over fewer days of the year.
From the 1950s to the 1970s, an average of about 69% of U.S. tornadoes occurred on days with fewer than 10 tornadoes, and about 11% occurred on days with 20 or more tornadoes. A 2019 study found that these rates have changed significantly in recent decades. The researchers found that on average since 2000, only about 49% of tornadoes have occurred on less busy days, and about 29% have occurred on days with 20 or more tornadoes.
“Now when tornadoes occur, they often occur in the environment where they occur,” said Tyler Fricker, an 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 the clustering behavior of tornadoes to human-induced climate change.
“The link between climate change and tornadoes is still very weak,” Dr. Fricker said. “It’s a really open and difficult question for us.” One difficulty is that tornadoes are too small and transient on a planetary scale to show up in global mathematical models that scientists use to study climate change.
But researchers can look at patterns in historical records. Busy outbreak days are getting busier as more tornadoes come together, according to Zoe Schroder, assistant professor of meteorology at Embry-Riddle Aeronautical University. “When these outbreaks occur, they often become larger. That means more tornadoes,” she said.
Scientists can also look at how atmospheric conditions that cause tornadoes change.
Jana Houser, associate professor of meteorology at Ohio State University, said tornadoes have two main ingredients. The first is atmospheric instability, which occurs when warm, moist air close to the ground meets cold, dry air above. The second is vertical wind shear, i.e. the change in wind speed and direction at different altitudes.
As the climate changes, the number of days you consume both of these ingredients may decrease. “But when we have favorable conditions, they almost supercharge, producing more tornadoes in fewer days,” Dr. Houser said.
Tornadoes are also spreading further east in an area people historically thought of as “tornado alley,” a region of the Great Plains that runs south to north from Texas to the Dakotas. (Tornadoes also occur in central Canada, but less frequently than in the United States.)
Over the past few weeks, tornadoes have struck the Great Plains, as well as parts of the Midwest, Appalachia and the Southeast.
Spring is typically the most tornado-prone time of year. “It’s no wonder we’re having a very active season right now,” Dr. Schroder said.
As of 2024, there have been a total of 639 tornadoes in the United States. That's slightly higher than average for this time of year, but far less than 2011, the worst year on record, when there were already 1,287 tornadoes as of May 7.
But Dr. Hauser emphasized that although there are seasonal and geographic patterns, tornadoes “can and do occur anywhere” in the United States.