Tornado season is here again, with twisters striking in Indiana, Pennsylvania, and Florida over the past few weeks. But while severe storms in spring are nothing new, there have been subtle changes in tornado patterns in recent years that portend a more dangerous future for communities across the country.

According to a preliminary count from the National Centers for Environmental Information, there have been 547 tornadoes documented from January through April 2024. That figure is higher than the year-to-date average — 338 — the organization calculated between 1991 and 2020 but in line with the number observed in 2022 and 2023 in the same time frame.

And even as the number of tornadoes has stayed relatively consistent in the last few years, experts say there have been key changes in their behavior over time that could have major consequences.

More tornadoes are now concentrated in fewer days, meaning they are less spread out and there’s a higher number occurring on the same day, according to a 2019 study published in Theoretical and Applied Climatology. A growing number of tornadoes are also occurring in the southeastern part of the US in addition to the Great Plains, where they have been historically most common.

There’s still a lot experts don’t know about why both these trends are occurring and it’s not clear if climate change is playing a role. What is more certain is that these shifts mean people will have to prepare for these natural disasters in new ways, with some communities enduring more severe storms in rapid succession and others being forced to build infrastructure for tornadoes they had rarely experienced before.

What we do — and don’t — know about why this is happening

Scientists have some information about why there are more concentrated tornadoes, or clustering, and why the locations of tornadoes have shifted slightly. With clustering, it’s tied to the presence of atmospheric and wind conditions that fuel dozens of tornadoes at once. And with changes in geography, it’s related to parts of the country drying out while other areas are seeing more rain.

The increase in tornado clustering has been observed since the 1980s and continues into present day, says Tyler Fricker, an author of the 2019 study on the subject. According to that study, while 11 percent of tornadoes occurred on days when there were 20 or more tornadoes from 1950 to 1970, now 29 percent of them do. The prevalence of low-pressure systems, warm moist environments, and high wind shear (changes in speed and direction of wind combined with height) all come together to fuel these clusters.

“We are seeing a reduction in the total number of days where there are tornadoes, but those that do occur are almost ‘supercharged,’ producing substantially more tornadoes than what we would otherwise expect,” says Jana Houser, an atmospheric scientist at Ohio State University.

What’s still unknown is whether such shifts are related to climate change as the Earth has gotten warmer due to human-generated greenhouse gases.

“It’s hard to pull out the different trends — maybe the natural variations are impacting tornadoes, maybe the broader climate change, maybe it’s a combination of both,” says Jase Bernhardt, a climatologist at Hofstra University. “We want more research done to understand why it’s happening.”

Experts similarly have some sense of why tornado geography has shifted, though they’re still working to untangle the factors involved.

Environmental conditions in the last few decades, for instance, have made the southeastern parts of the US more conducive to tornado formation. And as the Great Plains dry out more and parts of the Midwest and Southeast experience increases in precipitation, that’s made the latter regions more susceptible to tornadoes, Bernhardt says.

“Tornado Alley” is historically the corridor that runs from South Dakota south through Iowa to Oklahoma and Texas. Increasingly, scientists have noticed larger numbers of tornadoes occurring eastward in places including Tennessee, which is part of a collection of southern states known as “Dixie Alley.” According to a 2018 study from the National Oceanic and Atmospheric Administration, more tornadoes were starting to emerge in the Midwest and Southeast as they simultaneously declined in the Great Plains.

Concentrated tornado days could have consequences

These shifts in tornado patterns could have serious consequences for the communities that they affect.

When many tornadoes happen in one day, people may get buffeted with natural disasters and have to anticipate multiple events in a short period of time. That presents a challenge for forecasters, who have to put out rapid-fire alerts in a condensed window, says Fricker. People in the affected areas also have to prepare for a quick series of storms and ensure that they have shelter and supplies to endure all of them.

“You might have waves of these storms coming through your area, which means you can’t just be on for an hour or two, you kind of have to be on for the entirety of that day,” says Fricker. “And so from a human and from a property perspective, outbreaks are more likely to be disruptive, and they’re more difficult to really prepare for … because there’s so much more going on.”

Changes in the geographic distribution of tornadoes could also lead to long-term effects on larger population centers, and force communities that never had to worry about tornadoes previously to shore up their infrastructure.

“As tornadoes leave the great wide open of the Plains, they will encroach on places where more people live, often in mobile homes and other structures that aren’t prepared for them,” Mark Gongloff writes for Bloomberg. “Policymakers need to help vulnerable people prepare for a potentially more dangerous future and ensure that infrastructure and housing can better withstand whatever nature brings.”

Broadly, scientists’ research can help new areas of the country make preparations as they anticipate more tornadoes down the line. Doing so effectively, however, could require more support for the Federal Emergency Management Agency (FEMA), which doles out funds for rebuilding and preparation efforts. As Scientific American reports, FEMA’s disaster relief fund is due to run out of money partway through the year, a shortfall that also existed in 2023, which led to the agency putting thousands of projects on hold.

“Given the trend toward increasing tornadoes in areas outside the traditional ‘tornado alley,’” Houser says, “people need to be on their guard in areas that may not have normally expected to see tornadoes.“