US is experiencing more tornado outbreaks, despite fewer tornado days overall, researchers say

Although the number of days that there is a tornado in the U.S. is decreasing overall, the average annual number of tornadoes has remained roughly the same, according to the latest research.

Last week, more than 125 tornadoes were reported in the U.S. over a five-day period across states including Iowa, Missouri, Georgia and Mississippi – a continuation of a trend that shows more tornado outbreaks despite fewer tornado days, meaning days during which at least one tornado is reported, experts told ABC News.

The variability of tornado occurrences in the U.S. has increased over recent decades, in that there are now fewer days in which a tornado forms. However, there are more tornadoes on those days, Harold Brooks, senior research scientist on the climatology of severe thunderstorms and tornadoes for the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storm Laboratory, told ABC News.

Because of this, the total number of tornadoes over the course of a year, on average, has remained unchanged, Brooks said.

Researchers describe the shift as "a decline in the number of tornado days per year" but an "incline in the number of tornado outbreaks," Robert Trapp, professor of atmospheric sciences at the University of Illinois Urbana-Champaign and director of the School of Earth, Society and Environment, told ABC News.

New data from an upcoming paper that studies tornadoes through 2022 shows that this dichotomous trend is continuing, Trapp said. The new study also shows that the greatest number of tornado days are lost during the warmest months, particularly June, July and August.

"It's a pretty striking trend in in the long term," Trapp said.

Climate change could be affecting tornado behavior, Brooks said. As average global temperatures increase, they provide one of the main ingredients that allow tornadoes to form, Brooks said – namely, energy available for storms. A warming planet could create more favorable conditions for tornadoes to form because more moisture and atmospheric instability essentially fuels the storm systems in which tornadoes develop.

However, the changes of tornado behavior during the cool season – November, December, January or February and even early March – could be the biggest indicator to the impacts of climate change, Trapp said.

"When I think about how I explain the potential impact of climate change on tornado seasons, it's the cool season that really stands out," he said.

Data shows an increase of tornado activity during the cool season in the Southeast U.S., including in Mississippi, Alabama, Tennessee, Georgia, Kentucky and parts of southern Indiana. This is significant because the source of energy for the thunderstorms in which tornadoes form is typically absent in colder weather, Trapp said.

Climate change may also be expanding the geographic area of what is traditionally considered to be the so-called Tornado Alley, where the storms most commonly form, recent research shows.

Tornado frequency is slightly declining in the Great Plains and increasing in the East, a 2018 study by NOAA and Northern Illinois University found. The authors of the study added that further research is needed to determine for sure whether the shift is specifically driven by natural or human-induced climate change.

Additionally, analysis from Climate Central, a non-profit climate science communication organization, shows that since 1979, the frequency of favorable tornado days per decade has increased significantly in parts of Mississippi, Alabama, Arkansas, Tennessee, Kentucky, Missouri, Illinois, and Indiana, with a decrease observed in states like Oklahoma and Texas. The number of days conducive to severe weather is projected to increase as the climate continues to warm.

A large part of the increase in variability in tornado behavior is related to the increase in tornado occurrences in the mid-South region, Brooks said.

In addition, the geographic location with the most frequent tornado activity tends to change year over year, said Jana Houser, associate professor of meteorology at Ohio State University. One year, the Southeast may get slammed with tornadoes, while another year may see the activity concentrated further west, she said.

As the geographic location of where tornadoes tend to strike changes, so does the destructive potential of the twisters.. Many of the places where tornadoes are now occurring have much larger population densities and include infrastructure that is not durable enough to withstand tornadoes, such as mobile homes, the experts said.

Places like Mississippi and Alabama have a lot more people living in rural areas than do places in the Great Plains, like the western parts of Kansas and Oklahoma, Brooks said. There is also a lot more poverty and socially vulnerable populations in those Southern states, Brooks said.

Tornado seasons typically are not as well-defined as hurricane season is, Brooks said, adding that it is difficult to define trends for tornadoes because they're considered "rare events" at every location they strike.

Around the 1970s, the peak period for tornado activity shifted to earlier in the year, from mid-June to the last quarter of May, Trapp said. Last month, however, was the busiest April for tornado activity in the U.S. since 2011, even as the U.S. is now approaching the peak of strong and violent tornado occurrences, Brooks said.

Due to the unpredictable nature of tornadoes and the increase of extreme weather events overall due to climate change, residents need to be prepared for natural disasters more than ever before, American Red Cross spokesperson Stephanie Fox told ABC News in a statement.

"The climate crisis is forcing the American Red Cross to respond to nearly twice as many large disasters as we did a decade ago," Fox said, pointing to the organization's new climate crisis initiative that can help communities on the frontlines of extreme weather build resiliency.