The USA TODAY Network talked with scientists and experts and examined years of tornado data to understand how millions of Americans living in the South are at an even greater risk for tornadoes than those in the Plains. This is what we learned:
Tornadoes are occurring more often across the South than ever before.
The Enhanced Fujita scale classifies tornadoes into six categories. Our analysis focused on those rated EF1 and higher to provide better historical comparison of deadly storms. More than 60,000 tornadoes were reported across the U.S. from 1950 to 2019. More than half were EF1 or stronger.
This is the area historically known as “Tornado Alley,” spanning from Texas to South Dakota. We found that focusing on this zone fails to convey the full story of tornado danger in the United States.
Scientists are not exactly sure why some states have seen an increase and others have seen a decrease. But they do know that many tornadoes form in storms born in warm moist air off the Gulf of Mexico — and that the warming of Gulf waters is rapidly accelerating.
When lower-level Gulf winds meet cooler upper-level winds from the west, it can create instability and wind shear, key ingredients for tornadoes, according to Shawn Milrad, associate professor of meteorology at Embry-Riddle Aeronautical University.
When thunderstorms begin to form, wind shear, where winds of different directions and altitudes interact, can make the air spin on a horizontal axis. But winds alone don’t make tornadoes; something needs to tilt that spinning air.
Rising air in thunderstorms and daytime heating at the surface can create updrafts, tilting the spinning tube vertically. Warmer air rising from the surface meeting colder air above is the reason most thunderstorms happen in late afternoon. Tornadoes can form within the spinning updraft as a funnel cloud reaching up from the bottom.
Precipitation in a forward-flank downdraft ahead of the tornado and in a rear-flank downdraft function like a figure skater pulling in their arms. The downdrafts squeeze the rotating updraft to make it spin faster and form the tornado.
When comparing 2000-2019 with the previous two decades, we saw an increase in days with tornado outbreaks, or swarms — events where 10 or more tornadoes are spawned by the same weather system within a couple days.
Here’s a look at the last 40 years. Every mark represents a day when there was at least one tornado — you can see swarms showing up in the taller and lighter marks:
Though tornadoes can form any time of year, most occur from March through June. If you remove tornado data from the typical season, you can see that significant events have been occurring outside the historical season.
Overall, 20 U.S. states saw an increase in tornado activity when comparing annual data from 1980 to 1999 with 2000 to 2019. This include states outside historical “Tornado Alley” such as Alabama, Kentucky and Mississippi. Here’s a closer look at this state-level data — you can see the number of tornadoes spiking in some of the same states:
Some of the spikes include tornado swarms, such as the April 25-28, 2011, event when more than 360 tornadoes struck the U.S. The storms left more than 320 people dead across six Southeastern states.
A Dec. 16-17 2019, outbreak spawned at least 40 tornadoes across the Southeast, killing three. This event was way outside the typical tornado season.
When you compare the total number of people killed by tornadoes from 2000-2019 with the previous 20 years, many states in the Southeast saw significant increases. Many of the deaths can be attributed to major outbreaks. Alabama, Missouri, Tennessee and Kentucky had the biggest increases in deaths.
Regional risk factors contribute to increased fatalities. More people live closer together in the South than in the Plains, and that puts more people in harm’s way when a tornado develops.
“As you get east of the Mississippi, the population density increases,” said Victor Gensini, associate professor in the department of geographic and atmospheric science at Northern Illinois University.
“Tornadoes are more likely to hit things.”
Around 12:30 a.m. on March 3, 2020, an EF3 tornado ripped through Nashville, Tennessee. It was one of seven tornadoes that hit at night, killing 25 people across a heavily populated area. This tragic event highlights two other risk factors: visibility and time of day.
Most tornadoes strike between 2 and 6 p.m., so it makes sense that the high volume of tornadoes leads to more fatalities during the daytime.
But nighttime tornadoes are twice as likely to be deadly. One in 32 results in a death. If you can’t see a tornado coming, it is more likely to kill you, and even more so if you have already gone to bed.
Eastern tornadoes might also be obscured by heavy rain, and they’re more likely to be hidden from view by hilly terrain and trees.
“When most people hear a tornado warning, the first thing they do is confirm the threat,” said Stephen Strader, an atmospheric scientist at Villanova University.
“You’ve got a problem if there’s a lot of trees. That makes it very difficult to confirm the threat.”
Traditional homes can generally withstand wind speeds of 60-90 mph, but many trees cannot. They can crush homes and cars and limbs become dangerous projectiles.
A tree’s strength depends on its flexibility and the strength of both its trunk and root system. If the soil is saturated, it is less likely to withstand the forces of wind and gravity.
High winds can also overcome larger trees with issues like heart rot, a fungal disease that causes the core of a tree to decay from within. In general, a tree with more than two-thirds of its core infected is more vulnerable to high winds.
On March 25, 2021, a long-track EF3 tornado slammed central Alabama, traveling 80 miles over 98 minutes. The National Weather Service said the tornado snapped or uprooted tens of thousands of trees. Hundreds of residences were damaged or destroyed, either by wind or trees pushed over by the wind.
Even if you know a tornado is coming, you still need a safe place to ride out the storm, which can be a major problem for those living in mobile homes. And there are more mobile homes in the South than there are in the Plains and elsewhere.
Mobile homes, or manufactured housing, are typically not as structurally sound as traditionally-built homes or anchored into the ground, and they usually don’t have basements or shelters underneath them.
In January 2020, a Louisiana couple was found dead after their mobile home was demolished. The storm moved the home from its foundation to about 200 feet away.
As the Gulf of Mexico continues to warm, tornadoes may continue to expand eastward and across the calendar. And scientists will continue to study the terrifying storms, seeking ways to better understand what makes them so deadly and how to protect the communities in their path.
Read more in this series
SOURCE ‘Meteorology Today’; NOAA Storm Prediction Center; U.S. Census Bureau; Tree coverage map based on a NASA Earth Observatory map produced by Robert Simmon from data sets compiled by the Woods Hole Research Center and published Jan. 2012.
Contributing: Paul Woolverton, USA TODAY Network; and Javier Zarracina, USA TODAY