How the growth of Twister Alley will have an effect on different southern states

The USA TODAY Network spoke to scientists and experts and examined years of tornado data to understand that millions of Americans living in the south are at even greater risk for tornadoes than those in the plains. We learned that:

Tornadoes are more common in the south than ever before.

Total number of EF1 or higher by county.USA TODAY

The Enhanced Fujita Scale classifies tornadoes into six categories. Our analysis focused on those rated EF1 and above for better historical comparison of deadly storms. From 1950 to 2019, more than 60,000 tornadoes were reported in the United States. More than half were EF1 or better.

The Enhanced Fujita Scale classifies tornadoes into six categories.

The Enhanced Fujita Scale classifies tornadoes into six categories.USA TODAY

This is the area historically known as “Tornado Alley” and stretching from Texas to South Dakota. We have found that focusing on this zone does not convey the full story of the tornado hazard in the United States.

EF1 or larger tornadoes 1950-2019.

EF1 or larger tornadoes 1950-2019.USA TODAY

Scientists aren’t entirely sure why some states have seen an increase and others a decrease. But they know that many tornadoes form in storms that occur in warm, humid air off the Gulf of Mexico – and that the warming of the Gulf waters is accelerating rapidly.

When lower-level gulf winds meet cooler, higher-level winds from the west, it can lead to instability and wind shear, which are key factors in tornadoes, according to Shawn Milrad, associate professor of meteorology at Embry-Riddle Aeronautical University.

When thunderstorms form, wind shear, where winds of different directions and altitudes work together, can cause the air to spin around a horizontal axis. But winds alone don’t make tornadoes; something has to overturn this spinning air.

Windsheer on the left can rotate the air around a horizontal axis.  Winds from thunderstorms and heating surfaces can push the horizontal air vertically.

Windsheer on the left can rotate the air around a horizontal axis. Winds from thunderstorms and heating surfaces can push the horizontal air vertically.USA TODAY

Rising air during thunderstorms and daytime warming on the surface can generate updrafts that tilt the spinning tube vertically. Warmer air rising from the surface meets colder air above and is the cause of most late afternoon thunderstorms. Tornadoes can form within the rotating updraft as a funnel cloud that extends from the bottom up.

Precipitation in a forward flank downdraft before the tornado and in a tail flank downdraft function like a figure skater retracting his arms. The downdrafts compress the rotating updraft to make it spin faster and form the tornado.

Tornadoes can form within the rotating updraft.  Precipitation in a leading flank downdraft and a trailing flank downdraft compress the rotating updraft to spin it faster and form the tornado.

Tornadoes can form within the rotating updraft. Precipitation in a leading flank downdraft and a trailing flank downdraft compresses the rotating updraft to spin it faster and form the tornado.USA TODAY

Comparing 2000-2019 to the past two decades, we saw an increase in the number of days with tornado outbreaks or swarms – events where 10 or more tornadoes are generated within a few days from the same weather system.

Here is a look at the past 40 years. Each marker represents a day that there was at least one tornado – you can see swarms appearing in the larger and lighter markers:

Tornado events since 1980.

Tornado events since 1980.USA TODAY

Although tornadoes can form at any time of the year, most occur between March and June. If you remove tornado data from the typical season, you can see that significant events occurred outside of the historical season.

A total of 20 US states recorded an increase in tornado activity when comparing the annual data from 1980 to 1999 with 2000 to 2019. This also includes states outside of historic “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:

EF1 + tornadoes by state 1980-2019.

EF1 + tornadoes by state 1980-2019.USA TODAY

The peaks include swarms of tornadoes, such as the event on April 25-28, 2011, when more than 360 tornadoes hit the United States. More than 320 people died in the storms in six southeastern states.

An outbreak from December 16-17, 2019 spawned at least 40 tornadoes in the southeast, killing three. This event was way outside of the typical tornado season.

If you compare the total number of people killed by tornadoes from 2000 to 2019 with the last 20 years, many states in the southeast recorded a significant increase. Many of the deaths are due to severe outbreaks. Alabama, Missouri, Tennessee, and Kentucky saw the largest increases in deaths.

Change in deaths by federal state in comparison from 2000-2019 with 1980-1999.

Change in deaths by federal state in comparison from 2000-2019 with 1980-1999.USA TODAY

Regional risk factors contribute to increased deaths. More people live together in the south than in the plains, and that puts more people at risk if a tornado develops.

“As you travel east of the Mississippi, the population density increases,” said Victor Gensini, associate professor in the Department of Geography and Atmospheric Sciences at Northern Illinois University.

“Tornadoes are more likely to hit things.”

Population density of the United States.

Population density of the United States.USA TODAY

At around 12:30 p.m. on March 3, 2020, an EF3 tornado swept through Nashville, Tennessee. It was one of seven tornadoes that hit at night, killing 25 people in a densely populated area. This tragic event points to two other risk factors: visibility and time of day.

Most tornadoes strike between 2 p.m. and 6 p.m., so it makes sense that the high number of tornadoes results in more fatalities during the day.

But nocturnal tornadoes are twice as likely to be fatal. One in 32 leads to death. If you don’t see a tornado coming, the more likely you’ll kill it, and even more so if you’ve already gone to bed.

Night tornadoes are twice as deadly per tornado.

Night tornadoes are twice as deadly per tornado.USA TODAY

Eastern tornadoes can also be obscured by heavy rain, and they are more likely to be obscured by hilly terrain and trees.

“When most people hear a tornado warning, the first thing they do is acknowledge the threat,” says Stephen Strader, atmospheric researcher at Villanova University.

“If there are a lot of trees, you have a problem. That makes it very difficult to confirm the threat.”

Tree coverage in the USA.

Tree coverage in the USA.USA TODAY

Traditional homes can generally withstand winds of 60-90 mph, but many trees cannot. They can crush houses and turn cars and limbs into dangerous projectiles.

Wind speeds of 60 to 90 mph can cause trees to fall over or lose limbs.

Wind speeds of 60 to 90 mph can cause trees to fall over or lose limbs.USA TODAY

The strength of a tree depends on its flexibility and the strength of both its trunk and its root system. When the soil is saturated, it is less likely to withstand the forces of wind and gravity.

Rainfalls and wet roots have a significant effect on the strength of a tree.

Rainfalls and wet roots have a significant effect on the strength of a tree.USA TODAY

High winds can also overcome larger trees with problems like heart rot, a fungal disease that causes the core of a tree to deteriorate from the inside out. In general, a tree with more than two-thirds of its core infected is more prone to high winds.

On March 25, 2021, a long-range EF3 tornado struck central Alabama, traveling 80 miles over 98 minutes. The National Weather Service said the tornado tore or uprooted tens of thousands of trees. Hundreds of homes have been damaged or destroyed, either by wind or trees knocked over by the wind.

Even if you know a tornado is coming, you still need a safe place to weather the storm, which can be a big problem for those who live in RVs. And there are more RVs in the south than in the Plains and elsewhere.

US mobile home density.

US mobile home density.USA TODAY

Mobile homes or prefabricated houses are usually not as stable as traditionally built houses or anchored in the ground, and they usually don’t have basements or shelters underneath.

In January 2020, a Louisiana couple was found dead after their RV was demolished. The storm moved the house from its foundation to about 200 feet.

As the Gulf of Mexico continues to warm, tornadoes can spread further east and across the calendar. And scientists will continue to study the horrific storms, looking for ways to better understand what makes them deadly and how to protect the communities along their path.

Read more in this series

SOURCE ‘Meteorology Today’; NOAA Storm Forecast Center; US Census Bureau; Tree cover map based on a NASA Earth Observatory map created by Robert Simmon from data sets compiled by the Woods Hole Research Center and published in January 2012.

Contributors: Paul Woolverton, USA TODAY Network; and Javier Zarracina, USA TODAY

Published 19:58 UTC June 17, 2021 pm
Updated June 17, 2021 at 21:13 UTC

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