The Southern Tornado Alley: Where the deadliest twisters touch down in the US

By Amy O’Kruk, CNN

The Great Plains has long been considered the country’s tornado hotbed, thanks to movies like “The Wizard of Oz” and “Twister.” But in the past two decades, a swath of the southeastern US has proven much more deadly, resulting in close to a thousand deaths in the last 25 years.

Some climate scientists call it the Southern Tornado Alley (STA): An area above the Gulf of Mexico that stretches from the western borders of Arkansas and Louisiana to eastern Georgia, encapsulating Mississippi, Alabama and Tennessee in between.

Since 1998, the area has endured over 6,400 tornadoes – fewer than the close to 7,500 over the same time period in the Great Plains Tornado Alley (PTA) – the broad expanses of American flatland in states like Oklahoma and Kansas traditionally associated with storm chasers and behemoth twisters.

But in the Southern Alley, tornadoes are uniquely dangerous and often much more fatal. Tornados have killed about 940 people in the STA compared to roughly a quarter of that, 250, in the PTA, according to a CNN analysis of National Weather Service data since 1998 when Doppler radar was implemented countrywide.

“The Southern Alley is where you get the human population to meet up with the storms. Weather that may have missed 40, 50, 60 years ago, is now hitting these areas that are populated,” said John Gagan, a science operations officer for the National Weather Service based in Sullivan, Wisconsin, who’s studied the geography of US tornadoes. “And then there’s a lot of mobile homes, and these do not fare well at all.”

Mobile or manufactured homes – homes built in factories and transported to their final destination – contribute to a staggering number of tornado deaths. The states that fall in the

STA, such as Mississippi and Alabama, have some of the highest percentages of residents living in mobile homes in the US.

The materials and design that make mobile and manufactured homes light to transport, such as thin vinyl-over-gypsum walls, make them much less sturdy during severe weather. While residents in site-built, permanent homes are offered some protection in basements or interior, windowless rooms, nowhere in a mobile home is considered safe during a tornado. The National Weather Service estimates mobile home residents are 15 to 20 times more likely to be killed compared to those in permanent homes.

A big point of failure for mobile homes can be their anchoring systems. Many mobile homes use a pier-and-beam foundation, where a home is elevated on beams and secured to the ground. If an anchoring system is installed incorrectly or degrades over time, strong winds can easily lift and toss mobile homes, leading to fatalities.

And it doesn’t take a major tornado. Tornado strength is often measured using what’s called the Enhanced Fujita Scale or EF Scale, which rates tornadoes according to six categories from EF-0 to EF-5 based on estimated wind speeds and related damage. When it comes to mobile homes, Gagan said even an EF-2 tornado can cause total destruction.

“If you live in a mobile or manufactured home, you need to have a plan and you have to be able to exact that plan quickly,” said Gagan. “Because when it gets down to it, the minutes matter.”

Tornadoes that occur in the Southern Tornado Alley are far less predictable than in the Great Plains Tornado Alley, often making them more deadly.

Seasonally, the period of high tornado risk in the PTA is short, largely concentrated in the spring from April to June and tapering off the rest of the year. In the STA, the tornado risk also spikes in the spring, but persists in unexpected months too, especially in the winter from November to January.

In the STA over the last five years, about half of the 174 total fatalities (85) happened in March and April, with the other half (82) happening between November and January. Another seven fatalities were scattered throughout the rest of the year.

Time of day plays a role, too. While tornadoes in the PTA hit fairly consistently in the afternoon and evening when people are awake and alert, they strike much more often overnight in the STA, according to the same data dating back to 1998. About 18% of strong tornadoes hit overnight between 12am and 6am in the Southern Alley compared to just 5% in the PTA.

For now, Gagan said awareness is growing about tornadoes in the Southern Alley, but there’s still work to do reaching people who are likely to be caught off guard. He said that no matter where a person lives between the Rockies and Appalachians, a tornado can happen and all it takes is one.

“A lot of times people hear Tornado Alley and they just think Kansas. They think of that Wizard- -of-Oz type picture of a rope tornado,” said Gagan. “But there isn’t one alley, there are many alleys.”

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