Few people expect tornadoes in December, let alone one strong enough to collapse the 11-inch-thick concrete walls of a sprawling, new Amazon warehouse.
But the EF3 tornado that wrecked the facility near Edwardsville last month — one of several twisters that caused widespread damage in six states and more than 90 deaths — could be a sign of things to come.
While most people associate intense tornado outbreaks with spring, weather experts say both the timing and location may be changing.
Two decades ago, December tornadoes — if they occurred — plowed through fields and homes from eastern Texas to northern Florida, according to data from the National Oceanic and Atmospheric Administration, or NOAA. At that time, fewer storms occurred overall, and even fewer hit the nation’s traditional “tornado alley” — Kansas, Nebraska, Oklahoma and Texas. Recent unseasonable twisters tend to touch down in Mississippi, Alabama, Arkansas, Tennessee, Missouri, Illinois and Kentucky.
Scientists and those who study the phenomena agree that tornado outbreaks are moving east.
“Research has identified evidence of a ‘Dixie Alley,’ which represents an eastward extension of the traditional ‘Tornado Alley’ in the central Great Plains,” wrote Harold Brooks in a 2018 study on tornado spatial trends. Brooks studies tornadoes in NOAA’s National Severe Storms Laboratory, and said St. Louis has one of the most remarkable tornado histories of any place in the country.
In general, tornadoes are not happening more often — except in December. Professors and experts offer several reasons.
Tornadoes need two main ingredients: Warm air, and rapidly changing winds at different levels of the atmosphere, known as wind shear. Geographies with recent bursts in December tornadoes often have wind shear, William Gallus, professor of meteorology at Iowa State University at Ames, Iowa, said, but not the warm air that causes atmospheric instability. That’s when a parcel of air is warmer than surrounding air, at the same elevation, causing the parcel to rise.
He and colleagues were shocked by the dew points produced in Memphis and Kentucky in the recent tornado-causing systems. The one that passed through Iowa on Dec. 15 broke the record for single-day tornadoes in the state, Gallus said. And that system produced tornadoes in parts of the country that previously had no December tornadoes.
A link between climate change and an increase in extreme weather seems natural to some, such as Gallus. A warming planet means an often-warmer Gulf of Mexico, he said. Disturbances in the jet stream can result and can spur tornadoes — especially in December.
Yet others think different weather patterns, like La Nina, which causes wet Midwestern conditions to meet warm Southeastern conditions, are the cause. John Allen, professor of meteorology at Central Michigan University in Mount Pleasant, Michigan, said La Nina predicts a higher chance of severe winter weather, and consistently. The U.S. is experiencing a La Nina weather pattern this winter.
The most recent complete decade of data available from NOAA, 2010 through 2019, shows 221 December tornadoes rated at the Enhanced Fujita scale of 1 or higher occurred in the U.S., compared with 143 from 2000 to 2009, and just 78 during the 1990s. The majority of December tornadoes were rated at EF2 or lower, meaning weaker storms, but the percentage of strong or violent tornadoes increased to 10% from 4% between the past two decades.
And, in Midwestern-Southeastern states, excluding Kansas, Oklahoma and Texas — also known as “tornado alley” — December tornadoes increased to 189 twisters, up 78% from 106.
The NOAA tornado data has limitations, said Jana Houser, a professor of meteorology at Ohio University in Athens, Ohio. The first two decades were retroactively recorded using newspaper archives and other reports, Houser said. There are now more people to witness and report tornadoes, and more methods for them to report more easily.
In 2007, NOAA changed rating systems for tornadoes, shifting from only measuring damage after a storm to measure its strength to measuring wind speed and damage. Brooks said the change caused inconsistencies in the strength rating of storms throughout the data.
In their paper, Brooks and his co-author note the geographic trend could be a result of techniques to smooth reports over time and space.
Still, the number of December tornadoes continue to rise.
In December this year, strong tornadoes turned up the Midwest twice. The first aggressive system, on Dec. 10, affected Missouri, Illinois and Kentucky and killed eight people. The vicious winds blew homes off foundations and an EF3 tornado collapsed part of an Amazon warehouse near Edwardsville, killing and injuring workers trapped under concrete walls.
Meanwhile, a twister from the same system carved a 200-mile path through Kentucky, and potentially broke a 100-year-old record for time on the ground. The city of Mayfield, with a population of about 10,000 people, was devastated, with shredded trees and downed power lines littering the landscape. Windows and roofs were blown off buildings that were still standing. More than 70 were pronounced dead, and authorities are still working on recovery.
Five days later, a second storm system caused hurricane-force winds as far north as southeastern Minnesota and killed a man there, when a 40-foot tree blew into him outside of his home. Another person died when a semitrailer was blown on its side in Iowa; three others died in Kansas, where vehicle crashes resulted from high dust and low visibility.
Totals from all states that were hit reached close to 90 tornadoes, Gallus said.
On New Year’s Eve in 2010, an EF3 struck the St. Louis region, leveling homes, at least one business and knocking down utility poles in Sunset Hills. The storms continued and spurred tornadoes to touch down in north St. Louis before moving into southern Illinois.
“The weather was eerie, like it is now — warm — and just not normal for December,” said Pat Fribis, now mayor and then-alderman of Sunset Hills. “We were just shocked.” Fribis remembers looking at a house that had its whole facade taken off.
“It was like you were looking at a dollhouse,” she said. No one was hurt in the south St. Louis County community, but Fribis called the damage devastating.
Political issues stemming from the tornado caused Ann McMunn to run for office. She’s now an alderman representing the 1st Ward in Sunset Hills and recalls the New Year’s Eve tornado like it was yesterday. She was home alone in her three-bedroom house when a tree fell through the house and the storm leveled most of her street.
She remembers hearing two tornado sirens, at first. She ignored them until the third one went off, when she peered outside, and saw a green sky and rain falling sideways. She called her mother, who was at a Schnucks in Crestwood. Minutes later, she stood in the basement, still on the phone, when the tornado hit.
“There was a moment I was talking to my mom, and I couldn’t catch my breath,” McMunn said. “I said to my mom, ‘I think it must be right over me.’”
In 2000, an early December tornado in Alabama, near Tuscaloosa, killed 11 people and destroyed an entire shopping center before moving on to level homes. The storms overturned vehicles and injured 144.
“Ironically, the tornado dissipated as it moved into an open, unpopulated area,” the National Weather Service reported.
Have a plan
To understand the how and the why with December tornadoes, more research is needed. But collecting data on one of the fastest-moving and random weather events isn’t exactly easy, Gallus explained.
“I would have to be a trillionaire, and cover the entire country with weather instruments,” Gallus said. Even then, they’d likely get blown away before much meaningful data could be gathered.
Meteorologists can only recommend a few things to those potentially in the paths of unseasonal twisters: Prepare, have a plan and have a safe place to go.
To prepare, residents can build substantial shelters. And for those with bad knees, the space doesn’t have to be a basement.
Brooks has an above-ground shelter in his residence, with concrete walls reinforced by steel bars every six inches throughout, and a steel door.
Companies like Amazon can redesign their warehouses to make a safe space large enough for all employees.
“It’s hard to reinforce a building that size, but you can make smaller shelters for your entire population,” Brooks said. “You essentially spend the small amount of money to make part of the building really safe.”
In 2021, before the year-end data collection and cleanup, Gallus said the number of December tornadoes had already reached 160, from his tally of weather station reports around the country.
“It is probably safe to say the increase will continue in the decade 2020 to 2029,” Gallus said.