During winter, some may assume that the threat of tornadoes fades away as blasts of frigid air plunge into the Lower 48 states and snows blanket the northern landscape. But that chill doesn’t stop tornadoes in the central and southern United States; in fact, it can be a catalyst for spinning storms as it clashes with warm, humid air from the Gulf of Mexico.
History is replete with examples of devastating December tornadoes. But the tornado rampage from Arkansas to Illinois on Friday night and early Saturday morning rose to another level, unlike anything seen in modern records.
In particular, the violence and longevity of the tornadic storm that crossed four states, from northeast Arkansas to western Kentucky, was unusual for December or any time of year, if not unprecedented.
On average, about two dozen tornadoes form in the Lower 48 states each December. Most of them occur in the western Gulf Coast region and lower Mississippi Valley. Louisiana, Texas, Arkansas and Mississippi are particularly vulnerable. December tornadoes can be violent.
In 2014, Weather.com listed the five deadliest December tornadoes, summarizing devastating events such as the Dec. 5, 1953, Vicksburg, Miss., tornado, which killed 38 people, and a 1947 storm on New Year’s Eve in northwest Louisiana that left 18 dead. The year after Weather.com published that article, a swarm of tornadoes swept from Texas to Michigan between Dec. 23 and 26. Thirteen people died in Mississippi and Tennessee during tornadoes on Dec. 23, and 13 died in Texas on Dec. 26.
Just two years ago, 40 tornadoes tore across the South on Dec. 16 and 17, killing three people.
But none of those events were as deadly or destructive as what transpired Friday and Saturday. More than 100 people are feared to have been killed, making it the deadliest December tornado outbreak on record.
Beyond the human toll, the outbreak was exceptional for several meteorological reasons.
First, there is little precedent for the path length of the quad-state tornadic storm, which carved a 250-mile course through northeast Arkansas, southeast Missouri, northwest Tennessee and western Kentucky. The storm exhibited evidence of rotation even longer, for about 11 hours and 600 miles, according to Jack Sillin, a meteorology student at Cornell University:
Here's a look at last night's most powerful supercell from a radar perspective.— Jack Sillin (@JackSillin) December 11, 2021
The storm was a discernable entity for just over 11 hours as it traveled 600 miles from AR to OH.
During much of that time, the storm exhibited a classic "hook echo" indicative of strong rotation. pic.twitter.com/kBRcAhKLyv
While it is still not clear whether the storm spawned just a single tornado or several twisters, a rotating storm of that duration is very unusual any time of year.
The tornadic storm was extreme not only for its duration but also for its intensity. Evan Bentley, a tornado specialist at the National Weather Service Storm Prediction Center, tweeted that radar data would indicate it unleashed winds of 190 to 205 mph, suggesting it would rate as an EF4 or top-tier EF5 on the 0-to-5 Enhanced Fujita scale for tornado intensity. Meteorologists at the Weather Service are surveying the storm damage to assign an exact rating; the process could take a few days.
If the storm rates as an EF5, it would join only two other December tornadoes this strong.
Bentley also tweeted that the tornadic storm was rotating at an average speed of 94 mph for four hours, while noting that published research shows “only 1.5% of all tornadoes” spin at such speeds.
Radar data also revealed that the storm lofted debris for more than three hours, which is practically unheard of. Sometimes, radar detected debris above 30,000 feet, an incredibly rare occurrence. There have been numerous reports of items hurled by the storm found more than 100 miles away.
The storm activity also moved into areas unusually far north. While tornadoes are not uncommon in Arkansas and adjacent states to the south during December, tornadoes in Kentucky at this time of year are somewhat unusual. Noah Bergren, a meteorologist in Paducah, tweeted that from 1995 to 2020 the state saw a total of just 15 December tornadoes. "[I]n the past week we have already had 6, possibly even more to be confirmed,” he wrote. “December averages our 2nd quietest month for tornadoes annually.”
The Weather Service issued 146 tornado warnings during the event, the most on record during December.
Only a highly anomalous storm environment could support such an extreme situation. Often, in December, the amount of fuel available to storms is limited, which is why violent tornado outbreaks aren’t more common. But on Friday, temperatures over the zone where the storm erupted were record-setting. The high temperatures, 20 to 30 degrees above normal, fast-forwarded the atmosphere to conditions more typical of April.
Noting: Friday's tornado environment featured record-setting warmth. Memphis hit a record high of 79F (26F above normal), joining dozens of others. The links between climate warming and tornadoes are nuanced, but this does raise questions. https://t.co/TPZLLnyfuZ pic.twitter.com/XUIMQ4hx97— Capital Weather Gang (@capitalweather) December 11, 2021
The other key ingredient for tornadoes, wind shear, or a turning of winds with altitude, was also present in high quantities on Friday, as it often is in winter. The shear is generated as the jet stream, which separates cold air from warm air, dives into the Lower 48 states.
As the highly energetic winter jet stream dived into the central states and collided with this springlike environment, the atmosphere exploded.
Many other atmospheric intricacies also contributed to the severity of the event, but it would not have been possible without the record-setting warmth.
As temperatures warm because of human-induced climate change during winter, it might mean the odds of such events increase in the future.
A modeling study published in the journal Earth’s Future last month concluded that environments conducive to thunderstorms will increase 5 to 20 percent for every 1.8 degrees of temperature increase, “with higher latitudes, particularly in the Northern Hemisphere, showing much larger relative changes.”