A devastating outbreak of tornadoes swept through parts of the Midwest and Tennessee River Valley Friday night, and is poised to become the worst on record to strike the United States during December.
Kentucky Democratic Gov. Andy Beshear said he expects the death toll in his state alone to reach at least 70 to 100 people. If that’s the case, it will become the deadliest December tornado outbreak on record and among the 15 deadliest in any month.
“Last night was one of the most shocking weather events in my 40 years as a meteorologist — a violent tornado (in December!) drawing comparisons to the deadliest and longest-tracking tornado in U.S. history,” tweeted Jeff Masters, a meteorologist and expert on extreme weather.
The National Weather Service received 37 reports of tornadoes in six states Friday. The most destructive tornado, or series of tornadoes, carved an approximately 250-mile path through northeast Arkansas, southeast Missouri, northwest Tennessee and western Kentucky.
Should it be confirmed as single, continuous tornado rather than a series of twisters, the “quad-state tornado” would rank as the longest-track tornado in U.S. history and the first to track across four states.
The National Weather Service reported that the storm lofted debris into the sky for over three straight hours while declaring a tornado emergency in eight different locations; such alerts are only issued when a large, violent tornado threatens highly-populated areas.
As the tornado blasted through Mayfield, Kentucky, it sheared entire homes off their foundations and lofted debris more than 30,000 feet into the air, indicative of a twister of top-tier intensity. The Weather Service will survey damage over the weekend to assign the tornado a specific rating on the zero-to-5 Enhanced Fujita (EF) scale.
In addition to the quad-state storm, destructive and deadly tornado activity erupted in Illinois and there was one report of a twister in northern Mississippi.
More than 18 million people were under tornado watches amid the outbreak while the Weather Service issued 146 tornado warnings, the most on record during December.
Fueled by record-setting warm temperatures, the disaster was unprecedented in many ways for the time of year and is raising questions about the possible role of human-caused climate change.
The event was triggered by a powerful low-pressure system, which lifted from the Southern Plains into the Great Lakes. The low-pressure intensified as the polar jet stream, the high-altitude current along which storms track, plunged into the central U.S. ahead of a pulse of record-setting warmth.
As a cold front attached to the zone of low pressure encountered the exceptionally warm, unstable air Friday evening, numerous thunderstorms erupted.
The rotating thunderstorm that spawned the quad-state tornado(es) developed from a cluster of storms over central Arkansas. These storm cells congealed to the east of the cold front, separate from any other storms that could compete for available energy or disrupt it.
While passing through northeast Arkansas, the quad-state storm encountered an increasingly favorable tornadic environment, with extreme spin-inducing low-level winds. These ingredients remained in place as it tracked about 250 miles into western Kentucky.
The Weather Service in Memphis, Tennessee, issued the first tornado warning for the storm at 7:06 p.m. Central time, stating that a “confirmed large and extremely dangerous tornado” had formed. The storm moved over Monette, Arkansas, where it severely damaged a nursing home, at 7:24 p.m. Central time.
“This is a PARTICULARLY DANGEROUS SITUATION,” the Weather Service wrote. “TAKE COVER NOW.”
Meteorologists observed an intense velocity “couplet” on radar, in which air racing in one direction clashes with air racing in another. A sign of rotation, the couplet seen on the quad-state storm was among the strongest observed by modern radar.
The storm may have “cycled” once while passing over northwest Tennessee, a process in which the initial tornado dissipates and a new one develops. But by the time storm hit Mayfield at 9:27 p.m., the radar signature was once again indicative of a devastating tornado.
Aside from the extreme rotation indicated by radar wind data, a dramatic debris signature was seen as the storm approached the Kentucky city.
Not until after 11:30 p.m., when the storm was southwest of Louisville, did it stop rotating.
While intensity estimates could take many days to finalize, the radar signature was at times indicative of a violent twister, or an EF4 or EF5.
If rated an EF5, it would end a record eight-year drought in such top-tier tornadoes. It would be the first EF5 since the devastating Moore, Oklahoma, tornado on May 20, 2013, and the first in December since 1957.
If the tornado is rated at least EF4, it would be the first in the region in December since 2015 when a violent twister hit near the border of Mississippi and Tennessee. It would also be just the second EF4 tornado of 2021 in the United States; a typical year produces seven EF4 or stronger tornadoes.
There have only been 59 tornadoes prior with path lengths of 100 or more miles since 1950. That number shrinks to 12 for path lengths of 150 miles or greater. During the Super Outbreak of 2011 there were three such tornado paths, the last such instances.
The lengthiest paths on record top 200 miles, with caveats. A 235-mile track in March 1953 is the modern record holder, according to data from the Storm Prediction Center data. Since none of these tornadoes have happened since 1971, it is certainly possible some of them were tornado families, or cyclic tornadoes forming one after another, rather than one long-track event.
The 219-mile-long Tri-state Tornado of March 1925 is also considered the lengthiest tornado path by many sources. The deadliest tornado in U.S. history, it killed 695 people as it passed through Missouri, Illinois and Indiana during the overnight hours.
This event’s path length could exceed that. However, Rick Smith, a meteorologist at the Weather Service office in Norman, Oklahoma, tweeted that the quad-state tornado was more likely a series of twisters than just one. On-the-ground surveys to sort this out may be “a complex process that could take some time,” Smith wrote.
Even if the damage was caused by more than one tornado, the longevity of the intense supercell is unusual for any time of year, and likely unprecedented for December.
Victor Gensini, a professor of meteorology at the Northern Illinois University who has published studies on links between climate change and severe storms, said in an email that events like this may become more common in a warming world.
“[I]t’s absolutely fair to say that the atmospheric environments will be more supportive for cool-season tornado events,” he wrote in an email.
Meteorologists fear nighttime tornadoes the most, as they are the most deadly. People may be sleeping or lack the visual cues of twisters that approach during daylight. Considering the likelihood of at least 70 fatalities, Friday night’s outbreak was a testament to the peril of nocturnal storms.
Before Friday night, only 14 tornado fatalities had occurred in 2021, a below-average number. Previously, the deadliest December tornado occurred on Dec. 5, 1953, when 38 people were killed in Vicksburg, Mississippi.
Both of these numbers are expected to be easily surpassed. And this tornado event is likely to be the deadliest in Kentucky history.
It appears likely that this tornado will be the deadliest since the infamous Joplin tornado in May 2011 which killed 158 people.
In the modern record, since 1950, only 10 tornadoes have caused 50 or more deaths. Three of these happened in April 2011. Before that they all occurred before 1971.
The high death toll for this event occurred despite timely warnings from the Weather Service.
Roger Edwards, a tornado expert at the National Weather Service, tweeted that a significant death toll was practically unavoidable given the tornado’s ferocity.
“When a violent wedge tornado engulfs a densely populated area, devastation is assured. Even the very best warnings only minimize, not eliminate, casualties,” he wrote.