A severe weather outbreak in the mid-Mississippi Valley more than lived up to its well-predicted potential for strong tornadoes on Friday, December 10, taking lives and raking landscapes from Arkansas to Illinois. The worst toll was in Kentucky, where Governor Andy Beshear estimated on Saturday morning that at least 70 people had been killed, perhaps more than 100. Many of those deaths were in the devastated town of Mayfield (pop. 10,000), located about 20 miles east of the Mississippi River.

Friday’s tornadoes may be the nation’s deadliest and most destructive in more than a decade, since the catastrophic EF5 in Joplin, Missouri, on May 22, 2011.

At least 33 tornado reports had been catalogued by the NOAA Storm Prediction Center through Saturday morning. That number could drop over time, as some of the reports came from what may have been a single tornado – perhaps a tornado family – affecting four states, including western Kentucky. If that tornado’s death toll exceeds 100, it would put it among the nation’s 15 deadliest on record (see image below, produced by Jeff Masters).

No U.S. tornado is known to have killed more than 80 people outside the core tornado season from March to June.

Deadlest tornadoes in U.S. history, as compiled by the NOAA/NWS Storm Prediction Center. (Image credit: Jeff Masters, with SPC data.)

The vast bulk of Friday night’s destruction came from just two tornadic storms. One swept across northern parts of the St. Louis metropolitan area in Missouri and Illinois. The first tornado from this cell was given a preliminary rating of at least EF3, severely damaged part of a huge Amazon fulfillment center located in Edwardsville, Illinois. Heavy equipment was deployed in a prolonged rescue operation at the scene, where as many as 100 employees had been reported trapped. As of Saturday morning, at least one death at the Amazon facility had been confirmed by local police, according to the St. Louis Post-Dispatch.

About 100 miles to the south, an exceptionally intense and long-lived tornadic supercell thunderstorm spawned one or more tornadoes along most of a track extending more than 200 miles. The event is already being informally called the Quad-State storm, as it appears the supercell tracked from northeast Arkansas across the bootheel of far southeast Missouri into far northwest Tennessee and western Kentucky, moving across Kentucky Lake and Lake Barkley.

A nursing home in Monette, Arkansas, was heavily damaged, killing at least one resident. In Mayfield, a number of deaths appear to have occurred at a candle factory that was in operational swing-shift mode when the tornado struck, with an estimated 110 employees inside, according to Business Insider. NBC News reported that a survivor had indicated that employees were in the process of taking cover when the tornado struck.

The Quad-State event was extremely well predicted and warned, going back to a enhanced risk area issued by NOAA’s Storm Prediction Center (SPC) a day in advance, warning of the potential for strong tornadoes on Friday. A tornado watch was issued for western Kentucky at 3 p.m. CST Friday, extending to 11 p.m. The localized threat of strong, long-lived tornadoes in far western Kentucky was highlighted by SPC at 6:22 p.m. CST.

The NWS office in Paducah, about 27 miles from Mayfield, issued a “particularly dangerous situation” tornado warning for the Mayfield area at 9:04 p.m., roughly 20 minutes before the tornado reached Mayfield, noting that a “confirmed large and extremely dangerous” tornado was in progress. That warning was upgraded to a tornado emergency at 9:26 p.m., as the twister was passing through Mayfield.

An exceptionally strong and prolonged tornado

On radar, the Quad-State supercell thunderstorm exhibited some of the most violent tornadic fingerprints ever seen by meteorologists. This included debris vaulted as high as 37,000 feet and a pixel-to-pixel wind contrast possibly among the largest ever recorded in the history of National Weather Service Doppler radar.

The storm’s tornado vortex signature was virtually continuous for more than 200 miles and more than four hours. Although that signature by itself does not guarantee a continuous tornado path, it implies one or more very long-lived twisters likely occurred.

Based on the radar clues and the initial damage reports, especially from Mayfield, it seems very likely that EF4 damage occurred. It will likely take several days for National Weather Service storm survey crews to fully assess the complex track(s) left by this high-end tornado or tornado family. One of the markers of EF5 damage is a well-constructed home being destroyed and swept clean from its foundation. Naked slabs have already been found in Mayfield, but storm surveyors will need to assess whether the destroyed buildings were well built and well anchored in order to confirm EF5 damage.

“Please be patient with us,” tweeted the NWS office in Paducah.

On average, about one of the nation’s 1,000-plus tornadoes per year is rated EF5, the highest category on the Enhanced Fujita Scale. However, many years go without a single EF5. In fact, the most recent confirmed EF5 struck Moore, Oklahoma, on May 20, 2013. The subsequent EF5 “drought” of more than eight years has been the longest in U.S. tornado history, going back officially to at least 1950 and perhaps to the late 1800s.

A horrific multi-state analog

The Quad-State label alludes to the infamous Tri-State Tornado, an F5 tornado that killed 695 people (still the U.S. record for a single tornado) on a rampage from southeast Missouri to southern Indiana on March 18, 1925. Although the Tri-State Tornado was long considered to have a 219-mile path, a subsequent analysis identified many gaps in the damage path. That study concluded that a single tornado might have covered at least 174 miles of the path.

The month of December averages about two dozen U.S. twisters per year. On December 16-17, 2019, a swarm of 40 tornadoes tore across the Deep South, taking three lives. However, it’s quite uncommon for intense tornadoes to strike as far north as Illinois and Kentucky in early winter. According to Harold Brooks, of NOAA’s National Severe Storms Laboratory, the longest continuous path in meteorological winter (December to February) ripped across 122 miles of northern Arkansas on February 8, 2008. 

The closest recent analog to Friday’s long-track tornado(es) in the month of December occurred on December 23, 2015. An EF3 that tracked for 63 miles across northwest Mississippi, killing two, was succeeded within a few miles by an EF4 that covered 75 miles, killing nine.

Tornadoes in a warming climate

It’s truly shocking to see U.S. tornadoes this catastrophic in December, and yet this is a month that’s so far produced more than 3,000 daily record highs and just 11 record lows. That said, as discussed in a recent Climate Explained post, the links between tornadoes and climate change are more nuanced than for phenomena such as heat waves or extreme rainfall.

Fortunately, there is no sign that the number or intensity of the most violent tornadoes (EF3+) is increasing. However, tornadoes are becoming more tightly packed within outbreaks, and there are longer stretches in between, leading to more variability from quiet to violent periods and vice versa. Prior to Friday, the U.S. tornado death toll for 2021 was only 14, the third lowest in data going back to 1875. (The lowest on record was 10, set in 2018.)

There’s also been a distinct multi-decadal trend for recent outbreaks to shift into and east of the Mississippi Valley, particularly over the Mid-South, as opposed to the more traditional territory of the southern and central Great Plains.

Rural areas are typically more populous in the Mid-South than in the Plains, and this area is also more prone to tornadoes developing and striking after dark, a key factor in the Quad-State disaster.

As for seasonal timing, it’s never been impossible to get a violent tornado in December, even as far north as Illinois. At least two F5/EF5 tornadoes are on the record books for December: one in Vicksburg, Mississippi on Dec. 5, 1953, that killed 38 (the deadliest December tornado on record up to this year), and one on Dec. 18, 1957, that struck Sunfield, Illinois, as part of the state’s most severe outbreak on record so late in the year.

This December has been strikingly mild across most of the United States, and warm, moist surface air streamed into Friday’s tornadic storms, fueling their power. It’s not hard to imagine the springtime peak and the autumn second-season peak of tornado season edging closer to winter as greenhouse gases continue to warm our climate globally, nationally, and regionally. Such a shift in tornado timing has been difficult to confirm thus far, though.

(Addendum: The 2021 paper referenced in the tweet below uses high-resolution analyses from recent decades to provide evidence that tornado-producing environments are indeed increasing in the southern and eastern U.S. during winter, while summer may become less thunderstorm-favorable.)

Tornadoes need more than warm, moist surface air. They thrive in a complex stew of ingredients, such as strong vertical wind shear (winds that veer and strengthen with height) and a “capping” layer one to two miles above the surface that is warm and dry, but not too warm. These components are difficult to capture in forecast models, much less in long-term climate models. However, rapid progress in model development may lead to more insight into tornadoes and climate change over the next few years.

It’s certainly fair to say that Friday’s disaster should disabuse anyone of the notion that tornado “season” is limited to spring. Residents of the world’s most tornado-prone nation have to be vigilant year round, especially in a climate where winter warm spells are getting warmer.

Jeff Masters contributed to this post.

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Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...