A deadly late-season tornado outbreak, the deadliest on record in December, produced catastrophic damage and numerous fatalities across portions of the Southern United States and Ohio Valley from the evening of December 10 to the early morning of the 11th, 2021. The event developed as a trough progressed eastward across the United States, interacting with an unseasonably moist and unstable environment across the Mississippi Valley. Tornado activity began in northeastern Arkansas, before progressing into Missouri, Illinois, Tennessee, and Kentucky.
The most extreme impacts resulted from two long-track supercell thunderstorms that produced families of strong tornadoes. The first of these supercells produced tornadoes spanning four Mid-South states. The first notable tornado of the event began in northeastern Arkansas, near Jonesboro, causing major damage in and near towns such as Monette and Leachville, Arkansas, at EF4 intensity. It crossed the Missouri Bootheel, causing additional damage and fatalities near Braggadocio and Hayti. After crossing the Mississippi River into northwestern West Tennessee, that tornado dissipated, and a high-end EF4 tornado formed and moved through Western Kentucky, where the towns of Cayce, Mayfield, Princeton, Dawson Springs, and Bremen suffered severe to catastrophic damage.
Early estimates suggested that the tornado family—identified by some media outlets as a "quad-state tornado", due to the storm's long track, apparent continuity, and similarity to the tri-state tornado of 1925—might have cut a path of up to across the affected areas, making it the longest-tracked tornado in history. However, storm surveys found that the majority of the storm's path consisted of two distinct EF4 tornadoes, with three short-lived and weak tornadoes in between them in northwestern Obion County, Tennessee. The parent supercell that produced the two EF4 tornadoes, and eleven tornadoes in total, later became known as the "quad-state supercell". The second supercell and tornado family, produced an EF3 tornado tracking nearly in Tennessee and southern Kentucky, causing major damage in Dresden, Tennessee. It also produced a catastrophic EF3 tornado that moved through Bowling Green, Kentucky as well as other tornadoes in southern and central Kentucky. Other tornadic thunderstorms affected portions of eastern Missouri, Southern Illinois, West and Middle Tennessee, and western to central Kentucky during the late evening into the overnight hours of December 11, including two more EF3 tornadoes that struck Edwardsville, Illinois and Defiance, Missouri.
The death toll from the outbreak was 89 (with six additional non-tornadic fatalities), surpassing the tornado outbreak sequence of December 1–6, 1953, which caused 49 fatalities, as the deadliest December tornado event ever recorded in the United States. In Kentucky alone, 74 people were killed by three separate tornadoes. In addition, at least 672 people were injured. The tornado outbreak caused at least $3.9 billion (2022 USD) in damages. The outbreak set a new record for confirmed tornadoes in the month of December, with 71, a record that only stood until December 15, when a larger outbreak produced 120 tornadoes across the Midwest.
Meteorological synopsis
On December 8, the Storm Prediction Center (SPC) outlined a slight risk of severe weather across a broad area of the Mississippi Valley. Despite the potential for a higher-end severe threat to materialize, forecasters expressed uncertainty regarding the extent of instability, degree of directional wind shear, and late timing of potential storms. The following day, the SPC noted the increased potential for organized severe thunderstorms in the region extending from southeastern Arkansas northeast into southern Indiana, upgrading that area to an enhanced risk.
As an intense upper-level trough progressed across the High Plains, with robust instability and moisture return across the Mississippi Valley, the SPC expanded the enhanced risk and introduced a moderate risk area from northeastern Arkansas into southern Illinois on the morning of December 10. Forecasters indicated that atmospheric conditions favored the development of nocturnal supercells capable of producing long-tracked, strong tornadoes.
At 3:00 p.m. CST (21:00 UTC), the SPC issued a tornado watch across the highest risk area (encompassing central and eastern Arkansas, west Tennessee, northwestern Mississippi, southeastern Missouri, and southern portions of Illinois and Indiana), the first of eleven issued over subsequent hours over the middle Mississippi Valley. Initial storms developed across central Arkansas around 2:00 p.m. CST (20:00 UTC), with even weaker activity developing over central Missouri a little over hours later; additional clusters of thunderstorms developed over southwestern Missouri (forming between Bolivar and Carthage, eventually back-building into northeastern Oklahoma) and central Arkansas (forming southwest of Hot Springs) between 5:00 and 5:30 p.m. CST (23:00–23:30 UTC). Though this activity lacked much vigor at its onset due to a strong capping inversion, the convective cells began to show organization as they progressed eastward.
One such storm—which formed from the initial mid-afternoon activity near Arkadelphia, Arkansas—matured into a long-lived supercell as it progressed in an unstable, deeply moist, and highly sheared environment; this cell ultimately persisted for more than over several hours from eastern Arkansas to northeastern Kentucky, producing two large and intense tornadoes along its track, among eleven tornadoes in total. The cell started showing signs of surface-based rotation southwest of Searcy, Arkansas, around 5:30 p.m. CST (23:30 UTC). At 5:51 p.m. CST, the National Weather Service office in North Little Rock issued the first tornado warning associated with the storm for portions of Jackson, Lawrence, White, and Woodruff counties. One of the first tornadoes associated with the storm, an EF0, touched down in western Poinsett County (near Weiner) around 6:40 p.m. CST; about fifteen minutes later, storm spotters reported a large tornado near Greenfield, prompting a PDS tornado warning for portions of Poinsett, Craighead and Mississippi counties (including areas to the south of Jonesboro).
Doppler radar analysis estimated that the supercell maintained a nearly continuous high-end rotational vorticity signature, averaging for approximately four hours and 20 minutes, a rarity among thunderstorms that produce mesocyclonic vorticity exceeding such speeds (averaging 1.5% of all supercells). The only velocities below said average recorded along the storm track were observed between 8:44 and 9:01 p.m. CST [02:44–3:01 UTC] as the storm crossed from Obion County, Tennessee into Hickman County, Kentucky. This time frame coincides with the start of the Western Kentucky tornado, implying the supercell underwent a mesocyclone re-strengthening phase during this period. During this intense tornado's lifetime, peak gate-to-gate velocities of were recorded at 9:58 p.m. CST (04:58 UTC) over northeastern Marshall County, Kentucky.
Elsewhere, multiple lines of intense storms, some featuring embedded supercells, developed across the Mississippi Valley region through the overnight hours and generated other strong and long-lived tornadoes. By the pre-dawn hours of December 11, a decrease in instability led to a gradual weakening of a line of thunderstorms stretching along the associated cold front from eastern Kentucky southward into central Alabama.
The SPC issued a record-setting 43 mesoscale discussions (MCD) throughout the course of the day (12 UTC December 10 to 12 UTC December 11), all of which were associated with the broader storm system: 38 of the MCDs issued were convective discussions relating to the severe thunderstorm activity, and five were non-convective discussions relating to heavy snow associated with the system that concurrently fell throughout much of the Upper Midwest. The National Weather Service (NWS) issued a total of 149 tornado warnings throughout the night across nine states: Arkansas, Tennessee, Missouri, Mississippi, Kentucky, Illinois, and Indiana.
These included multiple 'particularly dangerous situation' (PDS) tornado warnings and tornado emergencies in Arkansas, Tennessee, Kentucky, and Missouri. Eight of the tornado warnings issued during the event by the NWS offices in Memphis, Tennessee, and Paducah, Kentucky, were tornado emergencies, the most ever issued during the month of December (breaking the previous record of three issued on December 23, 2015).
Effects from the system responsible for the outbreak extended into Canada, where the Meteorological Service of Canada issued wind and heavy rainfall warnings for portions of Ontario. However, no tornadoes were expected nor reported north of the border.
Confirmed tornadoes
With a total of 71 tornadoes, this was one of the largest December tornado outbreaks on record in the United States, only to be surpassed by an even larger outbreak just five days later which recorded well over 100 tornadoes in a shorter time period.
Northeast Arkansas–Missouri Bootheel–Northwest Tennessee
This violent, long-tracked, and deadly EF4 wedge tornado was the fifth of eleven spawned by the quad-state supercell, and the first of two violent tornadoes produced by the storm. It first touched down in Craighead County, Arkansas, just north of Bay, at 7:07 p.m. CST (01:07 UTC) on the evening of December 10. After initially only causing minor EF0 tree and outbuilding damage, the tornado moved through the south side of Bowman, inflicting EF0 to EF1 damage to roofs as well as downing power poles and tree limbs, and bending a radio antenna at a residence. The tornado then intensified to EF2 strength as it crossed AR 18 and passed northwest of Lake City, downing trees and damaging or destroying some metal outbuildings. A house also sustained minor damage in this area and AR 135 north of town was shut down due to downed power lines. The growing tornado then crossed County Road 505 as it continued northeastward, snapping power poles, unroofing a house, inflicting major roof damage to another home, overturning an irrigation pivot sprinkler, and damaging a silo. The tornado then reached EF3 strength as it moved through the western and northern fringes of Monette. Two industrial buildings were destroyed at the west edge of town, and many trees and power poles were snapped. The Monette Manor nursing home lost most of its roof and had masonry exterior walls knocked down, killing one person and trapping 20 other residents and employees, including five people who were injured. Multiple homes and mobile homes were damaged or destroyed in a neighborhood near the nursing home, several large grain silos were torn apart, and AR 139 was also shut down near the town due to downed power lines.