Tornado outbreak of December 10–11, 2021

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 December 11, 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 219-mile (352 km) tri-state tornado of 1925—might have cut a path of up to 250 miles (400 km) 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 123 miles (198 km) 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.