(This story, first published Monday afternoon, was updated Tuesday morning.)

A late-season tornado outbreak hit Illinois Saturday, dropping at least 26 confirmed tornadoes and injuring more than two dozen people in the Prairie State.

The surprise swarm of twisters marked the most to hit Illinois during a single December calendar day on record. The Storm Prediction Center had issued a “slight” risk of severe weather that morning, warning that “the development of afternoon thunderstorms in this region seems likely, with [wind] shear… favoring discrete supercell storm structures.” Supercell thunderstorms are the most dangerous form, with rotating updrafts that are the precursor to tornadoes.

Two monster thunderstorms spun up, one in west central Illinois and the other about 75 miles to the east. The behemoth tempests were flanked by smaller supercells on either side, their flying saucer-like clouds tracking hundreds of miles across the countryside northwest of St. Louis.

The western cluster dropped a total of nine short-lived tornadoes along the banks of the Illinois River from Beardstown to Havana.

The same storm yielded quite the photo op for one daredevil hunting mallards in Fulton County:

Farther to the east, a second strip of damage was observed between Springfield and Bloomington, west to Effingham County. At least one person was injured when their mobile home was tossed about in 110 mph winds from a tornado in Mason.

Possibly the strongest tornado of the day struck Taylorville, at times a half-mile wide as it plowed into the city of 11,000. Fox Illinois reported that 26 people were taken to Taylorville Memorial Hospital after sustaining tornado-related injuries. Six remain in critical condition. The windstorm also severely damaged 100 homes.

Despite the unusual time of year, the storm was well-predicted — allowing residents plenty of time to seek shelter. The lack of fatalities is a success story, a testament to the 41 minutes of lead time provided by the National Weather Service in Lincoln before the tornado struck. The office opted to issue a rare “Tornado Emergency,” warning of “a confirmed large and destructive tornado” barreling toward Taylorville.

Despite the enormous amount of damage, the twister has yet to be rated, as storm surveys continue across the area. However, it’s possible that winds topped 130 mph at times.


Three-dimensional radar scan of Taylorville tornado, on Saturday. (GR2 adapted by author)

From a meteorological standpoint, the supercell exhibited textbook structure with a corkscrew-shaped updraft towering six miles high. Storms on Saturday blossomed in an environment characterized by meager solar heating but plentiful shear — spin energy. That meant anything that fired would quickly rotate. Time-lapse video of the Taylorville storm shows that quite well, featuring beautiful striations about the cylindrically sculpted rotating cloud.

Meanwhile, Taylorville had a chance of snow flurries Monday afternoon.

Tornadoes in December aren’t terribly rare. The United States actually has a second, albeit less impressive, peak in tornado activity during mid-to-late fall. Instigating bouts of cold air mark the clash of the seasons, heralding the arrival of a winter air mass in the upper atmosphere. But Saturday’s episode was next level.

There is some indication that December tornadoes may be on the rise, though limitations in data make it impossible to know for sure. While the trend appears to reveal a notable increase in December tornado activity since bookkeeping began in 1950, there are concerns that many tornadoes before 2000 may not have been reported/counted due to lack of cellphones, cameras and social media.


(Data from TornadoHistoryProject.com)