Provisional data show December was among the top five warmest on record in Illinois, with no snow on Christmas, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey. The exact ranking of December 2021 will be confirmed later this month.

 The statewide average December temperature was 39.4 degrees, 9.6 degrees above the 1991–2020 average. This December was the warmest on record in Carbondale, the second warmest on record in Quincy, the third warmest on record in St. Louis, Springfield, and Peoria, the fourth warmest in Rockford and Champaign-Urbana, and the fifth warmest in Chicago.

December average temperatures ranged from the low 30s in northern Illinois to the high 40s in southern Illinois, between 6 and 12 degrees above normal. High temperatures regularly reached into the 70s last month in southern Illinois, including a 78-degree high in Randolph County on Dec. 4. Carbondale recorded 74 degrees on Dec. 25, one of the warmest Christmas Day temperatures on record in the state.

 Precipitation

Total December precipitation ranged from around 1 inch in far western Illinois to nearly 6 inches in far southeast Illinois. Most of western Illinois was 0.50 to 1.5 inches drier than normal, whereas eastern and southern Illinois were near to 0.50 inches wetter than normal for the month.

Snowfall was 1 to 8 inches below normal in December. Snow totals ranged from 4 inches in far northwest Illinois to 0 inches south of Interstate 72.

Many places did not record their first measurable snow until the last week of the month. Chicago recorded their first measurable snowfall of the season on Dec. 28, eight days later than the previous recorded latest first snowfall.

With the liquid water content of snow and rainfall taken together, the preliminary statewide average total December precipitation was 2.70 inches, 0.02 inches above the 1991–2020 average.  

Severe weather

Severe weather, including tornadoes, hail, and strong winds, tend to have the highest frequency in Illinois between April and June. However, unlike Atlantic hurricanes or extreme cold, Illinois does not have a “tornado season” because tornadoes and other severe weather can and do occur all year in the state.

 The most noteworthy event was on the night of Dec. 10, when severe thunderstorms moved across the southern Midwest and the mid-south, resulting in multiple very strong tornadoes in Illinois, Missouri, Arkansas, Tennessee, and Kentucky. The tornado outbreak was responsible for dozens of fatalities across the area, including six deaths in Illinois, many injuries, and damage or destruction to thousands of homes and structures.

Overall, the NOAA Storm Prediction Center shows nine tornado reports in Illinois for December, compared with an Illinois average of just over one December tornado per year between 1950 and 2020.

Outlooks

Near and longer term outlooks are indicating the potential for real winter weather in the Midwest. The Climate Prediction Center’s 8 to 14 day outlooks show elevated odds of below normal temperatures across the state for the second week of January, with higher odds of drier than normal weather over the same time period.

Meanwhile, outlooks for the entire month of January show the highest odds of near to below normal temperatures and above normal precipitation, still reflecting some of the weakening La Niña pattern in the Pacific.