The preliminary statewide average December temperature was 29.1 degrees, 2.5 degrees below the 1991–2020 average and tied for 53rd coolest on record going back to 1895. The preliminary statewide average total December precipitation was 1.50 inches, 0.93 inches below the 1991–2020 average and the 29th driest on record statewide.
Data are provisional and may change slightly over time.
Roller Coaster Temperatures
Day-to-day temperature variability in Illinois is highest in climatological winter, and December followed suit with huge dips and jumps in daily temperature, as shown in Figure 1 from Decatur. Daily average temperatures were 25 to 40 degrees below normal in the second week of the month and were 15 to 25 degrees above normal days around Christmas (Figure 1). Decatur broke its daily low temperature record with a -14-degree low on December 14th and broke a daily high temperature record with a 69-degree high on December 28th.
When taken altogether, December average temperatures ranged from low 20s in northwest Illinois to low 40s in far southern Illinois. December was 1 to 5 degrees cooler than normal in the northern half of the state, and near to around 1 degree warmer than normal in southern Illinois (Figure 2). The warmest place in the state last month was Olmstead in Pulaski County with an average December temperature of 38.4 degrees. The coolest place in the state was Stockton in Jo Daviess County with an average December temperature of 22.3 degrees.
The extremely mid days last month broke 43 daily high maximum temperature records and 45 daily high minimum temperature records. Morrisonville in Christian County broke its all-time December high temperature record with a 72-degree high on December 29th. Meanwhile the extreme cold mid-month broke 69 daily low maximum temperature records and 66 daily low minimum temperature records. Six places broke their all-time December low minimum temperature records, including -14 degrees in Decatur and -10
Overall, the preliminary statewide average December temperature was 29.1 degrees, 2.5 degrees below the 1991–2020 average and tied for 53rd coolest on record going back to 1895.
The Paradox of a Dry December with Plentiful Snow
December reminded us of the difference between total precipitation and total snowfall. Most folks think about snowfall as inches of accumulation, but for the climate and our water balance, the liquid water content of the snow that matters most. Case in point, northern and central Illinois had higher-than-normal snowfall last month, with totals ranging from 3 to 20 inches (Figure 3). Parts of central Illinois received over 400% of normal December snowfall, including 15.5 inches in Fisher and 12.8 inches in Bloomington. However, one inch of snowfall in Illinois usually yields between 0.05 inches and 0.10 inches of liquid water, meaning that even the highest snowfall totals last month equated to only around 1 to 2 inches of liquid precipitation.
Overall, December total precipitation ranged from around 4 inches in northeast Illinois to less than half an inch in southwest Illinois (Figure 4). Most of the northern half of the state was near to slightly wetter than normal last month, while the southern half of the state was 1 to 3 inches drier than normal. Morrisonville had its driest December on record (going back to 1948) with only 0.60 inches last month.
Overall, the preliminary statewide average total December precipitation was 1.50 inches, 0.93 inches below the 1991–2020 average and the 29th driest on record statewide.
Outlooks
Climate Prediction Center outlooks for January show an extension of colder-than-normal conditions from the northern Plains into much of northern Illinois and the Great Lakes region. January Outlooks show equal chances of above and below normal precipitation for the first month of 2026 (Figure 5).
