CHAMPAIGN, Ill., 1/4/21: December was much warmer and drier than average across Illinois, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.
The preliminary statewide average December temperature was 32.8 degrees, 2.9 degrees above the 30-year normal. Preliminary statewide average total precipitation for December was 1.78 inches, 0.91 inches less than the 30-year normal.
The atmospheric conditions that gave us a very warm November continued most of last month, resulting in above average December temperatures statewide. Average temperatures ranged from the high 20s in northern Illinois to the high 30s in southern Illinois, between 1 and 6 degrees above average.
Daily maximum temperature records were broken at 34 weather stations in December.
December total precipitation ranged from just under 3 inches in northwest Illinois to just over 1 inch in much of central and southern Illinois. The entire southern two-thirds of Illinois was drier than average in December.
Precipitation this fall season has been variable in northern and southern Illinois. Northwest Illinois was 4 to 6 inches wetter than normal in September, while most of southern Illinois was 1 to 3 inches drier than normal. This pattern flipped in October, and far southern Illinois observed 4 to 6 inches of additional precipitation.
The December 29 version of the U.S. Drought Monitor showed continued moderate drought across western and central Illinois, and severe drought persisted along the Interstate 72 corridor between Springfield and Decatur. These areas have significantly depleted soil moisture, and a wet end to winter and start to spring will be necessary to return soil moisture to near normal conditions.
Most of the state experienced below normal snowfall in December, following largely snow-free months of October and November. All but the northwest corner of Illinois had less than 5 inches of snowfall in December. These totals were between 1 and 10 inches below average, with the largest departures in northeast Illinois.
After an active start to January, the Climate Prediction Center’s 8- to 14-day outlooks indicate the highest odds of a change to warmer and drier than normal conditions across the state into the second week of January.
The outlooks for the entire month of January lean toward warmer and wetter than normal conditions, suggesting a change to more active winter weather around the middle of the month.
As La Niña conditions are expected to persist into early spring, forecasts from the North American Multi-model Ensemble for February through April favor warmer and wetter conditions through the winter-spring transition.