Following the second warmest December on record, below-average temperatures in January were a shock to Illinoisans. The preliminary statewide average January temperature was 21.1 degrees, 5.6 degrees below the 1991–2020 average, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.

January average temperatures ranged from the low teens in northern Illinois to the low 30s in southern Illinois, between 2 and 8 degrees below normal. Nighttime low temperatures frequently dipped below zero in central and northern Illinois, including...