Warm, dry December in Illinois concludes a cold, wet year

[January 09, 2020]    Temperatures in Illinois were well above average and precipitation was below normal in December, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey. More than 100 daily maximum temperature records were broken by month’s end.

The preliminary statewide December average temperature was 35.2 degrees, about 5 degrees above the 1981-2010 normal and the 18th warmest on record. Average temperatures in December ranged from the low 30s in northern Illinois to the mid-40s in southern Illinois.

Temperatures during the first half of December were very close to average, followed by a brief period of well below average temperatures caused by cold air from the north. On Dec. 20, the predominant wind direction changed to southwesterly, bringing warm, dry air into the region. Temperatures between Dec. 20 and 29 ranged from 5 to 25 degrees above normal across the state.

In total, 104 daily high maximum temperature records and 27 daily high minimum temperature records were broken over this time period, including a few dozen records on Dec. 25. This was the warmest Christmas day at 68 stations across the state. The daily average temperature in Decatur in Macon County on Christmas was nearly 20 degrees above the 30-year normal.

The station in Elgin in Kane County broke its previous Christmas day high maximum record by 10 degrees. The highest temperature recorded in the state was 70 degrees on Dec. 26 in Wayne County and again on Dec. 29 in Pope County. The lowest temperature was -4 degrees on Dec. 15 in Rock Island County.

December’s warm weather was an aberration in an otherwise colder than average 2019 in Illinois. Only three months this year–July, September, and December–exhibited a statewide average temperature above the 30-year normal.

December precipitation was below the long-term average for the entire state. The statewide average total December precipitation was 2.03 inches, approximately 0.66 inches below normal.

Areas in far southern Illinois received 2 to 3 inches less than average in December, approximately 50 percent of normal December precipitation. This was the 50th driest December on record in Illinois and marked the second straight month of below average statewide precipitation.

Preexisting moisture and reduced evaporative demand, typical for this time of the year, have prevented impacts from the prolonged dry conditions. Despite two straight months of well below average precipitation, streamflow and soil moisture were both near normal across the state.

Snowfall totals ranged from less than a tenth of an inch in far southern Illinois to over 10 inches in south-central Illinois. A strong system came through in mid-December and brought several inches of snow to an area spanning the St. Louis Metro East to the Champaign-Urbana area.

The highest 24-hour snowfall total was 5.6 inches in Lovington in Moultrie County on Dec. 17, although Mascoutah in St. Clair County and Columbia in Monroe County both recorded 7.5 inches on December 17.

The December snowfall glut in south-central Illinois turned into snowfall deficits of 8 to 10 inches in northern Illinois. This was only the 10th December with 1 inch or less of snowfall in Stockton in Jo Daviess County.

Despite the small snowfall totals, the seasonal total snowfall was above average for most of the state between interstates 80 and 64. A broad area between Peoria and the St. Louis metro east received over 4 inches of above average snowfall, whereas the Chicagoland region has so far this season experienced a snowfall deficit of 4 to 6 inches.

Short-term 8–14-day outlooks from the National Oceanic and Atmospheric Administration Climate Prediction Center show strongly elevated odds of both above normal precipitation and above normal temperatures. Thirty-day outlooks show elevated odds of wetter and warmer than normal conditions to persist throughout January in southern Illinois.

Outlooks for January through March and March through May continue to show elevated odds of above normal precipitation for the entire state.

[Lisa A Sheppard]