With warmer-than-average temperatures, December 2018 brought slight snowfall and a historic late-season severe weather outbreak to Illinois.
On Dec. 1, the National Weather Service confirmed 29 tornadoes in Illinois, an event that is considered the largest December tornado outbreak in state history, according to Brian Kerschner, a spokesman for the state climatologist office at the U. of I.’s Illinois State Water Survey. The second-largest number of tornadoes in December was 21 in 1957.
The most notable tornado occurred in Taylorsville in Christian County, ranked as an EF-3 on the Enhanced Fujita Scale. With a path of greater than one-half mile wide and wind speeds of 155 mph, the tornado caused major damage to 100 homes, and 22 injuries were reported. The worst of the severe weather was confined to west-central Illinois.
The statewide average temperature for December was 34 degrees, which is 4.2 degrees above the long-term average.
The highest temperature recorded for the month was 71 degrees on Dec. 2 at the Kaskaskia River Lock and Dam in Randolph County. The lowest temperature was 6 degrees on Dec. 28 at the Altona and Mount Carroll stations in Knox and Carroll counties, respectively.
December favored above-average temperatures for the entire Midwest, including Illinois. The largest temperature departures occurred in the upper Midwest and extended down into much of northern Illinois, where several locations finished the month 6 to 7 degrees above the long-term average.
The statewide average precipitation for December was 3.32 inches, which is 0.63 inches above the long-term average. The highest monthly rainfall total was 6.83 inches, recorded near Chester in Randolph County.
Two storm systems affecting the state during the last week of December helped to bring the monthly totals above normal for much of central and northern Illinois, where precipitation was lacking in the middle of the month. The highest widespread precipitation totals occurred around and south of I-70.
December was not an ideal month for snow lovers, as warmer temperatures and a combination of environmental factors kept December snow accumulation to a minimum. The highest accumulations were in the northeast and in central/west-central portions of the state, where a quick-moving cold front brought a majority of the snow accumulation during the first week of December.
The highest monthly snowfall total was only 3.1 inches, recorded at a station near Patoka in Marion County.
Looking ahead at the rest of January 2019, the Climate Prediction Center forecasts an above-normal chance for a drier-than-average January for most of the state. The highest probabilities are centered over northern Illinois and the Great Lakes. The highest probabilities for above-average temperatures are concentrated over the upper Midwest and North Dakota, with a smaller probability extending into northwest Illinois.