December – Page 2 – Illinois State Climatologist

December 21, 2011

December – Warmer and Wetter in Illinois So Far

Precipitation and Temperatures in December for Illinois

Based on data through December 21, the statewide average precipitation was 2.9 inches, 1.1 inches above average.
The statewide average temperature was 35.6 °F, 4.5 °F above average. That stands in stark contrast to last December when the average temperature was 21.8 °F through this date. Put another way, we are about 13.8 °F warmer this December than last December through the 21st of the month.

Where is the snow?

With the warmer temperatures this December, we have seen lots of rain but not much snow. The two maps show the snowfall for the period of December 1-21, 2011 and the same period for December 2010. Snowfall is lacking throughout much of the Midwest except for the one winter storm earlier this week. Southeast Illinois has reported no snowfall in December while much of the rest of state has seen two inches or less. By this time last December (second map), much of Illinois and the upper Midwest had experienced 10-25 inches of snow.

December 2011 snowfall — December 1-21, 2011 snowfall. Click to enlarge.

December 2010 snowfall — December 1-21, 2010, snowfall for the Midwest.

November 28, 2011

December snowfall in Illinois

The new 1981-2010 normal snowfall data have been released by the National Climatic Data Center. A map of those data for December are shown below. Normal snowfall ranges from barely an inch in far southern Illinois to 8 – 11 inches in many places in northern Illinois. Antioch is the snowiest station with 11.1 inches in December. Besides the total snowfall for the month, you can get maps of number of days with 0.1 inches or more, 1 inch or more, and 10 inches or more.
While southern Illinois gets less snowfall in December, they get more rain. In fact, the map of total precipitation (rainfall plus the water content of snow) shows that southern Illinois is almost twice as wet as northern Illinois. Normal precipitation amounts of 4 to 4.5 inches are common there, compared to around 2 inches for sites in northern Illinois. See second map.
So why is northern Illinois so “dry” compared to southern Illinois in terms of precipitation, even though northern Illinois gets much more snow? It turns out that the water content in snow is not that great. The rule of thumb is that 10 inches of snow melts into 1 inch of water (10:1). However, the actual ratio varies from storm to storm. I have seen the ratio vary from 4:1 for a wet, slushy snow to 20:1 for a light, powdery snow. So that 8 inches of snow in your driveway may not have any more water in it than the rain from a typical summertime thunderstorm.
You can explore all the new normals data in maps and tables for each station at the climate normals page on the State Climatologists website.

Illinois-snow-12DEC-normals — Normal snowfall (1981-2010) for Illinois in December. Click to enlarge.

Illinois-prcp-12DEC-normals — Normal precipitation (1981-2010) for Illinois in December. Click to enlarge.

September 15, 2011

October and Fall – Warm and Dry in Illinois

The NWS Climate Prediction Center has released their outlooks for October and October-December. For Illinois there is an increased chance that temperatures will be above-average in both the October and October-December time frames. Also there is an increased chance that precipitation will be below-average in both October and October-December.
As posted earlier, this forecast is consistent with the known impacts of the rejuvenated La Niña event occurring in the Pacific Basin. La Niña tends to give us warmer and drier than average conditions in fall. Last fall was a classic example of this with temperatures 1.2 degrees above average and precipitation 12% below average.

seasonal outlook — The October and October-December outlooks of temperature and precipitation from the NWS Climate Prediction Center. Click to enlarge.

January 2, 2011

December 2010 in Illinois – Cold and Snowy

December 2010 in Illinois turned out to be one of the coldest and snowiest on record. I updated the numbers on January 3 and the maps on January 5.

Temperature

The average statewide temperature was 24.0 degrees, 5.8 degrees below normal and the 12th coldest on record, based on preliminary data.

Snowfall

Snowfall was above-normal throughout the state in December and heaviest in a band from northwest Illinois into east-central Illinois (see maps below). Snowfall totals for December include 20.4 inches in Champaign-Urbana (snowiest December on record), 18.1 inches in Bloomington-Normal(second snowiest December on record), and 19.2 inches in Peoria (third snowiest December on record). Champaign-Urbana beat both Rockford (19.2 inches) and Chicago O’Hare Airport (16.2 inches) on snowfall totals. The largest snowfall total reported in the state was 30.0 inches near Galena by a CoCoRaHS observer. Streator reported 28.5 inches, the highest amount from a NWS cooperative observer.

Precipitation

Even though snowfall was above average in most locations, the average statewide precipitation was 1.8 inches, 0.9 inch below normal. Some of the snowfall did not have a high water content, especially if it fell when the temperatures were in the 20s. While the rule of thumb is that 10 inches of snow equals 1 inch of water, a 10 to 1 ratio, I have seen ratios ranging from 4 to 1 to 22 to 1 in Illinois.

December 2010 snowfall departure in Illinois — The December 2010 snowfall departures (inches) in Illinois.