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.
