Just a quick note showing how the recent winter storm played out. The snow was widespread across the upper Midwest, including northern Illinois. Amounts of 5 to 10 inches were common in northeast Illinois. Two sites in McHenry County had over 10 inches of snow: Harvard (IL-MCH-66) with 11.7 inches and Bull Valley (IL-MCH-13) with 10.8 inches. Central and southern Illinois were warmer and received mostly rain instead of snow. Rainfall amount in southern Illinois were in the neighborhood of 0.5 to 0.75 inches.
With the streak of warm weather this fall, thoughts of snow are far away – but not for much longer. The first significant winter storm for the Midwest is on the horizon on Thursday and Friday. It will likely hit Nebraska, the Dakotas, and Minnesota, but miss Illinois (blue shading on the map below).
So when can we expect to see that first measurable snowfall (0.1 inches or more) in Illinois?
Here is a map that we constructed a few years ago using data from 1971-2000. No surprise – the earliest dates are in the Chicago area and cluster around November 20. For the rest of the northern half of the state, the average date is towards the end of November. In central Illinois, I have always considered Thanksgiving to be the start of the snowfall season. The average dates get dramatically later as you go southward, getting closer to Christmas by the time you get to Carbondale and southward.
The impact of the February 1-2, 2011, storm on highways in Illinois was significant. Using snowfall data and Illinois DOT highway data, our GIS specialist Zoe Zaloudek was able to calculate the number of miles of interstate, US highways, and state roads covered by selected amounts of snow.
Below is the resulting map showing both the roads and significant snowfall. A table at the bottom of the map shows the number of miles affected by 6, 8 and 12 inches or more of snow. The impacts were felt all the way from Quincy to Chicago. We chose a starting point of 6 inches as the threshold for significant disruption of traffic and higher removal cost, based on earlier studies in Illinois.
For example, about 1,132 miles of interstate roads in Illinois received 12 inches or more of snow. Including 1,762 miles of US highways and 4,099 miles of state roads, it adds up to an incredible 6,993 miles of roads with a foot or more of snow to plow.
These estimates do not include the thousands of miles of county roads as well as city streets and alleys in the Chicago metropolitan area and elsewhere. However, we did not have a complete database of those road systems. Also roads covered by less than 6 inches of snow were treated as well for additional costs.