Besides the cold temperatures, one of the initial signs of winter is that first snowfall. Parts of Iowa, Wisconsin, and Indiana have already seen their first snow. In the figure below are the median dates of the first measurable snowfall (greater than or equal to a tenth of an inch) of the season in Illinois.
Think of the median date as the date by which we have seen snow in 50 percent of the cases during the period between 1971 and 2000.
In northern Illinois, the first snowfall occurs around Thanksgiving. In far southern Illinois, you have to wait until sometime in mid-December.
In about 10 percent of the cases, the first measurable snow can occur as early as November 5 in northern Illinois to November 20 in far southern Illinois.