Traces of snow were reported in Chicago and Rockford on October 4. A few years ago I did a post on the earliest and median dates for the first measurable snowfall of the season. Measurable means at least 0.1 inches or more. Snow flurries or traces of snow do not count since they have not been tracked closely in the historical weather records.
No real surprises for anyone who has experienced winter in Illinois. The earliest dates of measurable snow are in the late October, early November time frame. And the median dates range from late November in northern Illinois to the second half of December in southern Illinois. Even so, the dates can vary considerably between nearby sites since many of the early season snows are sporadic and not very widespread.
The median is also known as the 50th percentile and represents in the “middle” of the distribution of dates. In many cases it is a better measure than the average, which can be thrown off by some date that is extraordinarily early or late.
These maps are based on sites that reported snowfall regularly since 1981. The dates are the earliest and median dates of the first measurable snow (0.1 inches or more) for the standard averaging period of 1981 to 2010 (also known as “normal”).