While cold air moved in behind the low-pressure system that passed through the Midwest this week, it looks like Illinois will likely dodge the freezing temperatures for now. But someday they will come. Here are the median dates for when we see the temperatures drop to 32 or below in fall, as well as the earliest date in 1 out of 10 years.
In general, we hit 32 degrees in early October for northern Illinois and mid October for central and southern Illinois. In 1 year in 10, we have seen 32 degrees in the last week of September (second map) or the first few days of October. See more maps and discussion of frost on the State Climatologist web site.
If you look closely at the maps, you can detect noticeable differences in nearby locations. In many cases, there is nothing wrong with the reports. Many times our first episode of freezing temperatures in the fall is during calm, clear nights. In those cases, local conditions can make a huge difference (in town versus the farm, valleys versus hilltop, near or far away from buildings, etc.).