Here are the median dates of the first time we hit 32 degrees in the fall. Since we don’t have reports of when frost occurs, we use 32 degrees as a proxy. It is no surprise that the frost date occurs earlier in northern Illinois than southern Illinois.
If you look closely, you may see differences of several days between neighboring sites. Nighttime temperatures, especially on calm, clear nights in the fall, can be very sensitive to the local conditions. Sites in town tend to be a little warmer than those in the countryside, and result in delaying the arrival of colder temperatures.
Chilly temperatures have already visited the northern part of the state this weekend. Mt. Carroll reported a low of 29 on September 23 and Elizabeth reported a low of 28 on September 24.
You can read more about frost and see many more maps on the state climatologist’s frost webpage.