The latest NWS forecast has a frost advisory for northwest Illinois and an extensive area with freeze warnings that cover KS, NE, SD, MN, IA, and WI.
A frost advisory is issued when the minimum temperature is forecasted to be 33 to 36 degrees on clear, calm nights during the growing season. They are issued in the fall until the end of the growing season (marked by the occurrence of that first widespread freeze). Tender plants can be damaged by frost and should be covered or moved indoors.
Air temperature is measured at a height of 5 feet. Colder, denser air near the ground can drop below freezing even when the measured air temperature at 5 feet is 33 to 36 degrees.
A freeze warning is issued when significant, widespread freezing temperatures are expected. They are issued in the fall until the end of the growing season (marked by the occurrence of that first widespread freeze).
Most of Illinois has not seen 32 degrees yet, according to this map from the Midwestern Regional Climate Center’s VIP program. On average, most of Illinois will see their first fall freeze sometime in October.