A Frost Advisory is actually in effect between 1AM and 8AM Tuesday morning for Illinois counties along and north of Interstate-80 (blue-shaded area on the headlined map) where less cloudiness and more long-wave radiational heat loss is expected. Lowest readings in the Advisory area could drop into the upper 20s in frost-prone areas away from the lake and city.
A “dirty”, cold and increasingly drier Canadian-source high pressure with diminishing northerly winds will continue to build south into our area tonight, and temperatures are expected to fall into the 30s Chicago area-wide overnight. Temps will “bottom-out” in the hours immediately before and after sunrise Tuesday morning. The word “dirty” is used when upper-level or in some cases low-level disturbances cause imbedded cloudiness to occur in the high pressure.
Clouds could be a little more persistent south of Interstate-80 in northeast Illinois and northwest Indiana, and falling temperatures will likely be more inhibited by clouds. Even though a Frost Advisory will not be in effect over this area south of Interstate-80, there may be some locations with less cloudiness that could experience temperatures in the lower and middle 30s.
To be on the safe side, if you live in northeast Illinois/northwest Indiana, protect vulnerable plants wherever you can tonight. Note on the map, a Frost Advisory is also in effect for adjacent areas of northwest Illinois, southern Wisconsin, northern Indiana and southern Lower Michigan.