As a cold front swings through from the northwest, cloudiness and scattered light showers accompany the broad frontal boundary (weather radar mosaic below) tonight. Cold Canadian-source high pressure will follow into our area with winds shifting to the west-northwest and gusting.
With colder air riding the west-northwest wind, temperatures will struggle to reach the lower 50s Thursday afternoon even as the sun breaks through. Indications skies will clear and winds drop off Thursday night, allowing maximal radiational heat loss.
The Chicago National Weather Service Forecast Office has issued a Freeze Watch for late Thursday night into Friday morning for all of the Chicago area (northeast Illinois/northwest Indiana) excepting Cook County and the City of Chicago (the area under the Freeze Watch is designated in blue on the headlined map). The widespread freezing temperatures will likely kill unprotected plants. Temperatures will probably drop into the middle and upper 30s in the city, but away from Lake Michigan temperatures could fall off into the upper 20s and lower 30s
This cold coincides pretty well with the average period (Oct 11-20) when the first freeze most often occurs over much of our area (see average freeze map below).