CHICAGO -- A Freeze Warning is in effect for the entire Chicago area for later Friday night into Saturday morning (dark-blue-shaded area on map above). Temperatures are expected to drop into the middle to upper 20s across much of the Chicago area with upper 20s to lower 30s in the City of Chicago and right along the Lake Michigan shoreline effectively ending the 2015 growing season. Vegetation sensitive to freezing or sub-freezing temperatures will most likely be damaged or killed unless protective measures are taken. These temperatures will possibly occur again Saturday night/Sunday morning.
The coldest air of the season will hit as Canadian-source high pressure will drift east out of the central plains over the Midwest and western Great Lakes – centering over the Chicago area tonight. With this cold air mass overhead along with clear skies and light winds, conditions conducive to a hard frost/freeze will be maximized. As the high pressure moves slowly east, another night of very cold temperatures looks to be in store, until southerly winds and warming begins here later Sunday.
Note on the map below the widespread nature of this cold high pressure air mass with Frost and Freeze Warnings (Dark-blue-shaded areas)/Advisories (light-blue-shaded areas)/Watches (light greenish-blue-shaded areas) in effect from the northern plains all to way east to the central Appalachians. White, non-shaded areas in Minnesota, Wisconsin and Michigan previously experienced the effective end of their growing season.