According to Tom Skilling, AccuWeather and others, the Geneva area might be in for the earliest frost in many a year tonight.
"Patchy frost is possible west of a Crystal-Lake-to-Pontiac line after midnight," the NWS-C says. There's also a high risk of rip currents for the Illinois and Indiana shores of Lake Michigan.
That's the report, by the way, for Geneva—good old Latitude 41.93, Longitude -88.25, with an elevation of 758 feet above sea level.
Tom Skilling (whenever his name is spoke in weather circles, it's like when they mention "Randolph Scott" in Blazing Saddles) says the Coolest weather since late May threatens unseasonably early frost far west/northwest suburbs.
According to Tom, we could see frost Tuesday night (Sept. 18, 2012) in colder locations well west and northwest of the city. Areas at risk include the Fox Valley, DeKalb, McHenry County as well as the cities of Belvidere, Rockford and Beloit.
If I'm reading correctly, it looks like the earliest frost on record at Midway or O'Hare is Sept. 22, back in 1974.
MyChicagoGarden website says the earliest fall frost is Sept. 22 in Wheaton.
AccuWeather.Com says "temperatures are forecast to dip to frosty levels across much of Wisconsin and portions of Illinois" Tuesday night into early Wednesday morning.
If you have tender flowers or vegetable plants, cover them Tuesday evening or risk damage from frost, AccuWeather advises.
Temperatures are expected to dip into the middle 30s to low 40s early Wednesday morning, so bundle up if you're out and about.