The heavy fog and showers were causing hazardous driving conditions for the morning commute, the National Weather Service said. The advisory was in effect until 10 a.m. for Cook, Will, Kane, DuPage, LaSalle, Kendall, Grundy, Livingston, Ogle, Lee and DeKalb counties, though the fog could linger into the afternoon.
The fog is from a low pressure system that "provides a tug of war between winter and spring" across the area, the weather service said.
After temperatures in the 40s on Monday, the highs for the rest of the week will remain in the 20s with a chance of snow on Thursday.
A Flood Warning is in effect for Bureau, LaSalle and Putnam counties until midnight Monday. There are also ongoing flood warnings in Lake, Newton, Jasper and Porter counties in Indiana.
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A Flood Advisory is in effect until midnight Saturday for Grundy, Kankakee and Will counties.
A Flood Watch is in effect for Kendall, Lee, Ogle and Winnebago counties until 6 p.m. That extends until 7 p.m. for Boone, central Cook, DeKalb, Kane, Lake, McHenry and southern Cook counties and until noon Wednesday in northern Cook County.
A Winter Weather Advisory is in effect for DeKalb, DuPage, Kane, Lee, northern Cook, Boone, Lake, McHenry, Ogle and Winnebago counties until 7 p.m. because of possible freezing sleet and drizzle, ABC7 Chicago meteorologist Tracy Butler said.
Sun-Times Media contributed to this report.