The National Weather Service has issued a dense fog advisory for the Chicago area for overnight Sunday after an usually warm December weekend.
According to the advisory, "an unseasonable warm and moist air mass has settled over Northern Illinois and Northwestern Indiana. As widespread cloud cover clears out temperatures will drop quickly allowing for fog to form and quickly reduce visibility."
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Visibility in some areas could be less than a quarter of a mile, the advisory said.
McHenry, Lake, Kane, DuPage, Cook, Kendall and Will counties were all included in the advisory which is in effect until 8 a.m. Monday morning.
The National Weather Service cautioned drivers to use their headlights, drive slowly and leave plenty of stopping distance during the fog event.