Update 1:20PM CST…
As visibility improves in westernmost portions of the Chicago area, conditions deteriorate in the east. A Dense Fog Advisory continues until 3PM CST for McHenry, DeKalb and Kane counties and until 6PM CST for Lake, DuPage and Cook Counties (grey-shaded area on the headlined map).
The cold front continues to move east across northeast Illinois with visibility improving significantly about one to two hours after the front moves through a given location.
Reference the map below for current visibility across the Chicago area.
A Dense Fog Advisory is in effect until 2PM CST for Winnebago, Boone, McHenry, Ogle, Lee, DeKalb, Kane and LaSalle Counties (gray-shaded counties on the headlined map).
Dense fog has developed over an area along and just east of Interstate-39 in westernmost portions of the Chicago area. Visibility has dropped to less than a quarter-mile in that area, falling to near zero at many locations. If traveling in that area, be prepared for sudden changes to much reduced visibility and slow travel.
A cold front will move slowly east through our area today. Along and ahead of the frontal boundary scattered showers/drizzle will occur with temperatures and dew-points closing to near 100% relative humidity, enabling the fog to develop and persist. Once the front passes a given location, winds will shift to the northwest and the drier air will allow improving visibility.
Below is a map depicting current visibility…