Dense fog developing overnight

Update 6:45AM CST…

Dense fog has formed in the Advisory area early this Monday morning. Temperatures have dropped into the 20s where dense fog has formed with most of the airport observation sites also sensing a very light freezing drizzle. The visibility will be slow to improve in some locations and there could be some slick spots on secondary roads/sidewalks and parking lots.

Following is a list of area airports with poor visibility – all are reporting a very light freezing drizzle:

Location/visibility (in fractions of a mile) at 6AM CST

Rockford…1/8
Freeport…1/4
Rochelle…1/4
Peru/Ottawa…1/4
Aurora/Sugar Grove…1/4
Lansing…1/4
DuPage/West Chicago…1/2
Janesville,WI…1/4
Monroe, WI…1/4

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A Dense Fog Advisory is in effect for Winnebago, Boone, McHenry, Ogle, Lee, DeKalb and Kane Counties in northeast Illinois until 10AM CST Monday morning (grey-shaded areas on the highlighted map). Visibility is expected to fall as fog develops across these counties, perhaps to a quarter-mile or less before sunrise Monday. With the thickening fog and falling temperatures, glazing could occur on untreated roads/sidewalks, and other paved areas as temperatures approach or sink below freezing.

So individual traveling or outside during the early morning hours Monday, should be aware of the forecast fog and the possibility that poor visibility as well as potential slick spots could make for dangerous conditions. Note Dense Fog Advisories are also in effect to the north across southern Wisconsin and west into Iowa (see U.S. Map below). Fog could develop farther south and east closer-in to Chicago itself, and conditions will be monitored to determine if the Dense Fog Advisory will need to be expanded.

Temperatures over the Chicago have dropped into the lower and middle 30s this Sunday evening with dew points within a degree or two of the temperature in many locations. Fog has already started to form. As clouds slowly thin and move east, clearing skies will allow temperatures to fall and coincide with the dew points.