A Dense Fog Advisory has once again been issued for northern Illinois and south-central Wisconsin beginning Sunday evening and lasting through Noon Monday. Dry, sinking air throughout much of the atmosphere hasn’t been our friend as of late with the fairly cloudy skies and locally dense fog. Typically, when dry air and high pressure move into an area skies will be clear. But sometimes that dry air can develop over a moisture-rich air mass trapped right near the surface. This trapped moisture is in place because temperatures just above the surface are actually warmer, creating an inversion. The sinking air doesn’t allow the inversion to break, leaving the low deck of stratus clouds and fog to linger throughout the day.
This is exactly what we’ve experienced the last two days, and could very well see for another night or two. For Sunday night/Monday morning, visibility under a quarter of a mile at times can be expected, lasting through the morning commute. Temperatures will remain below freezing during this time, with freezing fog possible. Already, Freeport is reporting freezing fog. This could cause roads, bridges and overpasses to become slick with black ice during the evening, night and morning. As many get back into their routine Monday, please remember to take your time in the morning as the fog will be dense and it will be slick.