10:00pm Update: A Dense Fog Advisory has been issued for all of Northern Illinois and for Green and Rock counties in Southern Wisconsin. Visibility all across the area has fallen under half a mile, and with temperatures below freezing, freezing fog is possible.
Going into the night, dense fog is going to be our greatest concern. Just a slight chance for snow later tonight. More likely as we head into mid-morning Tuesday for precipitation.
Here is the latest thinking on what to expect Tuesday morning into Tuesday night.
This is a very dynamic late season storm system as on the cold side blizzard conditions will be felt from Iowa to Eastern Nebraska, and severe weather will occur from downstate Illinois and central Indiana on southward. In fact, a few rumbles of thunder may even be heard as close to Northern Illinois as the southwest suburbs of Chicago!
Strong winds in the lowest levels of the atmosphere will push warmer air northward overnight Monday into Tuesday. This will cause a thin band of light snow to develop before sunrise Tuesday, mainly affecting locations near the state line and northward. Drier air may actually be pulled in with an increase in surface winds from the East. This could actually hold some of the precipitation off until after sunrise and possibly until 9am or 10am. A great uptick in moisture will occur mid-morning Tuesday and this is when some of the heaviest precipitation will occur in Northern Illinois and Southern Wisconsin.
Colder air wrapping in Tuesday night and early Wednesday would support a rain/snow mix early morning with lighter snow showers through mid-morning Wednesday. Total snowfall amounts for far Northwest Illinois and Southern Wisconsin will range from 1" to 3". Although, if at the onset the precipitation is heavy it could add up a little more near the state line.
As always, this forecast could change slightly so it's important to continue to check back. But this is where it stands right now.