Midwest Winter Storm Update: Monday Evening

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. 

Green and Jo Daviess counties remain under the Winter Weather Advisory beginning Tuesday at 6am.  A slight shift in the storm track northwest has pulled Rock County out of the Advisory (which it was placed under earlier Monday morning).  Now this doesn't mean we won't see any wintry weather, but by far the heaviest snowfall will reside well north and west of Northern Illinois.

Surface low pressure is now moving out of the four-corners region in the southwest.  Ahead of the low, snow has quickly developed on the eastern side of the Rockies and in the western Plains.  This low is set to track from Kansas Tuesday morning to Northeast Iowa and Northwest Illinois Tuesday night and into Northern Michigan by Wednesday morning.  Now, there is still some time for this track to wobble a little so the forecast for Tuesday isn't set in stone, but we shouldn't experience too much of a shift from what we're thinking Monday 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.

At the onset, the surface warm front will be just south of the state line.  This means surface temperatures will be on either side of 32 degrees.  It looks like a burst of snow and sleet will occur in Northwest Illinois and continue through Noon.  As this occurs, warmer air will be pulled north causing temperatures to warm above freezing with a transition from snow/sleet to more of a snow/sleet, freezing rain and rain event.  Eventually, from Rockford on southward rain should become the dominant precipitation type through the afternoon until Tuesday evening.  North and west of Rockford...from a line from Monroe to Freeport...temperatures may be slightly cool enough to support snow just a little longer into the afternoon.  Everyone should experience a decrease in precipitation by 6pm/7pm Tuesday evening as drier air moves in from the southwest.  This is what we refer to as a dry-slot. 

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.