Late Season Winter Storm Update: Monday Evening

Nothing like coming off of record warmth Sunday to accumulating snowfall by Tuesday morning.  But it's already starting to move in and will be with us Monday night.  This winter event will roughly be about 12 hours, with most of the snow ending by mid-day Tuesday.

Your complete video forecast, and any further updates, will be found below.

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10:40pm Update: The Winter Storm Warning and Winter Weather Advisory remain in effect through Tuesday morning.  There have been no changes to that, and overall, no major changes to the going forecast this evening.

Warm air advection, known as WAA, has been developing ahead of the low pressure system to the west.  Wind flows counter clockwise around low pressure, so there is slightly warmer air being pulled northward this evening.  And while the radar shows light snow over far Northern Illinois, there have been reports of sleet and freezing rain mixing in on the southern edge of the radar snow returns.

The Rockford Airport is reporting freezing rain while the Albertus Airport in Freeport is reporting sleet.  To the north in Wisconsin, both Monroe and Janesville are reporting snowfall.

So the complete transition over to snow has slowed down just a little this evening and may take a little while to get going through Midnight.  This means we will also hold on to the mix a little longer over parts of Northern Illinois, which could cut back on overall snow totals.  It still appears that higher snow totals will be found close to the Wisconsin/Illinois border, but achieving totals over 6" may be a little harder to come by.  Especially if the warm air advection holds on longer.

9:00pm Update: Current thinking from earlier forecast hasn't changed much.  We had an initial band of sleet, freezing rain, rain and snow develop this evening along a frontal boundary over Northern Illinois a few thousand feet above.

Now, as we head further into the evening snowfall is expected to increase in intensity through 3am with forcing in the atmosphere increasing near the Wisconsin/Illinois border.  Isolated snowfall rates of one inch per hour can not be ruled out further into the night and early overnight.

Here is the technical discussion from the Storm Prediction Center:

SUMMARY...AREAS OF HEAVY SNOW ARE EXPECTED TO DEVELOP ACROSS THE
   REGION THIS EVENING...WITH LOCALIZED AREAS EXPERIENCING 1+ INCH
   SNOWFALL RATES THROUGH AT LEAST 09Z.

   DISCUSSION...RECENT OBSERVATIONS AND MODEL GUIDANCE SUGGEST
   DEVELOPMENT/EXPANSION OF WIDESPREAD PRECIPITATION IS CURRENTLY
   UNDERWAY FROM CENTRAL IOWA EASTWARD TO NORTHERN ILLINOIS.  THIS
   DEVELOPMENT IS IN RESPONSE TO LIFT ASSOCIATED WITH STRENGTHENING
   700-500 MB FRONTOGENESIS ACROSS THE REGION...WHICH MODELS SUGGEST
   WILL CONTINUE THROUGH THE OVERNIGHT HOURS.  ALTHOUGH MUCH OF THE
   REGION REMAINS RELATIVELY DRY IN THE LOWEST LEVELS /DEWPOINTS AS LOW
   AS 5-7F IN NORTHERN PARTS OF THE DISCUSSION AREA/... CONTINUED
   LIFT/PRECIPITATION WILL RESULT IN A GRADUALLY MOISTENING/COOLING
   BOUNDARY LAYER THAT WILL SUPPORT FROZEN PRECIPITATION.  THE PRIMARY
   PRECIPITATION TYPE FOR THIS REGION IS EXPECTED TO BE SNOW...ALTHOUGH
   LOCALIZED AREAS FROM EAST-CENTRAL IOWA INTO NORTHERN ILLINOIS MAY
   EXPERIENCE PERIODS OF SLEET AS PRECIPITATION FALLS THROUGH A MODEST
   WARM LAYER /AROUND 0C/ AROUND 850 MB.  OTHERWISE...AT LEAST ONE OR
   TWO BANDS OF SNOW ARE EXPECTED TO DEVELOP ACROSS THE REGION...WITH
   LOCALIZED AREAS EXPERIENCING GREATER THAN ONE-INCH PER HOUR SNOWFALL
   RATES OVERNIGHT.

7:45pm: Steady to moderate band of sleet and snow has developed over far Northern Illinois Monday evening.  I've received many reports of sleet in Stephenson, Winnebago and Boone counties.  But now beginning to see the sleet mix with snow as temperatures continue to cool.