Wintry Weather Returns Monday

 A Winter Weather Advisory has been issued for the following counties: Green, Rock and Walworth counties in southern Wisconsin and for Jo Daviess, Stephenson, Carroll, Winnebago, Boone, McHenry, Ogle and DeKalb counties in northern Illinois beginning Noon Monday.

A strong storm system will continue to develop and strengthen over the Plains Sunday evening, lifting into the Upper Midwest and Great Lakes by Monday afternoon. 

As this happens moisture will be pulled northward as temperatures reach the upper 30's and low 40's in northern Illinois and southern Wisconsin.  Initially, the air mass near the surface and a couple thousand feet above will start off dry.  As the moisture moves into this dry air it will evaporate, causing the surrounding air mass to cool, resulting in temperatures beginning to cool near the surface.  This is a process known as 'dynamic cooling'. 

Rain showers, with even a few embedded thunderstorms, will lift north through Arkansas, Missouri and Iowa late Sunday night and very early Monday morning.  Around sunrise Monday, the leading edge of the rain showers will encounter the drier air mass over eastern Iowa and northwest Illinois, causing the rain showers to mix with snow and sleet early Monday morning.  This won't happen for everyone, and not all locations will experience this Monday morning.  The greatest chance for areas to see this  mix will be west of a line from Janesville to Freeport.  However, the chance is still there for the entire Stateline.  If any wintry mix were to occur, accumulations would remain light.

Temperatures at the surface will continue to warm above freezing but begin to fall near freezing, and slightly below, a couple thousand feet above.  A very strong upper level low will move in from the northwest towards sunset Monday evening, causing a period of heavy precipitation during the late afternoon and evening hours.  Some of the precipitation will begin as rain, but with the combination of temperatures below freezing and a strong upper level low, the warm air may be overcome allowing for a period of wet, heavy snow to fall for a couple hours Monday evening. 

Road temperatures are above freezing so as the snow falls it will initially melt on the road surfaces before turning to a slushy snow.  Accumulations may begin on grassy and elevated surfaces first, accumulating up to a couple inches in some locations.  Areas most likely to receive accumulating snow will be north of I-88.  Between roughly I-88 and Highway 38 an inch of snow could fall.  North of Highway 38 to roughly Highway 20, one to three inches of snow could fall.  It's possible that areas very close to the Wisconsin/Illinois border, and into southern Wisconsin could receive several inches of snow by Monday night as a period of all snow will occur further north.

Impacts to the morning commute don't look too significant, but impacts to the afternoon and evening commute are most likely.

The upper level low then moves overhead Tuesday afternoon, bringing down a fairly cold air mass with it.  Scattered snow showers are likely Tuesday afternoon with bursts of heavier snow possible during the day.  An additional inch of snow by Tuesday evening is possible.