It's never cut and dry with winter systems. It takes a lot to get the ingredients for snow to fall... and it gets messy when you get anything else in the winter. Once again we're going to be dealing with a wintry mix through the day Monday.
Here's a look at the system from our short term model, the HRRR:
The timing of the precipitation and the temperatures at the surface will be critical and determining exactly what will transpire. There is a concern for freezing rain during Monday morning's commute. Temperatures in the morning will be below freezing and, because it's been cold this weekend, road temperatures will likely be near the freezing mark. (I will note any roads, sidewalks, driveways, etc that go untreated will be most at risk).
Low temperatures Monday:
The reason why freezing rain is going to be likely has to do with what is going on a couple thousand feet above our heads. Here's the projected 850 temperatures (in Celsius) at the same time in the morning:
Because the 850 temperature is above freezing (above 0° C) that's indicating there's a warm layer in the atmosphere. By examining soundings, it's evident there will be a large melting layer. Snow will be falling way up above our heads, begin to melt into rain and then freeze on contact at the frozen surface.
Even a little bit of ice accumulation can cause problems. A glaze to 0.05" of ice will be possible in parts of eastern Iowa into Illinois tomorrow morning (through noon). The National Weather Service has posted a Winter Weather Advisory - as Terry expected - for some of these areas.
I wouldn't be surprised to see this get extended further to the west by morning. As temperatures at the surface warm into the afternoon there will be a transition to sleet/rain. Then, as temperatures cool back down in the evening (at the surface and higher up in the atmosphere) there could be a transition to all snow.
There are some differences in the models on just how much snow accumulates, but there will likely be at least a dusting to a few inches of snow in eastern Iowa into Illinois and Wisconsin.
Here's the latest sweep. The GFS:
Wet, sloppy snow could also make for a slippery evening commute. Then, the precipitation ends and cold air settles in for the beginning of 2019.
Low temperatures Tuesday morning:
And the wind chills...
It's going to be a sloppy end to 2018 and winter will hang with us as we ring in the new year.