Complicated scenario

Update 10:02pm: GFS data has been coming in. There has been virtually no change in computer model solutions. I've also been looking at actual observed data. The information available to me does not raise any red flags to change the forecast. Rain is increasing in coverage over Iowa as I anticipated. That will move into our area overnight. Temperatures south of the IL/WI border should remain warm enough to support mostly rain, although some ice accumulation is possible, especially on bridges and overpasses. The higher risk for ice is still north of the IL/WI border.

Update 8:22pm: Precipitation is tapering off from SW to NE for the time being. Monroe reported light snow at 8:15; Janesville reported light freezing rain; here at the station, we ended the freezing rain with a few flurries.

We're entering a lull in the action. Looking at current surface observations, there is one low pressure located in eastern Kansas and another in northern Oklahoma. As the whole thing moves northeast, we should start getting enough lift to generate more precipitation. Some showers are developing in southern Iowa, and I suspect that is a sign that the main event is starting.

The evening model runs are coming in, by the way. The NAM has come in so far, and it has changed very little from the previous couple of runs. It handled the surface low placement much better than before, but everything else remained essentially the same. Since the run began more accurately, I have higher confidence in what it shows. At this point, the going forecast will remain the same. We'll see what the new GFS model shows in a little while.

Update 7:18pm: Still having areas of freezing rain. While the temperature may be just above freezing in some locations where rain is falling, it may still turn to ice due to the cold, snowy ground. The Storm Prediction Center has issued a technical discussion.

In addition, there are indications that the surface low will take a slightly more southerly jog. If this is the case, the winter threat areas will probably shift slightly southward. This puts southern Wisconsin in a greater risk of heavy snow, and far northern Illinois in a greater risk of additional icing. This possibility needs to be monitored over the next few hours. Of course, I will have updates as I have them.

Update 5:49pm: Freezing rain is accumulating enough to cause slick spots. While it's only a thin glaze at this point, be wary of icy conditions through the evening tonight.

Update 4:18pm: Winter Weather Advisory for northern IL along US 20... Winter Storm Warning for Wisconsin. I agree with the justification for the National Weather Service doing this. Northern tier of counties in Illinois should experience enough of a mix, then snow accumulation tomorrow, to make for hazardous but hopefully manageable conditions. In Wisconsin, the threat remains for not only some ice accumulation but also significant snow accumulation, especially just north of our viewing area.

The snowfall forecast map in the right-hand column on this page is my updated forecast. As I mentioned in my original post, I did move the snowfall swaths to the northwest a little bit. I expect Rockford to be somewhere in the neighborhood of 3-4" of snow, based on current trends and data.

However, I cannot stress enough that even very subtle changes in how this all plays out will have potentially significant consequences on precipitation type and amounts.

Original Post: As Adam pointed out earlier, much of the precipitation was evaporating before it was reaching the ground. However, this makes the atmosphere cooler - further complicating what type of precipitation will fall when. In addition, while warmer air is continuing to push northward, the warm front itself looks like it may be stalling for the time being.

Still many crucial details that need to come together just right... but a quick summary is that I will not be altering Adam's forecast very much. I may push the snowfall swath a little further northwest, but beyond that, we're entering the "wait and see" mode.

Mix changing over to rain in the south... mostly rain near US 20... and a mix north of the IL/WI border tonight. Some ice accumulation is possible, especially north of the border. The transition back to all snow will occur from NW to SE through the day on Tuesday. I'm expecting Rockford to transition back over to all snow between 10am and noon. Occasionally heavy snow is possible, especially in our northwestern towns (such as Monroe). Still going with 3-6" in Rockford; higher amounts NW, lower amounts SE.