Although the NWS in Chicago has yet to include its counties in the Winter Storm Watch, Stephenson, Jo Daviess, Carroll, and Whiteside counties are still in this watch on Thursday.
I can't criticize the good folks at the Chicago office for not including us in this watch. This is a very complicated forecast, perhaps the trickiest one we've had so far this "winter." There is still a lot of uncertainty as to where the surface low will track tomorrow, and this could throw a wrench into the above snowfall forecast.
Right now, this low is in southeastern Montana and is about to cross into northeastern South Dakota. Some of our models suggest this low will track along the I-80 corridor, just to our south. This solution would suggest light rain mixed with snow for the Stateline, with little or no accumulation. However, other model solutions suggest this low will move into central Iowa by morning, then re-develop further south over southern Illinois in the afternoon. If this happens, colder air would reach the area and most of our precipitation would fall as snow. Recent trends lead me to believe that the southern track will be closer to reality, which would mean wet, heavy snow for the Stateline area.
With a strong piece of energy moving through the jet stream late in the afternoon into early evening, snowfall rates could briefly approach 1" per hour, with some "thundersnow". Although this could lead to a quick 2-4" for some of us, snow accumulations earlier in the event will be limited by warm ground and air temperatures above freezing. That's why I think 3-6" will be the rule for most locations.
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