A WINTER STORM WATCH has been issued for all of southern Wisconsin and most of northern Illinois beginning either Monday evening or Tuesday morning. For Stephenson, Jo Daviess, Carroll, and Whiteside counties the watch will begin at 6pm Monday and last through Wednesday morning. For Green, Rock, Walworth, Winnebago, Boone, McHenry, Ogle, Lee, and DeKalb counties the WATCH goes into effect Tuesday morning and will last through Wednesday morning.
While some of the finer details with the winter storm are becoming a little clearer, the storm is still 24-36 hours away and subtle shifts and changes are likely to occur during that time. But here are some of the key messages we want to get out regarding impacts of the incoming snow:
The snow event will arrive in two rounds, with the first coming in late Monday evening and the second during the day Tuesday. Both rounds will feature a heavy and wet snow with temperatures in the low 30s. Monday night's snowfall will likely cause some impacts to Tuesday morning's commute, but greater impacts are likely for Tuesday afternoon and evening. It's during this time that a very heavy and wet snow will fall likely causing roads to become snow and slush covered throughout the day, reducing visibility, and making travel difficult to dangerous at times as the day wears on.
Low pressure currently over the Southwest will lift to the northeast through Plains Monday, and into the Great Lakes Tuesday evening. The current track of the low takes it from near St. Louis, MO Tuesday morning towards Chicago and northwest Indiana by Tuesday late afternoon and evening. This type of track does put areas of north-central and northwest Illinois, along with southern Wisconsin, in the favored location for heavy snow and high totals. But the absence of a cold air mass with the storm does raise some questions as to where exactly the rain/snow line sets up, which could ultimately impact totals overall. Over the last couple of days, the storm track has shifted some to the northwest, which pushes the freezing line closer to northwest Indiana and northeast Illinois. However, the heaviest snow usually falls just north of where the surface low tracks - placing areas from the Quad Cities up through Rockford in line for heavy snow. Temperatures during this time will be borderline freezing. In fact, temperatures will likely remain above freezing during the entire event. This raises some questions with regards to just how much of the heavy, wet snow will be able to accumulate during the event.
The snow to liquid ratio is likely to be lower than the average 10:1 - and probably closer to 8:1. This indicates that the snow will have a lot of moisture with it. Great for making snowballs and snowmen, but very sloppy and slushy. A heavy and wet snow doesn't accumulate as well as a drier and lighter snow. However, being just north of the surface low track the amount of lift in the atmosphere may be able to overcome the lower snow to liquid ratios by producing high snowfall rates - over an inch per hour - causing the snow to accumulate rather quickly throughout the day. If these higher snowfall rates do occur, travel would become very difficult from mid to late morning Tuesday through the evening.
Another key element to this winter storm is the rain/snow line. Should the track move a little further to the northwest tonight and Monday, that line would be pushed further inland and possibly impact areas to the east and southeast of Rockford from McHenry County down through DeKalb and parts of Lee County. There is likely to be a very sharp gradient of where exactly we see all snow, and heavy snow, to where we end up seeing a mix and lower snowfall totals. This gradient will become better defined over the next 12-24 hours.
As of Sunday evening, our preliminary snowfall forecast is calling for between 4-8 inches of snow across most of the region, with a snowfall gradient lining up from parts of McHenry County through southern DeKalb County. The greatest impacts are likely to occur from southern Wisconsin through most of north-central and northwest Illinois Tuesday with regards to snow accumulation and wind. And while there isn't a high chance for this, sporadic power outages are possible given the heavy nature of the snow and gusty winds Tuesday.
We will continue to monitor the latest trends and track of this storm and provide updates as needed. It may be a good idea to think about adjusting travel plans, especially Tuesday, if the current forecast remains on track. Most of the snow will be wrapping up Tuesday night with only a few flurries lasting into Wednesday morning.
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