Major winter storm on the way; 5-14” accumulations a good bet city, north and west -- tapering to an inch or two far south where heavy rains and flooding are a threat.
The fourth significant snow of the past two weeks, and potentially the winter’s biggest yet, is bearing down on the area. Chicago proper isn’t likely to escape a major accumulation. Current indications, gleaned from a suite of more than a dozen computer models, suggest the city is to register 5-10” totals—while areas just north and northwest, including McHenry, Lake, Ogle, Winnebago and even much of DeKalb and Kane Counties, may record SOME accumulations as high as 12-14” if current forecast trends verify. Any shift in the predicted storm track would necessitate adjustments to our prediction of accumulation and the placement of the heaviest snowfall. In addition, the potential for some embedded thunderstorms within the snow area isn’t out of the question. The intense bursts of snowfall such thunderstorms produce could produce locally higher totals, even in the city. The storm is likely to reach peak intensity in the hours before daybreak and through much of Wednesday morning. That’s when upward vertical motion generated beneath the nose of a powerful 110+ mph band of jet stream winds is to strongest. Upward motion enhances cloud and precipitation development and the potential for embedded thunderstorms, capable of especially intense bursts of snowfall, may occur. At that time, strengthening NE winds are predicted to reach 20-30 mph with gusts, which raises the risk for some blowing and drifting of the snow, especially in open areas.
With temperatures within striking distance of freezing during much of this precipitation event, the snow which falls is to be heavy and fairly wet -- what’s often referred to as “good packing” or “heart attack” snow. The weight of such snow means it must be shoveled with extreme care to avoid a heart attack. While many snowstorms produce a snow to water ratio of 10 or 12 to 1, it’s likely this storm’s snow/water ratio will be closer to 8 to 1.
South suburbanites, including many northwest Indiana suburbs and areas south of the city in Illinois, are likely to see MUCH lower snow totals if current forecast trends hold. Accumulations are likely to taper quickly as one moves south from Chicago to as little as 1-3” far southern sections toward Kankakee and Rensselaer, Indiana. In fact, it’s quite possible precipitation may still be falling in liquid form or as a mix as Wednesday begins, even as full blown snowstorm conditions cover an area from much of the city of Chicago on north and west.
Snow and sleet have already overspread the Chicago area as this update is being released at 4 p.m. Snow was first reported in McHenry County west to Rockford and across northwest Illinois in the last two hours. Rain is coming down across the southern suburbs. As the developing storm entrains more cold air, the mix of sleet, snow and rain is shift to ALL snow from Chicago north and west- with possible embedded thunder and lightning. Snow is to fall much of the day Wednesday, heaviest into the early afternoon then more occasional.
We continue to monitor this system and will have a complete update on our 5:55 pm WGN report and on Tuesday evening’s Nine O’Clock News.
Tom Skilling
Chief Meteorologist, WGN-TV/Chicago Tribune