The Chicago area and much of Illinois, northern Indiana. far southeast Wisconsin and southern Michigan is to experience first hand the rapid demise of an unseasonable 6-day late season warm spell before nightfall Wednesday ahead of what may become a major winter storm Thursday night and Friday. Thundery downpours have generated rainfall unofficially exceeding 4" in parts of the city this morning and early afternoon. Standing water and local flooding have been the result. And, additional driving downpours will sweep the area with regularity through a good part of Wednesday night. A lighter, less intense mix of sleet, snow and rain follows in MUCH COLDER air Thursday. The second major wave of storminess sweeps the Chicago area in much colder air Thursday night and Friday. That's when a growing number of forecast models suggest snow may become heavy amid howling NNE winds. In a worst-case scenario, Friday morning's rush hour could find the metro area at the height of the storm's wind and snow.
Winter storm watches in anticipation of the vigorous system arrival have been hoisted by the National Weather Service over much of central and southern Illinois Wednesday and are to be hoisted across the Chicago area in the late day forecast cycle covering Thursday into Friday. Though computer models have wavered on details of the storm's track in recent days, our in-house survey of 37 computer forecast model runsover that period--including the models run by the National Weather Service and others belonging to Environment Canada, the U.S. Navy, the UK Met office and the European Center for Medium Range forecasting--has consistently indicated the potential late week winter weather trouble. The most ambitious of the latest forecast models is suggesting powerful 30-40+ mph gusts could accompany heavy wet snow at the height of late Thursday night and Friday morning's storm and that significant accumulations may occur given the rate at which the snow is to fall. The ground and other outdoor objects are warm in the wake of a week of readings 20+ degree above normal. What's more, winds are to blow into Chicago off mid-40-degree lake waters until they back more NW Friday. This makes accumulation forecasts VERY tricky. But with the potential for embedded thunderstorms in the snow area suggested by the most energetic storm development scenarios, there's little doubt significant snowfall is possible. Our survey of the 8 most recent computer precipitation forecasts for Chicago proper reveals water equivalent precip totals during the period in which the atmosphere's thermal profile appears most supportive of snow ranges from as little as 0.04" to as much as 1.756". The average among these forecasts and 16 other recent projections we've examined covering the Thursday night/Friday storm period averages out to around 0.70". An 8 to 1 snow to water ratio would lead to 5-6" of snow using such an average. But, that same ratio with the heaviest precip totals generated by recent computer runs could lead to 14" tallies in at least sections of the metro area. This is all very tentative, but you can see why there is concern about the approaching system.
As always this far in advance of a major winter storm, MUCH CAN CHANGE. So, details of the forecast are hardly carved in stone at this early hour. Storm's like this, which have looked stunning on paper, have sheared out or taken a completely different track and missed the area. But, there's enough evidence at this point to remain watchful. More on the developing situation later on this blog, on our WGN Nine O'Clock News and in the Chicago Tribune and on the Tribune website.
Tom Skilling, WGN-TV Chief Meteorologist
