Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 542 AM CST Fri Dec 9 2016 .SHORT TERM... 410 AM CST Through Sunday Night... Main concern for the short term forecast period will be the developing winter storm which is expected to bring snow to the region as early as Saturday afternoon. In the meantime, today and tonight will be relatively quiet with mostly cloudy skies and unseasonably low temperatures. Highs today will be in the lower 20s with lows tonight in the middle teens. With winds of 5 to 10 mph overnight, wind chill readings will be in the single digits. As for the upcoming snow event, the longer range model guidance, with the continued exception of the NAM, are in relatively good agreement. Timing is still a bit tricky as the upper level pattern is evolving into relatively zonal flow and timing of individual shortwaves can be difficult. However, the GFS/ECMWF are in relatively good agreement on the timing and strength of individual waves, so confidence is increasing on the details and impacts of this system. There should be 2 relatively different phases to the pcpn. Initially, snow should begin to move into the wrn portions of the CWA by early Saturday afternoon. Conditions should still be sufficiently cold through a deep layer of the atmosphere that pcpn type is not in question and will begin as snow. The initial wave of pcpn will be driven by large scale warm advection in advance of a developing sfc trough over the wrn plains. As sfc high pressure settles off the Carolina coast, the GLFMEX will become more open, with deep layer moisture streaming nwd ahead of the trough. Upper level support should be somewhat weak as sheared out shortwaves track across the upper Great Lakes. From Saturday afternoon through early Sunday, the heaviest snow should be over the northern portions of the CWA, generally north of the I-80 corridor. Given the cold in situ conditions, snow to liquid ratios should be a little higher than typical, around 17:1. Current indicators suggest that 4 to 6 inches may fall by early sunday morning north of I-80 with amounts diminishing rapidly to the south. By Sunday afternoon, pcpn type becomes more of a concern. Sfc low pressure developing over the plains should approach the middle Mississippi Valley by mid day Sunday, further increasing the warm/moist advection. Sfc temperatures will begin to creep upward through the day, but with warmer air aloft overspreading a colder sfc, where temps should still be at or slightly below freezing. This will set up the potential for a at least a mix of fzra/ra or fzra/sn and a narrow corridor of freezing rain. Timing of the mix pcpn phase of the system remains the biggest uncertainty. The latest guidance would suggest that pcpn could be all snow into early afternoon with the changeover to mixed pcpn and then to liquid occurring over the srn portions of the CWA through the afternoon and into the evening hours. The sfc temperature trend should be atypical as well, with temperatures continuing to climb through the evening. At this point, it looks like the freezing rain could reach as far north as the I-80 corridor through the evening hours, while pcpn over the southern portions of the CWA becomes all liquid. The northern portions of the CWA, especially north of the I-88 corridor will likely remain all snow, though with the warmer air spreading north, liquid to snow ratios should decrease, dropping to 10-12:1. The greatest uncertainty in timing of the evolution increases through Sunday night as the ECMWF trends a bit faster than the GFS in lifting the sfc low across nrn IL and to the ern Great lakes. This appears to be a result of the GFS showing a bit more phasing of a middle stream shortwave across nrn IL and a nrn stream shortwave tracking across nrn Wisconsin. Given the relatively fast zonal flow aloft initially, would tend to favor the faster solution of the ECMWF, which would ultimately lead to pcpn tapering off a bit quicker than the slower, more phased GFS solution. But at this time range, the differences between these 2 solutions would ultimately only cause minor differences in eventual snow totals. Based on the currently favored solution, the northern portions of the CWA could see an additional 3 to 5 inches of snow, with snow amounts, again, rapidly dropping off to the south. For the entire event, currently expect 7 to 10 inches of snow north of the I-80 corridor, with the highest amounts closer to the Wisconsin border. The far southern portions of the CWA may only see 1 to 2 inches.
Friday will mostly cloudy and with well below normal highs in the low to mid 20s. An active winter pattern will develop this weekend into early next week, with an extended period of accumulating snow expected. The snow will begin Saturday afternoon come to an end by early Monday morning. Enough warmer air could get drawn northward Sunday afternoon and evening for the precipitation to mix with or change to rain or freezing rain, especially for locations south of I-80. A period of bitterly cold weather remains likely Tuesday night through Thursday. Please stay tuned for the latest forecast updates.