Update from the National Weather Service at 5:51 am this morning:
Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 551 AM CST Sun Dec 4 2016 .SHORT TERM... 324 AM CST Through tonight... Main forecast concerns/challenges are with the expected accumulating snow over much of northern Illinois and northwest Indiana this morning into the late afternoon. Main forecast changes this morning were to increase snowfall amounts and expand the advisory southeast to now include Livingston/Kankakee counties in Illinois and Lake/Porter counties in northwest Indiana. Everything still appears to be on track for the first accumulating snowfall today. Upper level trough noted on satellite imagery, with several embedded impulses quickly lifting northeast into the mid Mississippi Valley. Ahead of this feature, isentropic ascent is also helping to saturate the column. Although returns noted on radar, this saturation has not occurred quite yet, with snow not occurring at this time. Adjusted pops in the near term to reflect latest trends with upper level support providing the onset of snow across the far western CWA right around 12Z. This would include all areas in the west, along the I-39 corridor. Remaining areas to the east in northern Illinois still on track for right around a 15z time frame, and then soon there after for areas in northwest Indiana. Given this noted time line, have adjusted timing of the advisory, mainly to delay the eastern half until 15z. As this upper level trough swings through the area later this morning into the afternoon, expect a transition from light snow to more moderate intensity snow as large scale ascent really ramps up ahead of this feature. Most guidance in agreement with the increase in forcing due to many features coming together, which include a stout negatively tilted trough and more potential for instability aloft. Would anticipate many locations across much of northern Illinois and northwest Indiana to see this ramp up in snow intensity by midday, and even more so by early to mid afternoon. This not only includes areas along the I-39 corridor which was previously noted, but now also includes many areas in the northern two thirds of the CWA likely centered from just north of I-88 south to along/south of I-80. In these locations, strong forcing and more efficient snow production will likely allow for an increase in snow amounts even into the afternoon when the more intense snow will be able to offset melting. In the height of the more intense snow, 18-23Z, am a little concerned that the snow will come down hard enough to accumulate more on the roadways with at least slushy conditions possible. All of this and the increase in snow amounts, feel the advisory is still warranted and worthy of expansion. Now have snow amounts of 3-5 inches along the I-39 corridor, 2-4 inches elsewhere in the advisory in northern Illinois and northwest Indiana, and then 1-3 inches across the far southeast CWA. In this location, rain may briefly mix with snow but do think forcing will be strong enough even in this location to help the column/boundary layer to cool to support all snow in the afternoon. Snow will quickly taper off right around the 00z time and although some light snow may still be ongoing across the eastern CWA in the 00-03z period, it will likely be light enough to not warrant extension of the advisory. Rodriguez && .LONG TERM... 347 AM CST Monday through Saturday... Monday appearing dry/quiet on the backside of this system with rising heights and building high pressure. Continued WAA will usher in a warmer airmass but wonder what impact this will have on high temps with cloud cover possibly lingering and with a snow pack likely in place. Still keeping an eye on two possible systems to affect the area during the week. Confidence is still on the lower side with the first system arriving Monday night into Tuesday. Timing and placement adding to the low confidence as well as extent of the airmass which would be in place. Have continued to limit pops as well as continued to mention both snow and rain. Much colder airmass still expected later in the week, however, confidence also remains low with snow potential Wednesday night into Thursday. Still remains a possibility for snow but with varying guidance, no change in confidence. Much colder air and lake effect snow potential will then be concerns through the remainder of the work week, as well as snow potential returning CWA wide by the weekend.