We are continuing to keep a close eye and all of our attention on the latter half of this week and into the start of the weekend as it continues to look like a long duration winter storm is on the way into the Midwest, including here in Illinois. There will be a large area of snow accumulations from this system but there will also be some areas that receive sleet, some freezing rain, and plain rain as well.
Our system continues to inch closer to the coast and will be coming ashore later tonight. By tomorrow night, it will be coming out of southwest Canada after crossing the northern Rockies and heading into the northern plains. Even though the still will still be quite the distance from us in Illinois, it will begin already have impacts on the Midwest by tomorrow night. The evolution of this upper level low and at times with feature two smaller ones, is quite complex and is one of the reasons we could continue to see some changes among model guidance the next 36 hours or so.
As the upper low gets into the plains, low pressure will develop at low levels of the atmosphere with strengthening winds across the plains aiding in strong warm air advection up into the Midwest. This will lead to widespread precipitation developing to our west and southwest (area circled in orange) tomorrow morning and afternoon in response to that warm air advection and isentropic lift.
Below is one models depiction/animation of about the first half of this entire systems life cycle. Widespread precipitation will slowly inch its way into Illinois, mostly in the form of snow (but some mixed precip and rain south) later tomorrow night and into Thursday morning. Most of this snow will be on the lighter side with accumulations up to an inch or two possibly by Thursday afternoon. As the upper level low and surface low both strengthen and draw closer, we could see a better period of snow both Thursday night and again on Friday night. We cannot stress enough the slow moving nature of this system and someone getting two days of snow is definitely on the table. An interesting thing to watch will be what exactly surface temperatures will verify as. Some model guidance has temperatures right around if not just above freezing which would make snow accumulations difficult given the lighter nature/rates to be expected for a majority of this storm.
The thing to note though is that the forecast snowfall amounts below are not going to come fast and furious like we saw in the last system where areas got all of their snow in 6 hours or less. This system could lead to some areas seeing continuous or close to continuous snowfall for nearly 48 hours with the snow being more of the lighter type snow.