Over the last 24-48 hours, numerical model guidance has continued to both show a significant storm moving into the Midwest, including portions of Illinois along with coming into better agreement, at least to an extent on the overall evolution of this system as it moves into and through our area.
A fairly potent shortwave still out over the Pacific Ocean this morning is forecast to come ashore the Pacific Northwest tomorrow night and early Saturday morning and then dive southeast across the Rockies and emerge into the plains during the day on Saturday. It is after this where models start to diverge on exact solutions with regard to the exact track and strength of this system after crossing the Rockies, along the the potential for phasing with a northern stream wave that will be digging south and southeast into the northern Midwest from southern Canada. Even with these differences still at play, model guidance is consistent on a strong surface low moving east across the southern/central plains (the Kansas/Oklahoma border) later Saturday before turning it more northeast on Sunday.
As the strong upper wave dives southeast into the plains Saturday night, the strong surface low will continue east and northeast across the plains and into the western Midwest. As this is occurring, low level winds will be responding to pressure falls and this will mean stronger warm air advection and isentropic lift up and over the cold air, leading to a fairly widespread area of precipitation, some of it in the form of snow.
The surface low seen in the image above will then track into Missouri and eventually Illinois by during the day on Sunday, with a swath of accumulating snow from the central plains into portions of Iowa and Missouri, before moving into Illinois. As mentioned earlier, there still remains differences regarding the track and strength of this system and this has big implications on both snow amounts and the placement of the heaviest snow swath.
Timing wise for places in Illinois that look to see accumulating snow at this time would be during the afternoon and evening hours and possibly extending into the overnight hours as well depending on the exact speed of the system. Below is one models take showing rain changing over to moderate to heavy snow from west to east across western and northern Illinois by the late afternoon and evening hours and this is where several inches
Below is just one models take on forecast snowfall accumulations and is by no way a set in stone for sure forecast, so DO NOT TAKE IT VERBATIM at this range....