After a quiet and sunny day today across Illinois due to high pressure, attention is already looking out west at our next weather maker due in here tomorrow and tomorrow evening. It is a tricky forecast tomorrow regarding precipitation type and also amounts as I dig into the setup below

Currently across the area, high pressure is over the Midwest and will continue to slowly depart to the east by late tonight and into the overnight hours. Our wave (circled below) of interest is currently moving across Montana and into the northern plains as it crosses the Rockies. During the overnight hours as the high pressure moves east and our wave comes into the plains. Winds will respond and a low level jet out of the SSW will lead to strong warm air advection and isentropic lift into our area. Precipitation should develop on the nose of this advection during the early morning hours and this looks like it will be in the form of all snow. Given the very cold and dry conditions over northern Illinois, it could take awhile to fully saturate the atmosphere. Given this, only expecting light snow or flurries during the first part of this system.



Our shortwave will continue ESE into the plains and part of the Midwest by tomorrow afternoon.
Winds will respond and a low level jet out of the SSW will lead to strong warm air advection (below in red) and isentropic lift into our area. Precipitation should develop on the nose of this advection during the early morning hours and this looks like it will be in the form of all snow. Given the very cold and dry conditions over northern Illinois, it could take awhile to fully saturate the atmosphere. Given this, only expecting light snow or flurries during the first part of this system.
Below is a forecast loop of snow developing over northern Illinois tomorrow morning in response to that warm air advection.
 This system will come in two parts with the first smaller part early tomorrow morning with the snow chances. The second and larger part with a slug of moisture/precipitation forecast to move in from the southwest during the mid-late afternoon hours and last into the evening hours, especially across the southern portions of the Chicago area and south of I-80. This is where model guidance differs some with regard to precipitation type and this has to do with the amount of low level warming that is occurring in the models. Below are three forecast soundings for around the I-80 corridor at 21z tomorrow (3pm) As you can see, two models (NAM and NAM nest) show a warm layer just off the surface that would lead to more freezing rain where the GFS is a slightly colder solution.
The GFS is a little colder due to cooling processes through the lower part of the atmosphere and end up producing more snow given the slightly colder thermal profile. I am favoring a somewhat colder solution, especially north of I-80 including the Chicago metro area where most of the precipitation should be in the form of snow tomorrow afternoon and evening.



Right now thinking is that the best accumulating snow chances will exist north of I-80 up into I-88 corridor across northern Illinois with more of an ice threat south of I-80 down into central Illinois. A snow and ice accumulations map will lay out the details below.