We are still keeping a watchful eye out west and what is most likely to become a strong storm system as it moves into the Midwest late tomorrow night and more so during the day on Thursday across Illinois. Numerical model guidance continues to struggle with the handling of this system and several key features, but we are starting to see a little more consistency with regards to a shot of accumulating snow across the Midwest. Whether that is across parts of Illinois or elsewhere, remains to be seen at this time. I dig into the details below, including what we know at this stage and also what we don’t know moving forward.

Our southern stream upper level low is currently slowing spinning across the southern California coast late this afternoon and will continue to do so the rest of this day, before eventually turning more easterly as it moves across Arizona and into New Mexico by tomorrow evening. Model guidance is in fairly good agreement on this as we get into the day tomorrow.


Below is a loop showing this feature, the upper level low progressing eastward into tomorrow afternoon from southern California into New Mexico.


What happens next is what is giving the model guidance the most fits and what will determine how strong of a storm system this can become, along with accumulating snow chances as well. Sometimes in meteorology and when getting strong storm systems, two separate waves will eventually phase, or come together and become one larger system. Other times, they will remain separate due to poor timing or location of those waves and usually less phasing leads to a weaker storm system. So far, I’ve talked about one of the waves, the southern stream wave that is across the southern California coast. The northern stream wave aka the second wave of this puzzle is forecast to dig southeast from the Gulf of Alaska (where it currently is now) into the northern plains by tomorrow afternoon,

You can see this feature nicely below (circled in blue) on the GFS model which is valid for late tomorrow night as it moves southeast into the northern plains.


I threw in a comparison below between two models (the NAM and GFS) valid for midnight tomorrow night showing the key differences. As you can see below, I circled both waves (the northern stream and southern stream wave) on the NAM and GFS images. One model (the NAM) is phasing these waves more where the GFS isn’t as much, keeping them more separated. The amount of phasing that we see from these two weaves will determine how strong the surface low can get during the day on Thursday. The stronger the system, the faster it can A. pull in cold air from the north, and B. the stronger dynamics it will have to more so create its cold air. This is called dynamic cooling.


So as you can see, some models lead to a changeover to heavy snow across northern Illinois and the Chicago area, where other models are much weaker and/or keep the snow to our north across Michigan and Wisconsin. One model below (the NAM nest) is on the stronger and colder side of guidance which changes precipitation over the snow across northern Illinois during the late morning and early afternoon hours on Thursday.

Below is a quick 3 model comparison on snowfall amounts through Thursday evening across the area including Illinois, The NAM, GFS and Euro. As you can see, large differences still exist.


The bottom line: Rain is expected late tomorrow night through Thursday morning across most of Illinois, with a changeover to snow possible on Thursday afternoon across northern Ilinois (any accumulations/amounts still to be determined)

A timeline follows below:

8pm tomorrow-overnight: Rain and potentially thunderstorms will move into a good chunk of Illinos tomorrow evening and into the overnight hours.

Thursday morning: Rain and isolated thunderstorms continuing across central and northern Illinois. 

Late Thursday morning-through Thursday afternoon: Rain potentially changing over to moderate to heavy snow with accumulations possible


We will continue to have updates rolling out on this system and new model guidance continues to roll in whether it shows a higher likelihood of a snow event for portions of Illinois, or mainly a rain event. Stay tuned!