As we now have gotten one day closer to this potential weekend winter storm, we have been able to look at a full day of new model guidance to get some opinions as well as looking at the latest trends among said guidance from the last night and today so far.
What we know: Chances are increasing for a significant winter storm (6"+ of snow) to cross the nations midsection late Friday night and through the day on Saturday, one that could potentially impact portions of Illinois.
What we don't know: Exact timing, track of the storm system, placement of the heaviest snowfall amounts and snowfall amounts in general. Anyone out there trying to pass off snowfall accumulation maps are purely doing so for hype and attention. Again, yes...we are one day closer but that doesn't mean anything in regards to knowing expected snowfall totals. This system is still well out over the Pacific Ocean for cryin' out loud.
Let's dig a little more into the setup and some details below...
As mentioned above, our system of interest is still well out over the Pacific Ocean this Tuesday afternoon (as you can see in the image below circled in blue) and will continue to move in the direction of the West Coast of the U.S. and will continue in that direction over the next 24ish hours or so until it comes ashore tomorrow night.
Between now and Friday evening, this system and very strong associated mid-level jet (seen in the loop below) will not only come ashore out west, it will continue eastward and eventually cross the Rockies, before emerging into the plains as we head into this weekend. The main thing to take away from this system coming ashore is that once it does, it will be sampled much better for the weather models we look at. This means that all around the country, weather balloons are sent up twice a day in over 100 locations and these balloons record data that are then put into weather models. So, once our system is sampled better after coming ashore, numerical model guidance will be able to fine tune this forecast and get a better handle on things such as track and strength of this system.
By Friday night, our main upper level low will be located just on the other side of the Rockies before entering the southern plains, as current model guidance looks now. It is after this however where models begin to diverge on solutions. The further south the system goes, the further south chances for a significant winter storm will go, possibly favoring further downstate across Illinois. Further north, chances will increase for northern and central Illinois. All these questions will likely be ironed out in a few days.
Even so with the waffling solutions mentioned above, we are still seeing a good consensus on low pressure developing on the lee side of the Rockies and a strong low level jet/strong warm air advection aiding in developing a widespread area of snowfall along and north of the tight baroclinic zone (circled in red below), which is a fancy word for a sharp thermal gradient or contrast in temperatures aloft.
As low pressure begins to push east across the plains, so will the developing widespread area of snowfall to the north of the thermal gradient. Below is a 30 hour loop from late Thursday night into early Saturday morning should one possible solution as snow develops in a widespread fashion to our west across the plains, before moving east into at least the northern half of Illinois by late Friday night. As mentioned several times so far, this timing is by no way set in stone and could change.
After the area of low pressure gets through the plains, we begin to see more diverging model solutions with how they handle the strength of the low and more importantly, the track it takes as we go into and through the day on Saturday. Below is a loop showing the trend of the European ensemble members over the last five runs. Each red L on the loop below is one potential position for the surface low to be. As you can see over time, the L's begin to slowly converge on one area, becoming less spread out with possible solutions. They are not all in agreement yet but they are getting a little closer as we get closer to this event. This is a great way to visualize the number of possible outcomes and the "cone of uncertainty" that you get sometimes in forecasting.
Hopefully this explains things a little better regarding the track is still very much up in the air. Anyone posting snowfall totals map has no reasoning behind doing so as it will most likely change between now and Friday night. So as you can see below, there is the potential for a widespread, significant winter storm across the Midwest, including here in Illinois. Even so, if the area of low pressure were to take a different track, this would have big changes to the placement of the heaviest snowfall axis as well as who sees snow. If it were to track further north, more of Illinois would probably see rain while a further south track could mean more snow chances for the entire state. Stay tuned....