Still watching the weekend for potential winter system

As Meteorologist Brandon Arnold talked about Tuesday night, there are several different scenarios that could play out for this upcoming weekend in terms of a winter storm.  The potential  is there for several inches of accumulating snow, however, there's also the potential for more of a mix or even freezing rain scenario.

If you follow the First Warn Weather Team you know we are not about hype and want to make sure we receive the most up-to-date information before making any sort of snowfall forecast.  And while the uncertainty still remains for the weekend, I wanted to show you just how from one day to the next a weather model can vary in terms of the track of the low, intensity, amount of moisture, temperature, etc.

The graphic to the left is the American GFS model from Tuesday evening for 6am Sunday and there are a couple important features to point out.  First, notice the placement of the low...near the Indiana/Kentucky border.  This far southern track of low pressure is a little too far to provide a big, accumulating snowfall event for Northern Illinois.  Central Illinois and Indiana is a different story.  Second, the colder air (indicated by the blue, dashed lines, is just settling into Northwest Illinois meaning a higher potential of more of a mix.  Remember, this was last night's run of the GFS. 

Now, look at this morning's run of the American GFS model:  Very different.  This is the same time frame - 6am Sunday morning - as the previous image, but look at the placement of the low.  This particular run has the low tracking from the Texas panhandle Saturday night to the St. Louis area by midnight Sunday and then just south of Chicago by Sunday morning.  This is a dramatic shift north from the previous run indicating how inconsistent the models are this far in advance.  Something also to point out, the low then tracks from south of Chicago Sunday morning into Michigan by Sunday afternoon putting the axis of greatest lift, and snow, right over the Southern Wisconsin/Northern Illinois area.  Tuesday night's model run had the low in Ohio by Sunday afternoon!  Now, this is only one model of several that we look at and the GFS has been known for showing these wide variations this far in advance, but I think it's important to show that while there have been many posts and images of a significant snowfall it's also important to show that the forecast can, and will, change.  Some of the other models will be coming out within the next couple of hours and what we'll look for in those is consistency.  Not only between other models, but also within the model itself.  It's also important to point out that when trying to determine how much snow or what type of precipitation will occur, we need to look at where the upper level lows travel.  A general rule of thumb is that the heaviest snow will occur to the north and northwest of where the upper level low is located.  So, if the upper level low is located right over Northern Illinois, then the heaviest snow would likely be just to our north and northwest.

A couple other things to note with this next system are this: there will be a lot of moisture within the system and temperatures will be very close, if not a little above, freezing.  Climatologically, systems like this tend to have smaller snow to liquid ratios meaning the snow would be heavier than the snows we've had in the past.  Also, any sort of deviation of the track or increase or decrease of the temperature will have a huge impact on what occurs at the surface.  While it's hard to pinpoint how much snow we'll receive, the weather will impact this weekend.  So, if you've waited until last minute to finalize shopping I would say it may be best to get what you need done before this weekend.  Now, does this mean you need to stock up on break and milk, no.  But, if you have last minute Christmas shopping or travel plans, you will want to pay very close attention to this forecast.  Each day myself, Brandon and Max will have a little more information to provide to help clear up the picture for the weather this weekend!