How the Most Recent Snow Could Play a Factor into this Weekend's System

We weren't even finished with last weekend's snowfall before talk came up of another potential winter storm the following weekend.  "Are we going to get another half a foot of snowfall?" "I heard we will get more ice with this one"  "When is it going to be the worst?".  Those are some of the many questions I've heard regarding the weather this weekend.  So let's discuss this weekend's storm system a little more and look at how the recent snow could impact what we get this weekend.

When forecasting a potentially big weather event, meteorologists look for consistency within forecast models.  The less consistent models are from day to day, the less confident the meteorologist may become.  And really up until about a day and a half (sometimes even less) are we really going to know just how a storm system will track.  The low pressure system for the weekend is still well off shore.  And as you can imagine, models haven't been overly consistent with their projected forecast tracks going into the weekend.  However, they have been pretty consistent in wrapping up a decent amount of warm air ahead of the low somewhere in the Midwest and Great Lakes.  The problem this brings is if warm air gets wrapped up far enough north a transition from snow to freezing rain may occur.  And here's why...

The snow we have on the ground isn't going anywhere this week with temperatures staying below freezing.  While we may melt off a little bit of the snow by this weekend, we will still have a decent snow depth by Friday night/Saturday morning.  The snow cover acts like a refrigerator over Northern Illinois and Southern Wisconsin keeping the air mass cooler compared to where there is either less snow or no snow on the ground at all.  Storm systems develop because it's the earth's way of trying to

balance out temperatures over the United States.  That's why with the bigger storm systems during the transition seasons - spring and fall - we get such drastic temperature changes.  The same can be true on a smaller scale.  The difference in temperature between a snow covered and no snow covered ground can slightly influence the track of low pressure. 

Forecast models have been trying to tug warm air into Northern Illinois.  But with snow on the ground it's unlikely we'll see our surface temperatures warm near, or above, freezing.  Temperatures a few thousand feet above the surface could get close to the freezing mark, but we might fall just shy.  If temperatures all throughout the atmosphere remain below freezing there is a higher likelihood of the precipitation falling as all snow.  However, if the temperature at some point in the atmosphere reaches, or goes slightly above, freezing then this could create a situation where the precipitation may begin as snow but melt a little or all the way once reaching that warm layer.  If that's happens it will increase the chance of a mix or maybe even freezing rain over some part of Northern Illinois.  The fact that we do have snow on the ground could actually help steer the low a little more to the southeast increasing our chance for all snow for the weekend.