Sunday, December 4, 2016

First Snow of the Season Was a Big One

Mother Nature did not disappoint for the first snow of the season.  And it's not even 'officially' winter yet!  Snowfall totals from Southern Wisconsin and Northern Illinois ranged from 3" to 9".  Why such a big range?  A heavy snow band set up south of Rockford and was maximized on either side of I-88.  Most of the higher snow reports I've seen so far have been from Dixon, Amboy, Rochelle and DeKalb.  There, anywhere from 7" to 9" of snow fell. 

To understand why we received so much snowfall, we have to look at what was happening up in the atmosphere.  Early last week when the very first discussion of snow for the weekend was brought up, it was suggested that a very strong area of low pressure was going to develop as the northern and southern branches of the jet stream merged or 'phased' together.  As we got closer to the weekend it appeared that wasn't going to happen and the northern branch of the jet stream was going to become the more dominant branch locally - still pulling a strong low through.  When we look a looping water vapor image, the position of the trough ( or dip ) in the jet stream can tell us a lot.  The trough that moved through Sunday was negatively tilted.  Negatively tilted troughs tend to have more energy and instability associated with them.  And because of that a heavy band of snow occurred south of Highway 72 and really maximized on either side of I-88, producing the high snow totals.  Unfortunately these heavy snow bands are hard to forecast several days, and even sometimes hours, in advance.  And we don't know that they are going to happen until they're already underway.

Thankfully the snowfall has ended with only a few flurries east of Rockford.  Skies are beginning to clear west of the Mississippi River.  And with the recent snow and south winds, look for fog to develop and last through Monday morning.

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