Alright folks, I know the calendar just flipped to October, but apparently the weather pattern is about to kick it into high gear for the first invasion of chilly Canadian air late this week.  So what are the factors leading to this get out your warm coat type of weather? 

Let's begin with the good old jet stream.  As is the case during the fall season, the jet stream is beginning to shift back down to the south.  Not only does this introduce a better opportunity for precipitation every few days, but also allows for a great fluctuation in temperatures.  Rather chilly weather settled into the inter-mountain west last weekend as low pressure spurred up some rain and mountain snow.  In the process, the jet stream has carved out a trough in the western third of the US, which aligned the upper level winds to guide wet weather our way on Tuesday.  In the wake of this storm system, not much will change in the temperature department Wednesday.  However, the following cold front on Thursday, although not as great a rain producer, is going to usher 40 degree temps into the Wisconsin River Valley for daytime highs leading into the weekend.  As all of this is going on at the surface, the jet stream aloft will pull the trough that was out to the west into the mid-section of the country, producing an almost typical winter time positioning of the jet stream. 

To make things easier, below is the 300mb (jet stream) winds for the next 12 hours and then below that where the jet stream will be 5 days out.  These maps should update with each new model run of the GFS weather model. 

You may be wondering, how this translates to temperatures. Check out these 4 panels of the 850mb temperatures (~5000 feet up).  Notice the transition from the warmer yellows and greens to the darker blues and purple.  The temperature key in Celsius is at the bottom of the maps.  Needless to say, colder air is coming our way.

With all of that in mind, if another wave of low pressure decides to ride along in our direction from the southwest toward the Great Lakes this weekend or even early next week, the atmosphere would be cold enough to support snow flakes.  You won't have to break out the snow blowers yet, but don't be shocked if we have our first appearance of flurries or snow showers before Halloween.