Phase or No Phase? That is the Question

December 1st begins this Thursday and it's also the beginning of Meteorological Winter.  But if you've been wishing for any signs of winter the past couple weeks, we've been just out of luck.  The month of November has followed the months before in keeping temperatures above average.  Sure there have been some cooler days, but the majority of them have been well above average.

There's already been some talk about snow for the weekend, and now for next week already (something that I will get to in a later post).  But I want to talk about the system for this upcoming weekend and why we shouldn't place all our money on just one computer model.

What happens this weekend is very dependent on what happens with the jet stream.  Right now the jet stream is taking a major shift south in the southern states.  This is what's causing all the severe weather in Louisiana and Mississippi.  It's also what brought over a foot of snowfall to some towns in North Dakota!  The big dip in the jet stream weakens somewhat closer to the end of the week as models begin to forecast another strong low developing in the far southern Plains.  By this weekend, the jet stream begins to break apart into two branches: the northern branch and the southern branch.  The northern branch of the jet stream is what pulls down the cold air from Canada and the Arctic.  The southern branch of the jet stream is what supplies the moisture.  When these two branches work together, it can lead to big storm systems and that's what one computer model was forecasting for next Sunday and Monday.  And yes, it did suggest the possibility of snow.  However, another long range forecast model meteorologists use had a much different scenario.  It suggested these two branches do not work together (or phase) leaving us with a much weaker low pressure system and less precipitation.  Today the model that suggested a possible big snow was now indicating a weaker phase (coming together) of the two branches of the jet stream and suggesting a weaker low, but still with precipitation that would likely end up as a wintry mix.  And the model that had the weaker low to begin with is completely dry!

So before we get too excited about any winter weather making an appearance in Northern Illinois and Southern Wisconsin, we have to look at the overall pattern and what it means for the evolution of possible storm systems.  A phasing jet stream means a stronger low pressure system and a non-phasing jet stream means a weaker low pressure system.  Another factor in trying to determine precipitation type is the track of the low.  And trying to pinpoint that a week in advance is next to impossible!  As we get closer to the weekend we will definitely have a better idea of what will happen, but until then don't believe anything else you see suggesting significant snowfall.