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Friday, December 29, 2017

Will the caboose produce ? And what does that mean ?

A few spots found their way to -20 on the last Friday of 2017, making it one of the colder December nights in recent memory. We are basically in the middle of the current cold wave and are now anticipating two additional surges of arctic chill before temperatures are allowed to moderate somewhat. Though the lack of a holiday thaw was a welcome change, we could obviously use a bit of natural snow to go with it. Unfortunately, I can't promised much this holiday weekend, just some light snow Friday afternoon, amounting to about an inch. The PV will simply be too much on Saturday and Sunday especially in northern Vermont. Southern Vermont will likely see some more light snow Saturday from a decaying Clipper system but the underlying theme will continue to be the cold weather with readings struggling to get above zero by day and well below zero at night. More of the same is expected for Sunday night and Monday and although the snowbelt areas in New York state will continue to get snow, Vermont should stay mostly dry. Flurries are possible Monday but the layer of instability is too shallow to allow for any substantial terrain/lake snow in the high country.

So now to the real question on everybody's mind - will the caboose produce ? And what the hell does that really mean anyway ? When I heard that saying years ago, it referred to the last in a series of amplifications in an arctic pattern and the tendency for that final surge of cold weather to produce a storm before jet stream relaxes. Is this really a thing ? Yes, but it's not a hard and fast rule by any means. The combination of very cold air and the relative warmth in the Atlantic is the magic that produces the big snows in our neck of the woods but as evidenced by the current weekend, when the jet stream is too strong, nothing happens. As the jet begins to relax, even a little, often times it's enough to open the "storm door". 

The January 4th scenario is an interesting one since it involves subtropical energy over the
Bahamas getting sucked into digging polar jet energy and a clipper system. For a big storm to materialize, the polar jet will need to carve out a near bowling ball type structure. The Euro and Canadian models were all over that Thursday but the Thursday overnight models flipped around and the storm vanished. I wouldn't write the storm off yet however since this is a complicated interaction and may require a few more days to reach any certainty one way or another.

 This "final" amplification ensures that bitterly cold air remains in place across the region through first full weekend in January. Tightening in the jet stream across the Pacific will cause the polar jet to retreat but only somewhat. It's been pointed out by several weather sources that the coldest air in the world is clearly centered over North America. It will take a lot more than a neutral EPO/AO to dislodge this impressive area of arctic chill and neutral is about as adverse as it will get. That said, I envision a noticeable temperature moderation after January 6th, but arctic air will remain available. The most disconcerting aspect of the outlook between Jan 6-16th is the re positioning of the ridge/trough axis with the trough centered closer to the front range. This is great for snow starved Colorado but does raise the potential for ice/rain event somewhere in that time frame. I don't see an extended thaw right now however and I would expect a few chances for new natural snow. 

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