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If you build it, it will come. That saying applies to the cold air that's been building in Canada. Sooner or later the shear density of the cold will allow it to come plunging into the U.S. The unanswered questions are, when and where?

Just look at some of the readings in Canada early Friday evening. Already down to 20 below in the purple areas N/NE of Hudson Bay.

Snowfall accumulation has occurred at a rapid pace for so early in the season up north.

With the cold in place that snow's not melting. Here's the snow cover as of this morning. From the northern Plains and Upper Midwest snow extends all the way to the North Pole.

The GFS shows this for additional snow in the next 16 days. (just an estimate...)

The snow essentially acts as an ice box and feeds back generating even more cold, especially going into winter. It all feeds on itself until the cold grows so strong that it breaks free and moves south into the Midwest. At that point, snow can begin to accumulate here.

Looking forward most long range models are keeping the mother load of cold air about where it is for another 10-15 days. The EURO EPS ensembles show 850 temps that look like this November 18th. Look at the bulge dipping into the northern U.S. That is really ominous. That greenish/pink mass is the coldest air in North America as it lurks just to our north.

The 500mb jet stream flow for the same time (November 18th) shows a strong zonal flow (west to east) holding the cold at bay.

At some point this fast upper level flow should slow enough for a short wave to allow a buckle. When the dam breaks it won't be pretty. In my mind its not a matter of if it will break, it's merely a matter of when. I could see a big 1034 high like the one the GEFS shows below November 7th ushering in the chill.

I do want to stress this is just me speculating based on pattern recognition. There is nothing concrete indicating the cold shot and thus there is no way to accurately predict the timing at this distance. However, I would think the period just before or near Thanksgiving certainly is worth watching. By then the MJO is forecast to get back into colder Midwest phases vastly increasing the odds.

That's all I have for now. Enjoy your weekend and as always, roll weather...TS

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