Since
Portastorm mentioned it lets see how it looks/works! Stratosphere that is.
This is what it is now. (I'm going to use 30mb and 70mb since the warming will occur from the troposphere up)

Very cold up there and the vortex is fully intact, favors zonal flow over NA as all the cold is bundled up underneath it. Notice the patch of warmth in the far east, this is important.
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This is what is forecasted to happen within 7 days. Notice the warm spot grew into the Arctic through the Bering. That's the Aleutian ridging some have mentioned splitting the vortex. A chunk then slides into the Hudson bay, signal for a severely negative AO (already begun). The warming also is coming from the troposphere so a look little lower at 70mb (right), we see even more impressive warming.


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Another look at the heights. Explains the situation well. Verification for long range forecasts of the stratosphere is very good since there's little weather that actually goes on up there just winds and temps so don't worry too much on it being long range.


Here too notice on the right high pressure continues to bud into Alaska/Bering facing North America. This may be our best chance at a +PNA. And that (I believe) is one of the ways we can get a sudden stratospheric warming event.