If you’re looking for arctic cold mid winter, it might actually be preferable to have +WPO (and/or +PNA) dominate Nov/Dec, since that pattern opens conduits for +WAFz and a subsequent W1 response by the SPV. Given the -QBO, we’ll have a more favorable deposition of momentum (vs +QBO) should the SPV take a body blow.
There *are* other conduits to blocking and/or SSW that don’t require an unfavorable pattern for the west, but that is the most efficient one.
We saw it in Nov/Dec 2016, Jan 2017, Jan 2018, and Jan 2021. In the latter three cases it occurred so late in the winter the benefits didn’t show up until February.
In 2016 it was a tad early and weaker (no actual SSW) but that was a Niña/+QBO which can amplify the Aleutian/GOA High very easily.
Those Niña/+QBO years (thru 30-50mb) almost always perform in the PNW, regardless of strat dynamics..1995/96, 2008/09, 2010/11, 2013/14, 2016/17, 2020/21 were all Niña/+QBO.