Came across this post from a met named PBSwxPABlueRdige1310 on the WB forums. He's been a pretty good poster over the years and I always appreciate his explanations. It is a couple days old by now and I hope he does not mind me posting it here:
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The ridge is being tamped down now, though it appears temporary. I've been amazed at the lack of phasing over the past 6 weeks between branches of the jet. The low in the eastern Gulf will be booted out and bring rain/ice to SE NH and snow further north.
I don't actually think what's happening aloft is all that complicated. As of now and for much of the past few weeks, the deep troughing over BC/Yukon/NW Territories, to along and off the West Coast is - I believe - due to the anomalous cold water in the NE Pacific (in the order of 6-10F below 30-year climo). This is a powerful driver in North America and it's very, very hard not to have a robust counter to that troughing out east. Ridging over the south-central and SE is the classic response.
What we don't yet know is what will happen in a week or two. I still maintain that if the ridging can move a bit east to over mainland Alaska into NW Canada (and not over the Aleutians) then the persistent Arctic air over NW Canada will want to drain into the northern/central U.S. Meanwhile, the Greenland/Baffin Bay ridging is established now. That's a big change from a week ago when low heights were over that geographic area. However, all global ensembles show that ridging building heights further south into Hudson/James Bay in a week to two weeks. IF...IF that happens, then I believe it will be very difficult to sustain the pattern as we currently see it. The cold air will be forced to drain further south and press east. How exactly that transpires will depend on how much energy aloft is ejected out of the Western U.S. trough, and what happens upstream in AK...eastern Pacific. Without a +PNA ridge, indeed, the outcome for the U.S. East/Midwest is much less predictable.
Also of note, I've watched very warm, dry air build and move off the Mexican plateau and off the NM/CO mountains in this stream flow. That warm air has to be reckoned with until the heights build further south in NE Canada and that warm air can be suppressed by the weight of the pressing cold.
It looks to be like a mild to chilly outcome with high variability day-to day for the Mid-Atlantic to Lower Great Lakes over the next 7 to possibly 10 days, before a colder regime can take over. I would expect more wild swings and shifts in global modeling, especially in operation runs for the forseeable future, especially given building ridging in Greenland and over western AK. The models notoriously don't like high-latitude ridging, for it is anomalous.
Each of these perturbations in the stream flow will be hard to predict and the outcomes quite volatile with precipitation type, placement, and intensity, until further notice. The highest confidence right now is for a cold NW and northern Rockies into MN/ND with frequent snow in the west-facing mountain west, especially, CA/NV/WA/ID/MT/UT...western CO/western WY.
I would expect a much colder regime to penetrate the Mid-Atlantic/Midwest/southern Great Lakes very late this year or early in 2022. It's still my expectation that another cold surge, probably more intense, will visit late winter.
The idea - or notion - that the current regime holds for two or three months and it's mild all winter in these areas is not based in any sound meteorological data that I'm looking at.
