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Pattern January 2021 - Joyless January

I've always understood the +PNA to be favorable by just implying troughy weather downstream, but I've also recently begun to notice the stark impact that the Rockies have on eroding our cold air source, a secondary reason that a +PNA/tall west coast ridge is so important. A zonal westerly (or worse: SWerly) flow onto the west coast drives upstream flow directly perpendicular to the major axis of the Canadian Rockies in particular. If precipitation falls out of this orographic lift, then there is a strong adiabatic compression/warming east of the Rockies (a Chinook wind). This coupled with the strong -NAO block is leading to an accumulation of much warmer than average temperatures across Canada (still below freezing though), with effects extending southward into the US:
gefs_chinook_heights.png
gefs_chinook_temps.png
Note the sharp line of warmer than average temperatures east of the Canadian Rockies. Even on this frame though, there's evidence of below average temperatures associated with an active southern stream undercutting and moderating conditions across the southern US (e.g. anomalies in Texas and Florida). As the 12z GEFS rolls forward, a west coast ridge begins to appear just after day 10. The Chinook wind is still present, but it's clearly dampened here with more legitimate below average temperatures across the board:
gefs_pna_heights.png
gefs_pna_temps.png
These below average anomalies spread east and persist through the end of the model run.
 
RVD said:
I've always understood the +PNA to be favorable by just implying troughy weather downstream, but I've also recently begun to notice the stark impact that the Rockies have on eroding our cold air source, a secondary reason that a +PNA/tall west coast ridge is so important. A zonal westerly (or worse: SWerly) flow onto the west coast drives upstream flow directly perpendicular to the major axis of the Canadian Rockies in particular. If precipitation falls out of this orographic lift, then there is a strong adiabatic compression/warming east of the Rockies (a Chinook wind). This coupled with the strong -NAO block is leading to an accumulation of much warmer than average temperatures across Canada (still below freezing though), with effects extending southward into the US:
View attachment 61315
View attachment 61316
Note the sharp line of warmer than average temperatures east of the Canadian Rockies. Even on this frame though, there's evidence of below average temperatures associated with an active southern stream undercutting and moderating conditions across the southern US (e.g. anomalies in Texas and Florida). As the 12z GEFS rolls forward, a west coast ridge begins to appear just after day 10. The Chinook wind is still present, but it's clearly dampened here with more legitimate below average temperatures across the board:
View attachment 61317
View attachment 61318
These below average anomalies spread east and persist through the end of the model run.
Click to expand...

I don't think this necessarily means we need a strong or persistent +PNA though. Having deep vodka cold in Canada is not necessary to pull off a big storm... at the very least since we don't live in Canada, and colder usually implies drier. We will need some west coast ridging to pull of a storm, though, even if transient. Long range ensemble models are beginning to hint at a +PNA though, in part as the high latitude block retrogrades, which is great. Hopefully that evolution sticks around.
 
Myfrotho704_ said:
I wouldn’t think we’d do it with some of the first waves but clearly we’re getting close to something hereView attachment 61328View attachment 61325View attachment 61326View attachment 61327
Click to expand...

Already seeing close calls/favorable trends before any +PNA pops is a nice sign. I'd be impressed if this one works for anyone outside the NC mountains, but I like its implications for a setup with a bit more cold air (which, if the long range ensembles are right, could be as soon as the 10-15th).
 
Myfrotho704_ said:
It’s like the better pattern is gradually speeding up and sneaking up on us, lol
Click to expand...

I'm planning on heading back to Raleigh for school sometime around the 10th, though I can wait a few days longer. It sure would be nice to sneak in something before I have to go willfully degrade my snow chances.
 
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