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Seems like conflicting views and opinions, even from various well respected mets regarding the - NAO/block/ AN heights near and around Greenland. Some say what tombo stated, others are gung-ho about real storminess and snow prospects in the East.

Great thread. Worth a read. Way more to.it than what I posted. Anthony Masiello @antmasiello · 1h A tradition of sorts: to take the ECMWF zonal wind anomaly charts during a SSW and show the zero-line descend through the stratosphere. GIF @antmasiello · 1h **correction, that is not a zonal wind anomaly but mean zonal @antmasiello Replying to @antmasiello the mean zero-wind line determines the level at which future waves break (RW can only upwell through westerly flow) and deposit their easterly momentum. The descent in this one seems quicker than usual, but I do not have numbers to back that up. 12:06 PM · Dec 28, 2020·Twitter Web App @webberweather · 1h Replying to @antmasiello JRA-55 SSWE compendium composite shows the zonal wind reversal propagating from the stratopause to ~20mb in a matter of 3 days (ish) during major sudden warming. Looks about right to me @antmasiello · 1h thanks for the composite, and your conclusion is probably right, but that doesn't show the zero-line reaching the 50-100mb depth, probably from event smoothing.

No time for greater detail but, wow to the new AO and NAO ensemble forecasts. Really diving down, looking better and better for a SECS based solely on those 2 indicates moving forward into January.

The Ural blocking has diminished by hour 576 ( accept at own risk ) which was responsible for the East Asian Mountain torque. Hopefully models go to a more robust + PNA , and post the SSWE we may get the vortex to settle further South in time. May seed areas to our NW with a very cold air mass by the third week of Jan.

Crazy look here with the Pac and the height distribution from Greenland to the US SE Coast. A week beyond this date is when things might start shifting. On a unrelated note, Ocean temp off Southern New Jersey according to Magic Seaweed is 55 degrees F. If correct rather mild for this time of year.

Some feel the opposite of this view. Yet, as mentioned, we may still get some good events. As griteater posted earlier, mid to late Jan for an improved Pac. That seems to be the trend during the last couple of days. Anxious to see if the modeling does a big change for the better in the next several days, as it possibly could latch on to the time period and evolution of the SSWE. Of course the outcomes doesn't have to be one of colder, however, I am hoping for improved looks in the PNA domain. David Gold @dgoldwx2112 Replying to @antmasiello I think in the winter mean the tropical heat source will be situated well west of where it typically aids in setting up the kind of pattern that eastern winter weenies love. That of course doesn’t preclude some good events. 5:05 PM · Dec 26, 2020

Agreed, wondering if in the end our savings grace will be the very thing that was an obstacle. Hopefully we score when the Pac improves and we take advantage of the active pattern.

All and all we are in trouble. Blocks mean little, folks neglect temps. This winter the new player is source region, damn I missed that one.

Excellent info, thanks.

Last thing, if we have a favorable SSWE for example whether it be a displacement event, or a split, of combination is it possible that under certain circumstances it can create its own regime, over-powering say even the Pac influences or is that not a valid possibility ? I ask because HM alluded that the strat may be able to create its own regime under certain circumstances in a recent post several days ago. Thanks

Tom, in your view, can there be too much of a +EAMT event? Meaning, having multiple events is it a net plus or minus to a desired Eastern cold state in time?

Only this year, and in this new warmer background state, does a deep - AO and -NAO produce no significant snowfall for us. Source region concerns continue, and even though the +EAMT events are helping with the -NAO they are also bringing in copious warmth to Alaska . At our latitude we need perfection and it is just not there. The fast flow and uncooperative Pac is over-powering the - AO and -NAO. How long this lasts is still uncertain , but hopefully by mid month the Pac changes up a bit. At the moment the really cold air is in Asia, as you have read about the very strong high pressure areas there.

We need at @Isotherm to come in here and do a little updating. I believe according to his seasonal forecast , January held the highest probabilities of colder with normal to above normal snowfall for our area.

Goes to show, both humans and models still possess debatable value beyond a certain point.

Ark like rains here , warnings flying across the radios and TVs, Rudolph is wearing scuba gear.

Hard to be a believer after a run like that.

EPS at 360 hours has an awesome Greenland block in the Davis Straights with another incoming +EAMT event. Maybe spur a +PNA . Mid to late Jan. could rock.

Donald Sutherland did some posting about the CFS last winter, and its accuracy for the month ahead. What he found was it really gets into its wheelhouse of accuracy around this time of the month for the month ahead. So, it would appear you can place a little more faith on the CFS at this time.

Hearing about concerns related to source regions from several mets. This went in the wrong direction, hopefully by mid Jan this looks much different. CFS progression makes sense, so it is not out there alone with bringing improvements in the EPO region.

This is a long range thread ? Gathering unique, useful data and weighing it for long range outcomes is the fun part of long range forecasting.

7 x more likely. Good odds. @SimonLeeWx Replying to @SimonLeeWx The Arctic High regime is ~7x more likely to occur if the lower-strat vortex is weak than strong (https://doi.org/10.1029/2019GL085592…) - and that's certainly consistent with this forecast. Suggests that the accuracy of the stratospheric forecast may be important here for the regime forecast. Wintertime North American Weather Regimes and the Arctic Stratospheric Polar Vortex agupubs.onlinelibrary.wiley.com

Overall probability is reassuring that the Greenland blocking regime is indeed real. @SimonLeeWx Tropospheric-led but then subsequently likely reinforced by the weakened stratospheric vortex, the GEFS 35-day shows an unusually strong signal for the Arctic High regime (Greenland blocking) to dominate the North American regime space over the coming few weeks. 11:12 AM · Dec 23, 2020·Twitter

Progression could be cuter, cuter. Mid Atlantic snowstorm then clipper, then classic Miller A third week of Jan. It fits evolution of NAO block retro and then Pac improvement based on forcing moving into more favorable area / +EAMT events and then a rise off of a deep -NAO sort of similar to a HA event.

Damn !!!! I like!!! Eric Webb @webberweather This is the composite of Jan-Febs that had late Dec-Jan SSWEs & whose tropospheric pattern in the 3 weeks leading up to the SSWE bears some semblance to this yr (+WPO/+EPO/-NAO/-AO/+SCAND). Very NINO heavy composite as you'd probably expect given the deep, precursor Aleutian Low 11:42 AM · Dec 23, 2020