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Met Winter 20-21: Pattern Drivers & Evolution

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While there's reasonable agreement on a -AO and -NAO persisting into early January, there is some uncertainty WRT whether or not the Pacific side offers any help or not. The EPS develops a +EPO into e

I'm starting to get a little more excited for early-mid December. It looks like the Pacific jet will retract significantly, which is the real reason for the retrograding Alaskan LP. Upper level diverg

Hey all!   I apologize for my absence as i'm currently in the "heat" of finishing up my undergrad, working on some side projects, and working with my advisor regarding, well grad school; tha

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17 minutes ago, PB GFI said:

Weeklies old week 3.




New week 2 





I think they are just useless beyond week 3.

It got the Aleutian Low however. Which is the critical part of this pattern anyway.

Don’t worry about the CONUS 500mb maps, look at the driving forces more generally.

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19 minutes ago, Snowy Hibbo said:

It got the Aleutian Low however. Which is the critical part of this pattern anyway.

Don’t worry about the CONUS 500mb maps, look at the driving forces more generally.


Yeh, I pointed out a few days ago in its old day 16 , that it had the trough set back in the Aleutians and one into Europe,  so I expected one to show up on the EC.


It did finally correct. 


It would be nice if we could keep it there because once it centers itself over Fairbanks the ridge finds its way into the GL.




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Tomorrow will be fair but cool. Most of the region will see high temperatures top out in the middle and upper 40s. Wednesday should be somewhat milder. Readings will rebound into the upper 50s and lower 60s for Thanksgiving Day as a storm passes to the north and west of the region.


Tomorrow through Thursday, a storm will move out of the Plains States toward the Great Lakes Region and into Canada. This storm could bring a significant rainfall to the Central Plains States into the eastern Great Lakes Region. North and west of the storm track, including southern Minnesota, southern Wisconsin, Michigan's Upper Peninsula, and parts of Ontario (excluding the greater Toronto area), a moderate to significant snowfall is possible.


December could see above normal temperatures redevelop during the first week of the month after starting off on a cool note. However, the possible development of a period where the AO is negative and PNA is positive could delay the warmup. However, both historic experience following exceptionally warm November cases and the latest weekly and monthly guidance still suggest a warmer than normal December is the base case.


The ENSO Region 1+2 anomaly was -0.7°C and the Region 3.4 anomaly was -1.5°C for the week centered around November 18. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.90°C and the ENSO Region 3.4 anomaly has averaged -1.42°C. La Niña conditions will likely prevail at least through the winter.


The SOI was -7.83.


Today, the preliminary Arctic Oscillation (AO) figure was +2.264.


On November 22 the MJO was in Phase 3 at an amplitude of 0.962 (RMM). The November 21-adjusted amplitude was 1.344.


Based on the latest guidance, no significant stratospheric warming event is likely through the end of November.  


Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall.


Since 1950, there have been four La Niña winters that started with a warm December in the Northeast and warmth across much of Canada, as is the current forecast on the monthly EPS, latest weekly EPS and latest CFSv2 monthly guidance: 1974-75, 1998-99, 1999-00, and 2011-12. All featured a warmer than normal winter and among the winter months that followed December, only January 2000 was colder than normal in the East. Median seasonal snowfall figures were as follows: New York City: 12.9" and Philadelphia: 13.1".


Since 1970, there were 9 winters that saw the AO and EPO average +0.25 or above. Mean snowfall for Boston, Harrisburg, New York City, Philadelphia, and Washington, DC averaged 50% of the most recent 30-season mean. The largest snowfall deficits relative to the most recent 30-season mean figure were located in the Philadelphia to New York City corridor. In addition, 33% of cases saw less than 10" seasonal snowfall in New York City and 44% saw less than 10" seasonal snowfall in Philadelphia.


Based on sensitivity analysis applied to the latest guidance, there is an implied near 100% probability that New York City will have a warmer than normal November. November will likely finish with a mean temperature near 51.6°.


The implied probability for a November mean temperature of 50.0° in Central Park is now approximately 95%. Since 1869, there have been 19 cases where November had a mean temperature of 50.0° or above. 14/19 (74%) went on to have a warmer than normal December, including 9/19 (47%) with a December mean temperature of 40.0° or above (2.5° or more above normal). The winter mean temperature for those 14 cases was 37.5° (2.2° above normal). The winter mean temperature for the subset of December cases with a mean temperature of 40.0° or above was 38.9° (3.6° above normal) and four of those cases wound up having an average winter temperature of 40.0° or above.


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1 hour ago, OHweather said:

14-km EPS Global Northern Hemisphere 500 hPa Height Anom.gif


What I think will be a key feature to watch is the TPV that begins near Greenland at the beginning of the loop, drifts across the pole, and then drops into the Okhotsk Sea or eastern Asia during the first week of December. It seems like that feature moving by AK is part of what causes the +EPO and flood of Pacific crap in the mid-range...but, when it moves away, it may combine with a strong positive East Asian Mountain Torque and convection over the western Pacific to really enhance the jet over East Asia and the West Pac, with an opportunity for wave breaks to raise heights near AK and drop the EPO.


This whole set-up is riddled with sources of model uncertainty, as it's wave breaks near Scandinavia that give the TPV its kick towards eastern Asia...and then wave breaks that may drop the EPO in early-mid December. But, I do think a lot points to the EPO eventually dropping at some point, more a question of when and how beneficial it can be at that time. I'm eying the second or third week of December as the greatest potential for an EPO-induced cold shot into the CONUS. Until then, the pattern is interesting with ridging over the top and a bevy of slow-moving sub-tropical jet disturbances, though the marginal amount of polar influence will make it so that we need things to phase just right for a substantial snow to occur somewhere. It's certainly possible, but it's not that hard for it to fail either. 


CFS weekly.jpg

Cohen 1.png




Moving forward, as I mentioned in my "outlook" write-up over the weekend, a signal continues to show up on most extended guidance for a pattern featuring an Aleutian low and Scandinavian-Ural high late November into a good portion of December. This is a pattern that is a pretty good match to what has been observed prior to stratospheric warming events, and the newest run of the ECMWF weeklies show a noticeable signal to weaken the PV mid-December into early January (though, not every member does so):


EPS 10mb wind.png



December will likely be mild as a whole across the CONUS...though, most of that warmth is focused well west of where most of this forum lives, and there probably will be some snow in the eastern U.S. While the early December +PNA pattern is interesting, I'm more intrigued for the second or third week of the month when the EPO may drop, especially if the sub-tropical jet remains reasonably active. The signs of a weakening polar vortex later in December into January may well deliver a much colder month of January overall across the CONUS than December if it comes to fruition...though, where exactly that cold dumps in would need be worked out as we get closer. 


Watch the trend on the zonal mean. Last 3 updates: 16, 19, 23 in order









The trend is your friend. While not trusted makes you wonder if something is up.

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The overall higher 500mb look we always see in lower to mid latitudes is likely the result of climate change.


Heights are being compared to the 30 year average, and it has been documented that average heights across the northern hemisphere and the globe in general are higher than the long term average.


So there's always that persistent light orange everywhere.  



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  • 33andrain changed the title to Met Winter 20-21: Pattern Drivers & Evolution
  • NJwxguy78 changed the title to Mid/Long Range:Met Winter 20-21: Pattern Drivers & Evolution

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