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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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10 hours ago, Event Horizon said:

Honestly, it seems that the QBO, ENSO, and Stratospheric backgrounds early on in cold season could be starting indicators to what we could have for winter. I'll need to find that site that averages those tellies. 

They certainly are, but to what affect it has on winter, level/range of predicability & accuracy are three of the biggest areas of uncertainty/still evolving. As you know, the atmosphere is so fluid and dynamic in nature it is going to take time to fully grasp how everything is teleconnected, and even if/when we do understand these better it still may be hard to forecast with certainty. Looking forward to this website you speak of. I'm interested!

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4 hours ago, Above The Storm said:

They certainly are, but to what affect it has on winter, level/range of predicability & accuracy are three of the biggest areas of uncertainty/still evolving. As you know, the atmosphere is so fluid and dynamic in nature it is going to take time to fully grasp how everything is teleconnected, and even if/when we do understand these better it still may be hard to forecast with certainty. Looking forward to this website you speak of. I'm interested!

 

Here it is. This one. 

 

https://psl.noaa.gov/data/usclimdivs/

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Temperatures rose in the 70s across the region today. Tomorrow will likely be somewhat warmer with more sunshine than today.

 

In the Rockies, Denver reached 91° today. That surpassed the previous daily record of 89°, which was set in 1992. It was also Denver's 75th 90° or warmer day. The previous annual record was 73 days in 2012.

 

Temperatures will likely remain at generally above normal levels for the remainder of September. A cool shot is possible to open October.

 

Phoenix has an implied 91% probability of recording just its 13th September on record with a mean temperature of 90° or above. The most recent such occurrence was 2018 with a mean temperature of 91.8°. Phoenix will very likely record its hottest June-September period on record with a four-month mean temperature of 95.3°-95.5°. That would exceed the previous June-August record of 95.1°, which was set in 2013 and tied in 2015. The existing record is 93.9°, which was set in 2011.

 

The ENSO Region 1+2 anomaly was -0.4°C and the Region 3.4 anomaly was -0.8°C for the week centered around September 16. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.93°C and the ENSO Region 3.4 anomaly has averaged -0.68°C. La Niña conditions have developed and will likely prevail through the remainder of autumn.

 

The SOI was +14.26.

 

Today, the preliminary Arctic Oscillation (AO) figure was +0.211.

 

On September 23, the MJO was in Phase 5 at an amplitude of 1.368 (RMM). The September 22-adjusted amplitude was 1.168.

 

Since 1990, there have been 11 La Niña events, 6 of which followed an El Niño winter. 10/11 (91%) case saw warmer than normal September. All 6 following an El Niño winter were warmer than normal. September mean temperatures for New York City for those cases were: 11 cases: 69.9°; Subset of 6 cases: 70.8°; Entire 1990-2019 period: 69.0°. The September mean temperature for all La Niña and neutral-cool cases following an El Niño winter (1950-2019: n=13) was 69.9°.

 

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. 9/10 (90%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO.

   

Based on sensitivity analysis applied to the latest guidance, there is an implied 70% probability that New York City will have a warmer than normal September. September will likely finish with a mean temperature near 68.4°.

 

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A -4sd ao in December during a la nina or near la nina year usually means heavy snow...there are exceptions but very few,,,

12/13/1966 the ao goes below a -4sd...the same time NYC is getting snow and rain but NYC got 7" of snow on 12/24...

12/19/1995 the ao goes below a -4sd...the same time the city is getting an 8" snowstorm...

12/29/2000 the ao goes below a -4sd...NYC got a foot of snow the next day...

12/18/2010 the ao goes below a -5sd...NYC got 20" of snow on 12/26-27...

1950 which is another la nina year had a -4sd ao on the 27th...at the same time NYC was getting 3" of snow and the coldest temp of the winter...9 degrees...not great but it was the best part of that winter...

 el nino Decembers I found  with a -4sd are...

12/20/1963...ao was below -4sd on the 20th...NYC got 7" of snow on the 23rd...

12/21/2009...ao was almost -6sd...NYC got 11" of snow on the 19th-20th...

four more years I found with an ao -4sd in December...1961 was almost a -4sd...

12/26/1961...almost -4sd...6" of snow on 12/23-24...

12/31/1962...below -4sd...no heavy snow that winter...Just record cold...13 degree max in NYC 12/31/62...

12/27/1968...below -4sd...light snow soon followed...it took till Feb to get a heavy snowstorm...

12/29/1976...below -5sd...light snow that day but the winter had record cold...

12/27/1978...below -4sd...it took till Febfuary to get heavy snow...

 
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3 hours ago, donsutherland1 said:

Temperatures rose in the 70s across the region today. Tomorrow will likely be somewhat warmer with more sunshine than today.

 

In the Rockies, Denver reached 91° today. That surpassed the previous daily record of 89°, which was set in 1992. It was also Denver's 75th 90° or warmer day. The previous annual record was 73 days in 2012.

 

Temperatures will likely remain at generally above normal levels for the remainder of September. A cool shot is possible to open October.

 

Phoenix has an implied 91% probability of recording just its 13th September on record with a mean temperature of 90° or above. The most recent such occurrence was 2018 with a mean temperature of 91.8°. Phoenix will very likely record its hottest June-September period on record with a four-month mean temperature of 95.3°-95.5°. That would exceed the previous June-August record of 95.1°, which was set in 2013 and tied in 2015. The existing record is 93.9°, which was set in 2011.

 

The ENSO Region 1+2 anomaly was -0.4°C and the Region 3.4 anomaly was -0.8°C for the week centered around September 16. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.93°C and the ENSO Region 3.4 anomaly has averaged -0.68°C. La Niña conditions have developed and will likely prevail through the remainder of autumn.

 

The SOI was +14.26.

 

Today, the preliminary Arctic Oscillation (AO) figure was +0.211.

 

On September 23, the MJO was in Phase 5 at an amplitude of 1.368 (RMM). The September 22-adjusted amplitude was 1.168.

 

Since 1990, there have been 11 La Niña events, 6 of which followed an El Niño winter. 10/11 (91%) case saw warmer than normal September. All 6 following an El Niño winter were warmer than normal. September mean temperatures for New York City for those cases were: 11 cases: 69.9°; Subset of 6 cases: 70.8°; Entire 1990-2019 period: 69.0°. The September mean temperature for all La Niña and neutral-cool cases following an El Niño winter (1950-2019: n=13) was 69.9°.

 

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. 9/10 (90%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO.

   

Based on sensitivity analysis applied to the latest guidance, there is an implied 70% probability that New York City will have a warmer than normal September. September will likely finish with a mean temperature near 68.4°.

 

Ouch. 

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9 hours ago, OHweather said:

Hey all. Work has frankly sucked out most of my desire to discuss and look at weather during my free time much of this summer (something you never think is possible when you're a weenie posting on the Accu forums 10+ years ago!), but the early frost to start the week and prospect of flakes potentially flying before too much longer is starting to slowly give me the urge to not be a WxBoard hermit anymore. I'll try to post a somewhat more routine commentary over the next few weeks and much more detailed / comprehensive write-up at some point in late October when time allows. At this point, a few scattered thoughts based on what we've been putting together at work to this point...

 

Given a La Nina, westerly QBO, climate trends, and majority of long range guidance at this point, a warm winter has to be favored across the southern and eastern U.S. at this point, especially compared to the 1981-2010 normals which are almost obsolete at this point given how much warming we've continued to see this decade. I don't think that can be ignored and is the logical starting point. Warm in the southern U.S. overall, mild up the east coast (but not as constantly warm farther north), and cold across the northwest-north-central U.S. 

 

cdas-sflux_ssta_global_1.png

 

Singapore_u (1).png

 

With that said, La Ninas with a westerly QBO tend to feature more ridging near Alaska and generally more cold farther southeast into the CONUS than easterly QBO La Ninas (as @griteater pointed out in the outstanding post at the beginning of this thread). Also, this La Nina is currently centered over the eastern Pacific, which tends to open the door for a colder outcome compared to La Ninas that are more basin-wide. An extraordinarily warm and large western Pacific warm pool and negative IOD may encourage an active MJO this winter over the western Pacific, and the persistent western Indian Ocean standing wave that severely limited Pacific forcing last winter through this summer may finally be weakening (not 100% sold on that yet, but there appears to at least be some more variability in the near future). Warm water near the Maritimes can be dangerous, as sub-seasonal forcing in that area is warm in the winter for us (and remember, I think the baseline here is a mild to warm winter across the southern and eastern U.S.), but the warm West Pac may encourage an active enough MJO to give us windows of opportunity. While the PDO is rather close to neutral, waters are currently quite warm over the NE Pacific...we'll see if that changes in October though with more storminess in that area. The warm waters over the NE Pacific and extensive western U.S. drought are a result of a persistently ridgy pattern there this summer...does that continue into the cold season, or will outside forces coerce a change? 

 

So, off the cuff, there's an urge (especially after last winter) to forecast an all-out torch-fest. There are several reasons for optimism...and the pattern in the short-term, with an amped Pacific jet resulting in an amped west coast ridge and hints of Scandinavian blocking, would be beneficial both short and long-term if it ends up being a semi-regular occurrence over the next few months, as it is not warm as it occurs and also isn't favorable for the stratospheric polar vortex strengthening uncontrollably. Is this just a brief mid-fall blip, or is it a sign of things to come? Remember, we had a hell of a blip / head fake last November and early December...I bit on it myself. 

 

Looking at some analogs, there are some very warm winters that show up in here, actually a couple of block-busters, and a few OK years. The first set focuses on west QBO La Ninas that reasonably match in at least 2 of the following 4 areas: Indian Ocean (negative IOD), east based La Nina, warm West Pac, and Warm NE Pacific. As a note, 1973-74 would have made this set, but it was an exceptionally strong La Nina so I chose to exclude it. 

 

The DJF 500mb composite and temperature/precip anomalies:

 

W QBO LN DJF.png

 

Temps with 1981-2010 climo:

 

set 1 DJF 81-10.png

 

Went back and forth on which climo period is best to use given 55-56 was in a noticably cooler climo, so for completeness here is the same map with a 1951-2010 climo:

 

set 1 DJF 51-10.png

 

Precip:

 

set 1 DJF precip.png

 

The bulk of the cold in these winters is frontloaded, with November and December both colder than normal for much of the northern and eastern U.S:

 

W QBO LN ND.png

 

January is a more mixed month, with February featuring an incredible warm signal over most of the southern and eastern U.S. March is more mixed and leans cooler over the NE. 

 

A second set of years I played with focuses less on SST distribution and more on QBO and early-mid fall patterns and tropical forcing. There were few that matched all areas (though, 95-96, 10-11 and 17-18 were at least reasonable in all fields analyzed, even if not perfect). They show a similar (but slightly muted) signal overall for the whole winter, and still show a preference for any cold being more front-loaded with a strong warm signal for February:

 

Fall 500 VP DJF.png

(because all but 1 analog are after 1980 and this is a larger set, will not include the longer climo base period map...but it would make it appear warmer compared to the climo as it did to the other set)

 

set 2 DJF temps.png

 

set 2 DJF precip.png

 

Fall 500 VP ND.png

 

Some overall takeaways...yes, a mild winter is favored (temperature wise) over the southern and eastern U.S., but there are ways to get cold air at times. Analogs are not as warm as you'd think (a disclaimer about ENSO-based analogs generally being too cold the last few winters is needed here, though last winter was exceptional due to the record wind speeds and cold observed with the stratospheric PV between January and March), and there is a strong signal for early cold and snow potential over the Great Lakes, Ohio Valley, Northeast and Mid-Atlantic among the analogs. Given the global pattern setting up heading into October, I think the idea of an early blocky / cold pattern is there if we can get sub-seasonal forcing into the western Pacific between late-October and some point in November. However, for as surprisingly chilly as the analogs may be early, they're more mixed in January and are exceptionally warm overall for February (and mixed / leaning cool for March in the Northeast), underscoring this type of winter does have "warm risks" that may be substantial. 

 

So, I'm not overly pessimistic or optimistic at this point. I am curious though. 2017-18 is a decent analog (that winter held onto a +QBO below 40-50 mb all the way through), and I can see how an overall similar evolution to that winter plays out (though quite possibly without the SSW and hence, a more typical March).


Fantastic post, in agreement with your early thoughts. I believe we we looking at warmer than average winter, however things will line up on occasion for some cold periods (think 2-4, 3-6”

clipper events with these passes) and a chance or two at a KU-esque event.  It won’t be as brutal from a wx community aspect as last year, that’s for sure. 

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Tomorrow will be mainly cloudy with perhaps a few showers and a little cooler than today. In general, temperatures will likely remain at generally above normal levels for the remainder of September. A cool shot is possible in the opening week of October, but warmer readings could return during the following week.

 

Phoenix has an implied 90% probability of recording just its 13th September on record with a mean temperature of 90° or above. The most recent such occurrence was 2018 with a mean temperature of 91.8°. Phoenix will very likely record its hottest June-September period on record with a four-month mean temperature of near 95.3°. That would exceed the previous June-August record of 95.1°, which was set in 2013 and tied in 2015. The existing record is 93.9°, which was set in 2011.

 

The ENSO Region 1+2 anomaly was -0.4°C and the Region 3.4 anomaly was -0.8°C for the week centered around September 16. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.93°C and the ENSO Region 3.4 anomaly has averaged -0.68°C. La Niña conditions have developed and will likely prevail through the remainder of autumn.

 

The SOI was +16.88.

 

Today, the preliminary Arctic Oscillation (AO) figure was +0.613.

 

On September 24, the MJO was in Phase 5 at an amplitude of 1.436 (RMM). The September 23-adjusted amplitude was 1.370.

 

Since 1990, there have been 11 La Niña events, 6 of which followed an El Niño winter. 10/11 (91%) case saw warmer than normal September. All 6 following an El Niño winter were warmer than normal. September mean temperatures for New York City for those cases were: 11 cases: 69.9°; Subset of 6 cases: 70.8°; Entire 1990-2019 period: 69.0°. The September mean temperature for all La Niña and neutral-cool cases following an El Niño winter (1950-2019: n=13) was 69.9°.

 

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. 9/10 (90%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO.   

 

Based on sensitivity analysis applied to the latest guidance, there is an implied 79% probability that New York City will have a warmer than normal September. September will likely finish with a mean temperature near 68.6°.

 

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Under partly to mostly cloudy skies, temperatures rose mainly into the middle 70s across the region.

 

In parts of South America, a historic early spring heatwave toppled monthly records. Some monthly records included:

 

Asuncion, Paraguay: 108°
Corrientes, Argentina: 109°
Foz Do Iguacu, Brazil: 102°
Pozo Hondo, Paraguay: 114° ***New all-time national high temperature record***

 

Meanwhile, in parts of Europe, early-season snows blanketed even the lower elevations of the Alps.

 

Temperatures will likely remain at generally above normal levels for the remainder of September. A cool shot is likely during the opening week of October, but warmer readings could return during the following week.

 

Phoenix has an implied 95% probability of recording just its 13th September on record with a mean temperature of 90° or above. The most recent such occurrence was 2018 with a mean temperature of 91.8°. Phoenix will very likely record its hottest June-September period on record with a four-month mean temperature of near 95.4°. That would exceed the previous June-August record of 95.1°, which was set in 2013 and tied in 2015. The existing record is 93.9°, which was set in 2011.

 

October could be wetter than September in and around the New York City area. Since 1950, 10 of the 12 (83%) of the La Niña cases that followed an El Niño winter saw October receive more precipitation than September, including 7 of the 8 (88%) cases since 1980. October 1954 and October 1998 were the exceptions. The largest increase in precipitation occurred in 2005. September 2005 saw just 0.48" precipitation in New York City. October 2005 received 16.73" precipitation.  

 

The ENSO Region 1+2 anomaly was -0.4°C and the Region 3.4 anomaly was -0.8°C for the week centered around September 16. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.93°C and the ENSO Region 3.4 anomaly has averaged -0.68°C. La Niña conditions have developed and will likely prevail through the remainder of autumn.

 

The SOI was +16.11.

 

Today, the preliminary Arctic Oscillation (AO) figure was +0.683.

 

On September 25, the MJO was in Phase 5 at an amplitude of 1.350 (RMM). The September 24-adjusted amplitude was 1.442.

 

Since 1990, there have been 11 La Niña events, 6 of which followed an El Niño winter. 10/11 (91%) case saw warmer than normal September. All 6 following an El Niño winter were warmer than normal. September mean temperatures for New York City for those cases were: 11 cases: 69.9°; Subset of 6 cases: 70.8°; Entire 1990-2019 period: 69.0°. The September mean temperature for all La Niña and neutral-cool cases following an El Niño winter (1950-2019: n=13) was 69.9°.

 

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. 9/10 (90%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO.   

 

Based on sensitivity analysis applied to the latest guidance, there is an implied 85% probability that New York City will have a warmer than normal September. September will likely finish with a mean temperature near 68.6°.

 

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47 minutes ago, Thundersnow3765 said:

A quick reminder of how incredibly snowless last winter was, and another reminder not to make a kneejerk reaction forecast to it. Odds are very favorable that this winter ends up better than last :) 

Image

At least both the Euro and Ukie like February to be above normal snow. 

 

3BA46691-4B38-4E83-B31E-159CDDCBCCED.jpeg

04065B65-40E5-4F08-B817-B05370837480.png

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The euro and ukie seasonals seem to agree on a backloaded winter with December being abhorrently disgusting and January being a transition month. Honestly, haven’t all of our recent winters been forecasted to be backloaded? Sometimes that transition happens... sometimes it does not. Much has to do with the strength of the PV in October and November and the frequency and intensity of attacks on the trop and strat pv beginning in early December. 

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3 hours ago, 33andrain said:

The euro and ukie seasonals seem to agree on a backloaded winter with December being abhorrently disgusting and January being a transition month. Honestly, haven’t all of our recent winters been forecasted to be backloaded? Sometimes that transition happens... sometimes it does not. Much had to do with the strength of the PV in October and November and the frequency and intensity of attacks on the trop and strat pv being in early December. 

I like what the pattern looks like going into october.  Of course, that doesn't necessarily mean that two months from now it will look the same, but I'd rather see this than something that looks totally hopeless.

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4 hours ago, 33andrain said:

The euro and ukie seasonals seem to agree on a backloaded winter with December being abhorrently disgusting and January being a transition month. Honestly, haven’t all of our recent winters been forecasted to be backloaded? Sometimes that transition happens... sometimes it does not. Much had to do with the strength of the PV in October and November and the frequency and intensity of attacks on the trop and strat pv being in early December. 

So far it looks like the PV will be weaker than normal for now. Let’s what happens in November 

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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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