40/70 Benchmark

December Appears Poised To Evolve Colder Than Anticipated "La Niño"Pattern To Drive Cold Regime Into Mid Month Recent Corrections Amongst Guidance There has been a major shift amongst seasonal guidance over the course of the past ten days towards a colder month of December than originally anticipated. Here is the CFS forecast monthly 500mb and temperature departure anomalies for the month of December issued on November 16th. This pattern shown is reflective of a -WPO/-PNA configuration given the warmth over the eastern and southwestern US, as outlined in the Eastern Mass Weather Winter Outlook. However, there have been incremental adjustments over the course of the latter half of November that have culminated in this most recent run of the CFS forecast for the month of December. This new forecast is still indicative of a negative phase of the WPO given the continued warmth over the southwestern third of the US in response to the above average heights in the vicinity of the Bering Sea. However, the marked change towards lower heights and colder temperatures over the east and southeastern third of the nation is representative of a +PNA, which is a significant departure from the previously advertised -PNA (RNA). This is obviously both unexpected and uncharacteristic of a burgeoning cool ENSO event, and in fact, it is more representative of the type of regime expected during a warm ENSO event. Growth of La Nina Interrupted by Hostile Zonal Wind Pattern The anomaly in region 3.4 as of October 23 was -0.5C and all indications were that at least a modest La Niña was developing. However, as of November 20, the anomaly has warmed to -0.1 This is due to the fact that the zonal wind pattern across the tropical Pacific throughout the month of November has been largely devoid of the easterly trade winds necessary to upwell the subsurface cold pool, and thus sustain the growth of La Niña. Note the stark contrast in the zonal wind anomalies during the month of November to the composite of those preceding the past five cool ENSO events. Perhaps the most telling sign of the growing tropical discontinuity is the fact that this past month of November featured one of the most powerful westerly wind bursts on record for the month of November. Note that 5/6 seasons in the above annotation are El Niño, and 4/6 are very powerful warm ENSO events at that. Interestingly enough, the sole cool ENSO season of 1998 also took place on the heels of a potent El Niño and on the cusp of a longer term, multi-decadal shift in the PDO. It is no surprise that the 850mb zonal wind composite of these months of November is much more redolent of November 2024 than the recent cool ENSO composite. Both this growing discontinuity throughout the tropics, as well as last year's potent El Niño within the secondary nadir of a prominent Pacific Cold phase are signs that a major shift in the multi-decadal phase of the PDO maybe imminent. While this is unlikely to have major ramifications for the coming winter, the ongoing tropical deviation from the cool ENSO regime that has been so prevalent for the past decade most certainly will, for at least the first half of the December, anyway. December 2024 Preview Perhaps the purest manifestation of this unexpected and historic westerly wind burst during the month of December is with respect to the Global Angular Atmospheric Momentum, which continues to be positive and more indicative of a warm ENSO event than an incipient La Niña. This is likely the primary reason why this deviation from the Maritime Continent forcing, which was expected to take place in January per the Eastern Mass Weather Winter Outlook, has taken place earlier than expected and appears rather stable. Note the persistence of this -WPO/-EPO/+PNA paradigm throughout the first half of December. While the MJO has not been a major influence on the pattern of late due to the overwhelming +atmospheric momentum, it may represent a subtle constructive interference with the ongoing regime early on in December with its passage though phase 5. Regardless, the early month pattern is of high confidence with multi-ensemble suite consensus. There is also strong agreement that this pattern will remain in place until at least the end of the second week of the December. However, mid month is when the plot thickens, so to speak, as there are mixed signals abound. The trade winds are forecast to reinvigorate, as the atmosphere makes one final attempt to ignite La Niña. This late reinvigoration of a modest La Niña is similar to November 2008, when the region 3.4 anomaly warmed from -0.5C on October 1 to -0.1C on November 5th. Note the similarity in the modest subsurface cold pools below region 3.4. Concurrent with the expected resurgence of La Niña mid month is the potential propagation of the MJO into phase 6, which features an Aleutian ridge oriented, RNA pattern that is much more reminiscent of cool ENSO. The caveat being the issue of whether the MJO wave will make it into phase 6 with a enough amplitude to over power the residual, waning positive atmospheric momentum, as guidance indicates that the MJO impulse may decay before reaching phase 6. The early December 2010 is a prime example of a meager Maritime Continent wave being insufficient to de-constructively interfere with ample antecedent positive global angular momentum. However, even the latter portion of December 2010 featured a return to an RNA pattern and the ensuing resurgence of the moribund La Niña should ensure a similar occurrence this month before another potential deviation beyond the new year. In the mean time, a few higher terrain locales may experience a sneak preview of winter on Thanksgiving day, with perhaps up to 1-3" in the highest terrain of the Worcester hills and up to 2-5" in the Berkshires for a transition to rain. Thereafter, the region awaits the first potential major and widespread wintry threat along or about December 7th.

December update. https://easternmassweather.blogspot.com/2024/11/december-appears-poised-to-evolve.html

Whooooosh. He was mocking Woflie.

That's what I was referring to. Can you link that site?

Two weeks takes us to the period everyone has been alluding to....12/7-10. No one is arguing for much before then.

Look no further than this.

This is an El Nino pattern.....mother nature seems to be engaging in some regression after last year's "El Nina"...

Even the Dec 2019 event that crushed me...it was gone in a like a week.

We need a Will intervention, desperately.

We all fall into the trap sometimes....we're human haha.

If you really believe he is acting as you say he is, probably best not to engage.

Thanks, buddy. Happy Holidays, for real. haha

Even when you get buried in really cold patterns....its still relatively "dry", so to speak....there was very low LE relative to the amount of snow in the huge pack of Feb 2015. Jan 1996 was the same way.

We deperately need snow, snows, snowing

That isn't what people mean and you know this...."dry" in this instance means a dearth of major storm activity, not dry relative to a swampass July.

Where did you get the extended version of that product?

2018-2019 was a textbook epOOOO Nooooo season.

epOoooooo Noooooo!!!

Luke, I understand that concern...if you had included that in the intial response, then I would have reacted differently. I have 3 little ones with a 4th imminent, so I'm not that engaged on here when its quiet. I have no clue what you have or haven't been clear about. It looked to me just like a butt-hurt, eastern Mass envy post. I agree that its not perfect, but we have a fighting chance, which is a welcomed change from the past two seasons and really the majority of the past decade.

Looks like I will clearly be too warm for December, but hopefully I'm not wrong about January, as I felt that would be the best month going in.

Euro looks much more realistic now...1-3" for N ORH hills and maybe 2-5" in the Berkshires before the turkey day rain.

Man, I like the angle of the dangle....that PV is oriented right in our direction. Definitely going to at least feel wintry.

You did.

We need a seperate seasonal banter thread for all of the neurotic, fatalistic defense mechanisms born of the past several seasons of trauma that are sure to foster incessant bickering.

You implied as much.