December 2020 Discussion

[Baroclinic Zone]

What a great 7 day stretch this has been.  Even got to enjoy the warmth on Christmas with the rain in between.

2020-12-22	47	24	35.5	4.6	29	0	0.00
2020-12-23	42	23	32.5	1.8	32	0	0.00
2020-12-24	59	23	41.0	10.6	24	0	0.01
2020-12-25	64	41	52.5	22.3	12	0	1.53
2020-12-26	41	28	34.5	4.6	30	0	0.00
2020-12-27	41	22	31.5	1.8	33	0	0.00
2020-12-28	53	22	37.5	8.0	27	0	0.00

[Ginx snewx]

Just now, Baroclinic Zone said:

What a great 7 day stretch this has been.  Even got to enjoy the warmth on Christmas with the rain in between.

2020-12-22	47	24	35.5	4.6	29	0	0.00
2020-12-23	42	23	32.5	1.8	32	0	0.00
2020-12-24	59	23	41.0	10.6	24	0	0.01
2020-12-25	64	41	52.5	22.3	12	0	1.53
2020-12-26	41	28	34.5	4.6	30	0	0.00
2020-12-27	41	22	31.5	1.8	33	0	0.00
2020-12-28	53	22	37.5	8.0	27	0	0.00

Yea absolutely fantastic for all the kids looking to enjoy their winter toys. Nothing better than brown sticks and mud. Perfect Christmas week weather 

[Baroclinic Zone]

Just now, Ginx snewx said:

Yea absolutely fantastic for all the kids looking to enjoy their winter toys. Nothing better than brown sticks and mud. Perfect Christmas week weather 

Kids play online games now.  No mud to be seen anywhere around me.  I don't mind the landscape without snow.

[Ginx snewx]

Just now, Baroclinic Zone said:

Kids play online games now.  No mud to be seen anywhere around me.  I don't mind the landscape without snow.

Thousands and thousands of kids play outside.  I feel bad for any kid whose parents don't kick their asses outside to play in the snow. Stick season sucks up and down

[Baroclinic Zone]

Just now, Ginx snewx said:

Thousands and thousands of kids play outside.  I feel bad for any kid whose parents don't kick their asses outside to play in the snow. Stick season sucks up and down

Put it his way, I see more kids playing outside in our neighborhood when there is no snow vs when there is.  We have an open recreation lot next to our house and they'll be out there for hours playing football or baseball but nothing when there is snow on it.

[CoastalWx]

48 minutes ago, Ginx snewx said:

Yea absolutely fantastic for all the kids looking to enjoy their winter toys. Nothing better than brown sticks and mud. Perfect Christmas week weather 

It’s terrible. Mud everywhere. Snow is much easier.

[Torch Tiger]

fantastic winter

[Baroclinic Zone]

3 hours ago, CoastalWx said:

It’s terrible. Mud everywhere. Snow is much easier.

Where?  In the deep woods?  

[Ginx snewx]

Jerry @weathafella you could have been picking your pack in Pittsburgh 

[Ginx snewx]

Woah ended up with  whopping 10.46 rain month for Dec. Holy crap

[Ginx snewx]

Geezuz 

 

[Baroclinic Zone]

1 hour ago, Ginx snewx said:

Woah ended up with  whopping 10.46 rain month for Dec. Holy crap

Damn yo.  That's double what I got and still more then the last 2 months combined here.

[Ginx snewx]

2 minutes ago, Baroclinic Zone said:

Damn yo.  That's double what I got and still more then the last 2 months combined here.

The ASOS is wrong Bob

[Baroclinic Zone]

5 minutes ago, Ginx snewx said:

The ASOS is wrong Bob

Not seeing any missing days.

 

[Ginx snewx]

1 minute ago, Baroclinic Zone said:

Not seeing any missing days.

 

Undercount, wind, snowmelt. Trust the Stratus from Cocorahs everytime

[CoastalWx]

Yeah Steve is right. It can be underdone a bit.

[40/70 Benchmark]

I was pretty happy with the December forecast locally, but nationally that PNA really flipped the script on the forecast. For once the polar domain was not the issue.

December 2020 Verification & January 2020 Preview 

December 2020 Verification & January 2020 Preview

Well Behaved Arctic; Problematic PNA

The main premise of the December portion of the 2020-2021 Winter Outlook was the re emergence of high latitude blocking as a prominent winter feature.

 

The following is an excerpt from the actual Winter Outlook released in early November.

 

"There has only been one winter (DM) month, March 2018, that has averaged a negative NAO since February and March 2013. It is postulated that there is a high likelihood that we will add to that list this season. Most likely in December, but possibly March or even both. "

-Eastern Mass Weather 11-5-20

 

This was an especially bold call considering the state of long range guidance at the time, which was suggesting the development of a rather formidable polar vortex once again. 

However it worked out reasonably well, as the NAO averaged at least modestly negative.

 

 

And the polar vortex underwent consistent assaults, as evidenced by the extremely negative AO.

 

 

Guidance ultimately adjusted, and the consensus then scrambled to follow suite.

Although the forecast worked out fairly well locally, there was some limitations with respect to the Pacific pattern that were of a much larger detriment to the national forecast.

 

Here if the forecast temperature anomaly map from early November:

 

 

"The month of December should average 1-2 degrees below normal for northern New England, near normal to a degree above normal for central southern New England, and 1-2 degrees above normal from north to south near New York City points southward into the mid atlantic. Cold will dump over the high plains and bleed eastward".

-Eastern Mass Weather 11-5-20

 

The temperature forecast verified locally across southern New England and the Boston area.

 

 

(Courtesy of Eric Fisher via Twitter)

 

But there was a significant RNA pattern forecast that is quite typical of significant la nina, but what actually verified was a formidably PNA that is more representative of an el nino. This wreaked havoc with the national temperature pattern. 

 

 

 

 

Note that while the forecast was for cold centered in the northern plains, and the anomalies generally configured in a cool north, warmer south gradient typical of la nina, the opposite actually verified. Due to the PNA pattern that materialized, cooler anomalies verified to the south with warmer anomalies to the north, which is observed more frequently during el nino events. This variation in the pattern ultimately had important ramifications on the anticipated forms of cyclogenesis.

 

December Snowfall

The forecast was for month was for the developments of storms of fairly moderate intensity, since the forecast higher heights in the southeast would work in concert with the expected negative NAO shear amplifying shortwaves. 

 

"It should be an active period marked by above average precipitation, especially for points north of Washington, DC. Primary modes of cyclogenesis include Alberta clippers, southwest flow events, and Miller B redevelopers, the latter of which especially prevalent during periods of negative NAO. New England should be favored for snowfall, especially central and northern parts of the region. The mid atlantic will need breaks in the RNA to coincide with negative NAO flex for significant snowfall, which is not exceedingly likely, but possible. A White Christmas is more probably than climatology would suggest this season. While the month is likely to average a negative NAO in the mean, this is not meant to imply that excessive blocking will reside for the entirety of the month".

-Eastern Mass Weather 11-5-20

 

 

 

 

 

 

Since the anticipated southeast ridge never materialized, one major event did in fact impact the northeast US near mid month, which is when the pattern was expected to become more active.

 

 

                                    (Courtesy Eric Fisher via twitter)
 

 

Although this system deposited over 12" of snowfall over a relatively large area just one week prior to the holiday, much of the area was denied a white Christmas, nonetheless. This was due in large part to some idiosyncrasies regarding the positioning of the higher heights in the NAO domain.

 

Much like ENSO Events, Location Crucial for Blocks

The month of December was not quite as active in terms of winter weather across the area as implied, in large part because the large Christmas event this past week was entirely rainfall across the region. The easy first guess would be because the NAO was not predominately negative, as expected, but this is not true as previously discussed.

 

 

A more exhaustive review of the data reveals why.

Latter December has been characterized by a "thumb ridge" of higher heights across the north atlantic to the south of Greenland, as opposed to a true block over Greenland.

 

Although technically registering as a negative NAO due to higher heights in the southern periphery of the NAO domain, this is not a block because it does nothing to "block" a storm from cutting inland. This was evidenced by the inland track of the Christmas low, however, long range guidance suggests that this will change heading into the month of January as NAO becomes more ideal.

 

 

January 2021 Preview

Here is the original forecast release from early November:

 

January Forecast Temperature Composite:

"Temperatures should be near normal to 1 degree above across northern New England, 1-3 degrees above normal in central and southern New England, and 2-3 degrees above in the mid atlantic. Any storm activity will entail large precipitation type issues near the coast, with the most snow reserved for northern New England. The RNA pattern should only strengthen, as overall storm activity wanes".

 

Eastern Mass Weather 11-5-20

 

 

 

January Forecast Precipitation Composite:

A lull in winter for the northeast, especially below the latitude of northern New England and east of New York state, should develop during the month, as storms becomes less frequent and the predominate track shifts inland. Some seasonal cold bouts should be sufficient for some mountain snow, and perhaps some "front end" wintery precipitation further to the south.

 

The original forecast called for a recovery of the PV in the latter portion of the month. However, that looks as though it may be delayed by disruptions to the PV that may potentially be more severe than originally forecast. This in large part due to intense warming in the polar stratosphere. The implication of this is that we are currently more than likely in the midst of the "lull" in the winter pattern, which began with the shift of the NAO block to the south and the attendant "Grinch Storm" in latter December, and will likely persist through the first week of the  new year.   The month may also average closer to normal than expected, and colder further to the south across the eastern US.

This "lull" period also consisted of the light rain event last night, and what will be a primarily rain event tomorrow night, although it is likely to begin as a brief period of snow and ice across the interior

 

 

It is after one final mix to rain event this weekend that the plot may being to thicken. Although Sudden Stratospheric Warming events are relatively infrequent during la nina events and westerly QBO seasons, the emergent el nino like tendencies continue to imply that this may be a viable option beyond the first week of the month.

 

Currently, the stratosphere remains fairly weak, which creates the opportunity for cold to escape into the mid latitudes. However, that does not necessarily mean that it will escape into north America. In fact, currently the vast majority of the cold is focused in Asia.

Remember not to generalize all weakening PV episodes.

 

 

 

However there is now growing confidence that the balance of cold will begin to shift toward North America as we enter the new year.

 

 

 

 

 

 

 

 

 

 

This sets the stage for what currently looks to be a classic pattern for a major east coast snow storm during the second week of January.

 

While a true Sudden Stratospheric Warming is remains somewhat dubious for reasons specified in the winter outlook, there does exist strong multi cluster support among ensemble guidance for such a a pattern to materialize.

Guidance continues to emphatically suggest the development of a west-based NAO beyond the first week of the month.

 

 

 

There maybe additional system worth watching early next week for wintery implications before the true fever pitch for the second week of 20201 and beyond.

 

 

[Greg]

On 12/28/2020 at 5:38 PM, TheSnowman said:

I have had 22 20" Storms (from what I can tell by alllll my research over the years) since 1897-1898.  BUT!!  10 of those 22 Since 1992-1993!!  

Some of those are because of measuring technique also. People tend to forget that. How much is debatable but noteworthy. Will is correct though, with normal measuring, it's pretty tough to get 30" of snow on the nose. The Northshore since 1870 has seen a few since then as Will stated. A recap: 1898 (February (Lawrence, MA/ Northshore)),1969 (February),1978 (February), 2003 (December), 2005 (January). They are pretty rare/hard to come by. That's why they are usually deemed "Historic" in nature when they do occur.