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Met Winter 20-21: Pattern Drivers & Evolution

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Speaking of the SSW, there has been a trend as of late on the GFS for a less dominant Ural vortex, and less flattened out secondary ridge over the Atlantic. These seem a bit related, and it is a trend away from a displacement into the other side of the globe, instead turning to twin vortexes suggesting amplification of the weaker ridge between. Perhaps looking closer to a split than thought before



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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

I think that we are going to see a pretty impeccable 500mb pattern after January 10th.    The -NAO originally forced by wave breaking (which is occurring in the very near future) will become

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Also noticed something while poking around. Even with a mediocre pattern especially in the Pacific, there's decent split flow and a decent southern stream there. So, there are storms, we just need improvements so those can produce snow



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After a cold start, temperatures moderated this afternoon. Temperatures generally peaked in the middle and upper 30s across the region.


Meanwhile, on account of the strong storm that impacted the region on December 24 into December 25, more record warmth occurred in parts of Canada. That storm also brought blizzard conditions to parts of Nunavut today while eastern portions of that Province experienced unseasonable warmth.


Daily record high temperatures included:


Badger, NL: 51° (old record: 42°, 2001)
Churchill Falls, NL: 36° (old record: 22°, 2011)
Corner Brook, NL: 52° (old record: 41°, 2010)
Hopedale, NL: 42° (old record: 36°, 1976)
Pangnirtung, NU (66.15°N, 65.71°W): 39° (old record: 26°, 2000)
St. John's, NL: 50°/10.0°C (old record: 50°F/9.8°C, 1977)


Tomorrow will be partly sunny and noticeably milder before cooler air returns to the region.


The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait.


There is growing ensemble support for the latter outcome. The probability that the block will be anchored near Newfoundland and Labrador in the extended range can inhibit opportunities for snowfall. This can especially be the case should the block attempt to link up to the western Atlantic ridge. In coming days, it will become clearer whether the recent and still ongoing shift in the ensemble guidance toward that outcome will become the most likely scenario for the first half of January.


Under this scenario, the first 10 days of January could be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada.


The ENSO Region 1+2 anomaly was -0.8°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 16. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.57°C and the ENSO Region 3.4 anomaly has averaged -1.17°C. La Niña conditions will likely prevail at least through the winter.


The SOI was +15.15 today.


Today, the preliminary Arctic Oscillation (AO) figure was -1.954.


On December 26 the MJO was in Phase 2 at an amplitude of 0.165 (RMM). The December 25-adjusted amplitude was 0.194.


Based on the latest guidance, a significant stratospheric warming event near or just after the start of January is likely. The highest levels of the stratosphere will warm during the closing days of December. The warming will begin to descend during the opening week of January. In addition, the mean zonal winds at 1 mb and 10 mb will likely reverse while the mean zonal wind at 30 mb will likely slacken considerably.


There is the proverbial fly in the ointment. It appears that this warming event could produce a split in the polar vortex. Typically, when the polar vortex splits, Eurasia is favored for cold. Displacement of the polar vortex often favors North America.


The recent snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue into the start of January.


Based on sensitivity analysis applied to the latest guidance, there is an implied 99% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.0°.


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Here we go.


The way to a Neg EPO isn't just getting  the " PAC " to slow down its diverting the pac which in itself slows it down.

A monster block displaces the low heights back towards the Aleutians and then the heights lower to the S of Alaska.

And you end up forcing cold HP around the jet.


Also the result is that it cuts the pac air that you see roaring over BC.


Also when you bring the block back over HB you develop colder anomalies in the mid latitudes and you force LP south of you.







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Wait for the period starting around the 10th onward. 


Work the block back to HB and watch as it kicks the low heights out of Alaska, it stems the PAC into W Canada and develops your storm track to the south. 


It's the look you want to see in mid January.

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19 minutes ago, Allsnow said:

Verbatim that 630 hour silliness map is not a “great look.” Cold dumping in the west with a hint of a south east ridge....


It depends on your location lol not bad for my location. 

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Just now, Event Horizon said:


I've seen that before. Isn't that a SWFE look?

Yes, 100% correct. Better pattern then we have now obviously. It would offer overrunning, front end thump, And redevelopment Chances with a hint of a -nao

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4 minutes ago, Allsnow said:

@Analog96 please don’t argue with me over a 630 hour map! I let you take the cheap shot at me. This forum is open to all types of views, even if they may not be the most popular. I think with the way the next two weeks look to play out I deserve the benefit of the doubt on reading a h5 map. 

You def deserve 👏👏. Good job as always man

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Since 1950, there were 13 cases where the EPO was +1.000 or above while the AO was -1.500 or below for at least three consecutive days during the January 1-20 period. In 3 cases, the Arctic block extended southward into the Middle Atlantic region (January 1-4, 1955; January 15-17, 1998; and, January 1-3, 2003). Below are the composite 500 mb height anomalies and temperature anomalies for that cluster:




Mean temperatures were the following:
Boston: 33.0°
New York City: 37.7°
Philadelphia: 40.5°
Washington, DC: 43.9°


Only Boston saw accumulating snow during the above cases. Its biggest snowfall was 3.5" during January 15-16, 1998.


The dynamical guidance is even warmer than these composite anomalies. The closest match from the above cases is the January 1-4, 1955 period.


In sum, the forecast pattern is one that will likely favor above to much above normal temperatures and little or no snowfall through at least the first 10 days of January in much or all of the Middle Atlantic Region and southern New England.


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38 minutes ago, Allsnow said:

@Analog96 please don’t argue with me over a 630 hour map! I let you take the cheap shot at me. This forum is open to all types of views, even if they may not be the most popular. I think with the way the next two weeks look to play out I deserve the benefit of the doubt on reading a h5 map. 


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