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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

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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3 minutes ago, Webberweather said:

 

This is arguably the most epic E Asia Mtn Torque event I've ever seen. Coupled w/ the potential for a sub 925mb sfc low near the Aleutians in a few days, that alone tells you how insanely strong these stationary, planetary wave fluxes are gonna be the next week or so, which makes a Sudden Stratospheric Warming Event all the more likely in early January.

Doesn't look like we're done with EAMT events in the near future either if the GFS is to be believed....

gfs_mslpa_wpac_fh0-384.gif.fbfae0fe49fbc6c3ac174cadf1a02e09.gif

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1 minute ago, Webberweather said:

 

Yep, I see no end in sight either as long as the Ural Sea block is there, we'll continue to see big Siberian highs descend into E Asia and continuously reinforce the N Pacific jet. One of the strongest looking & classic precursor signals you'll ever see to a sudden stratospheric warming event

gfs_Tz10_nhem_33.png

Yup. Corresponding GFS strat forecast unsurprisingly looks like this.

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Following a frontal passage, the sun returned and readings rose into the upper 40s and lower 50s. Tomorrow will be fair but colder with highs mainly in the upper 30s and lower 40s.

 

As of December 28, the preliminary monthly Arctic Oscillation (AO) average has been -1.560. 75% of days have seen the AO at or below -1.000. This is a dramatic change from Winter 2019-20 when Arctic blocking was virtually non-existent. The last comparable December was December 2012 when the December 1-28 average AO was -1.786 and 79% of days saw the AO at or below -1.000. However, that December was even warmer, on account of a mainly positive EPO and negative PNA.

 

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.

 

Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, there is some indication on the EPS that a transition that could lead to a trough's moving into the East could get underway.

 

The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter.

 

The SOI was +19.15 today.

 

Today, the preliminary Arctic Oscillation (AO) figure was -3.452. That is the lowest figure since November 26, 2018 when the AO was -3.629.

 

On December 27 the MJO was in Phase 6 at an amplitude of 0.061 (RMM). The December 26-adjusted amplitude was 0.165.

 

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. Toward the end of the first week in January, the top layers of the stratosphere could begin to cool.

 

The significant December 16-17 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 through the first half of January.

 

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 December. December will likely finish with a mean temperature near 39.0°.

 

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Another thing about the pattern change I just realized thanks to @Analog96 - you'll be able to tell when things are changing once you see the storm path shift. Right now, definitely a cutter pattern. But, once we get the shift happen, you could get things like a low out of Canada diving SE as the PNA finally flips positive. Once we see the storm track shift away from cutters, I believe it'll be time for the real action to begin

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2 hours ago, MR FREEZE said:

Just wondering if anyone knows what the highest pressure ever recorded in NYC was?

 

1 hour ago, ModRisk said:

 

I'm not sure but i found this article. 

 

https://www.nytimes.com/1981/02/14/nyregion/pressures-of-a-31.08-barometer-reading.html

 

Which would be 1052.49 mb.  

Would love to see one of those anchored in place with a juicy low riding up the coast.  It might result in suppression.  That can be equally debilitating as snow changing to rain.  In Jan 2010, there was a storm that kept inching up the coast fighting suppression.  The cutoff was literally at the Verrazanno bridge.  Over 6 inches worked thru parts of Staten island but just flurries made it across the bridge into Bay Ridge Brooklyn coating the ground.  After swallowing that bitter pill, we more than made up for it from that point on.  The rest is history!

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How I have the next 45 days temp wise..

Jan 1 - 10 NYC + 8 

 

11-14  will transition to N 

 

Jan 15 - Feb 15 KNYC - 3 to - 4.

I think this is the core of what's coming ( and that's a really cold and snowy stretch during the coldest Ns of the year ).

 

But I like the strat to eventually couple so if it doesn`t happen on day 1 I am still confident you will see it during the core period. 

 

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Now this should put to rest once and for all whether or not the trough is coming back under an extreme ridge around the 10th.

 

The last week in here has been difficult to say the least explaining  how easy this was to see.

 

Now in my opinion this is the beginning of A KU pattern, but to be fair I have no way of knowing if it's between the 10th - 20th or 20th - 30th that it occurs ( I always like the pattern the get established first ) but when was the last time you saw the start of pattern begin with such a look.

 

 

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