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Met Winter 20-21: Pattern Drivers & Evolution


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1 hour ago, brooklynwx99 said:

Talk about a trend, the GEFS is almost in line with the GEPS and EPS. This is getting close to a KU pattern.

 

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 Exactly many seem to be worry about a return of La Nina and that the positive may roll over. Well that doesn't look like that will happen. You seem to be an expert on the Polar Vortex. Just wondering if let's say a SSW happens mid December how long do they typically last?. Can we only get one SSW event in Winter?.  What would that mean for the rest of Winter?. 

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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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4 minutes ago, Oliviajames3 said:

Absolute opposite scenario of what we dealt with last year and the weeklies.  Might be getting ahead of myself but I am hopeful goi g forward 

 

We look to be good through week 3 of December.

 

Then we will see if what @CCB!posted  happens 

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1 hour ago, PB GFI said:

 

We look to be good through week 3 of December.

 

Then we will see if what @CCB!posted  happens 

 

 

Even though week 4 is not reliable what does it show?. I hope we can hold onto this pattern and just keeps getting better until Mid Jan then we can have a mini January thaw and reload. 

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4 minutes ago, leo2000 said:

 

 

Even though week 4 is not reliable what does it show?. I hope we can hold onto this pattern and just keeps getting better until Mid Jan then we can have a mini January thaw and reload. 

 

The skill score past week 3 is awful. 

 

Just look at the old week 4 vs the new week 3 I posted.

I ignore them after week 3.

 

 

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34 minutes ago, Heavy snow said:

Joe b just posted that we better cash in because he thinks we warm by Christmas 


Considering what I posted earlier from Paul Roundy, I would be inclined to say we need to cash in the 2nd or 3rd week at the coast.  After that it appears we will torch and have to wait until after the new year for next opportunity of wintry weather at the coast.

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Temperatures soared well into the 60s across the region this Thanksgiving Day. Tomorrow will be partly cloudy and still unseasonably mild.

 

Late this weekend into early next week, a storm could bring moderate to heavy rainfall to the Southeast and then up the East Coast. Following the storm, the Southeast could see unseasonably cool temperatures. The core of this colder air will likely stay south of the region, but somewhat below normal to near normal readings are likely in the Middle Atlantic and southern New England areas.

 

Overall, December could start off mild before that period of cooler than normal to near normal temperatures commences. This cooler period could still give way to warmer readings at some point during the second week of December, but there is considerable uncertainty. The now likely development of a AO-/PNA+ pattern has shifted the outlook toward colder temperatures during the first half of December (consistent with statistical guidance). The duration of the AO-/PNA+ pattern could delay any warmup until near mid-month.

 

Statistical guidance based on the ENSO state and teleconnections would typically favor a colder regime for the first half of December. The dynamical guidance has moved closer to this idea for the Middle Atlantic region. Both historic experience following exceptionally warm November cases and the latest weekly and monthly guidance suggest that a warmer than normal December remains the base case even if the first half of the month winds up colder than normal. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above.  

 

The ENSO Region 1+2 anomaly was -0.7°C and the Region 3.4 anomaly was -1.5°C for the week centered around November 18. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.90°C and the ENSO Region 3.4 anomaly has averaged -1.42°C. La Niña conditions will likely prevail at least through the winter.

 

The SOI was +5.21.

 

Today, the preliminary Arctic Oscillation (AO) figure was +2.227.

 

On November 25 the MJO was in Phase 4 at an amplitude of 0.937 (RMM). The November 24-adjusted amplitude was 0.705.

 

Based on the latest guidance, no significant stratospheric warming event is likely through the start of December.  

 

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. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall.

 

Since 1950, there have been four La Niña winters that started with a warm December in the Northeast and warmth across much of Canada, as is the current forecast on the monthly EPS, latest weekly EPS and latest CFSv2 monthly guidance: 1974-75, 1998-99, 1999-00, and 2011-12. All featured a warmer than normal winter and among the winter months that followed December, only January 2000 was colder than normal in the East. Median seasonal snowfall figures were as follows: New York City: 12.9" and Philadelphia: 13.1".

 

Since 1970, there were 9 winters that saw the AO and EPO average +0.25 or above. Mean snowfall for Boston, Harrisburg, New York City, Philadelphia, and Washington, DC averaged 50% of the most recent 30-season mean. The largest snowfall deficits relative to the most recent 30-season mean figure were located in the Philadelphia to New York City corridor. In addition, 33% of cases saw less than 10" seasonal snowfall in New York City and 44% saw less than 10" seasonal snowfall in Philadelphia.

 

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 November. November will likely finish with a mean temperature near 52.2°.

 

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25 minutes ago, uofmiami said:


Considering what I posted earlier from Paul Roundy, I would be inclined to say we need to cash in the 2nd or 3rd week at the coast.  After that it appears we will torch and have to wait until after the new year for next opportunity of wintry weather at the coast.

Just enjoy a somewhat favorable pattern as we get into the second week of December.  If it materializes and delivers awesome if not we gave it a try.  Don’t worry about a month from now at this point.  

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2 minutes ago, Analog96 said:

FWIW DT seems to think most winter forecast consensus is WAY wrong.

Do you have a link to his forecast or Twitter? It would be interesting to read. There’s still no standard approach to seasonal forecasts, so more thoughts are always welcome.

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