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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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14 minutes ago, leo2000 said:

-EPO too?

 

Yes, the deep negative is in a perfect spot.

 

It pumps the ridge poleward and that map ends with a horseshoe block.

 

The ridge at 60 will deliver Arctic air into that pattern.  That's really good source region air with negative on the east coast at the end of the run.

 

It's great if it's real. 

 

 

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3 minutes ago, Allsnow said:

No. But the Flow from the pole because  of the +pna would make it cold in eastern Canada/US

 

The heights are lower IMO out over the Aleutians than in N Alaska.

 

I would classify it as such. 

 

But I agree with you the positive shoots poleward and that's cold underneath. 

 

 

20201121_142115.jpg

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12 hours ago, OHweather said:

I will post the rest of the technical write-up during the day Saturday (it's too much for me to proof right now), but here is the month-by-month summary and maps at the end of my personal winter outlook. My apologies for being a bit tardy, but I've been busy and this took a while to put together. It's almost easier to do this in October, as enough happened this month to change the outlook and add quite a bit to the technical discussion.

 

December:

 

December Map.png

 

A +PNA to end November and start December leads to a very mild / warm start to the month across the CONUS (slightly cooler over the Southeast / East Coast)…however, an active STJ and a wavier pattern than the ensemble means will show at this distance may allow for enough polar influence for an opportunity or two at a winter storm across the central or eastern U.S. Given the +PNA and active STJ, this may occur in areas such as the Ohio / Tennessee Valley, Appalachians, Southeast or Mid-Atlantic, farther south than what you may expect in a La Nina and more typical of an El Nino. A +PNA, split flow, and active STJ are features that are frequently seen in an El Nino!

 

This window is brief, the first week or so of the month and with a questionable amount of polar air to work with, so it may not work, but there should be a bit to track on the models as this comes into range. Thereafter, the SPV moving away from Alaska, persistent +EAMT, and convection getting into the 120-150E area (and likely persisting a bit more as the GEFS has due to the lower frequency forcing in that region this fall) likely allows for a window of -EPO, and a pattern more typical of a December La Nina. I think there’s still enough momentum in the Pacific jet that this dumps into the Midwest and Northeast as opposed to the Rockies and Plains as typically occurs with EPO shots.

 

This trends towards cold edging towards the Northwest / northern Rockies into late December, with a Southeast Ridge cropping back up due to forcing becoming more focused on the Indian Ocean, and due to Indonesian convection beginning to teleconnect to a Southeast Ridge in later December.

 

With the start of end of December quite possibly warm for a large portion of the country, the month as a whole will be mild across the CONUS.

 

While I think the NAO likely ends up positive for the month, if we take advantage of the potential to develop a -NAO in early or mid-December, it would up the ante for snow in the Northeast and Mid Atlantic in that window, and may slow the warm-up later in the month. Even without a -NAO, I think there are opportunities for some snow in December in the central and eastern U.S. (even the southwest may have a window with the STJ, the NW and Rockies will be active and trend colder later in the month as well).

 

January:

 

January Temps.png

 

January may start cold in the Northwest U.S. and warm across the southern and eastern U.S. However, I think the MJO has another opportunity to propagate east during the first half of the month. I believe the PV will be weaker and more receptive to the MJO producing a blocking response than it is in early December. This may cause another period of -EPO, -NAO, and a somewhat active sub-tropical jet for a couple of weeks starting the first or second week of January. This will bring another window of winter farther southeast towards the Ohio Valley, Mid-Atlantic, and perhaps even the Southeast.

 

I think we start trending towards a Southeast ridge later in January as the MJO fades and Indian Ocean forcing crops back up. How much blocking develops in the first half of January will of course influence how quickly this warm-up occurs.

 

February:

 

February temps.png

 

This is generally the warmest month in the analogs, with an amped central Pacific ridge dumping cold into Alaska, western Canada, and the northwest / north-central CONUS. If we have persistent Indian Ocean forcing and a strengthening PV to open the month, that will probably be the case again this year. If there’s cold available in Canada, the +NAO may lead to confluence east of New England that leads to high pressure over eastern Canada that can cause cold air to seep down into New England and perhaps the Great Lakes in February, causing some wintry threats in these areas…however, the pattern I envision for February is not snow-friendly in the Ohio Valley, and especially the Southeast and Mid-Atlantic.  

 

March:

 

March temps.png

 

Cold air likely remains available in Canada, so if the MJO becomes active again in late-February or March it may shake-up the pattern enough to bring it south and bring one last shot of winter in March. If this does not occur early enough, the Southeast Ridge dominates much of March and keeps the cold over the Northwest, Plains, and Midwest / northern Great Lakes / northern New England.

 

Seasonal snow:

 

Seasonal Snow.png

 

EDIT: Here is a link to the entire technical discussion: https://jimsullivanweather.com/2020/11/21/jim-sullivans-2020-21-winter-outlook-write-up/

As I uploaded all of the discussion to my blog, I realized it was much too long for me to double post the whole thing on various forums (it takes a considerable amount of time to upload all of the images. So not to come off as lazy or promoting my blog (that I never use except for these), but here is the link to the technical discussion: https://jimsullivanweather.com/2020/11/21/jim-sullivans-2020-21-winter-outlook-write-up/

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23 minutes ago, OHweather said:

As I uploaded all of the discussion to my blog, I realized it was much too long for me to double post the whole thing on various forums (it takes a considerable amount of time to upload all of the images. So not to come off as lazy or promoting my blog (that I never use except for these), but here is the link to the technical discussion: https://jimsullivanweather.com/2020/11/21/jim-sullivans-2020-21-winter-outlook-write-up/


you’re fine Jim. You share so much here that we’re always happy to link directly to your content. we appreciate it!

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12 minutes ago, 33andrain said:

Would technically need the trough axis further east to get snow to the coast.

 

Trough looks pretty broad so at least in that depiction it may still be good for the coastal tristate. 

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  • 33andrain changed the title to Met Winter 20-21: Pattern Drivers & Evolution
  • NJwxguy78 changed the title to Mid/Long Range:Met Winter 20-21: Pattern Drivers & Evolution

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