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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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Joe D`aleo.

 

 

Years with major events in December in the NESIS list since 1956 has winters with 2 to 5 major events. JB had an early version in a video that had a mislabel for 1960/61 (1950/61). He caught it. The winter of 1990/91 was not memorable for snows on the east coast but was on the expanded NCDC version of NESIS that expanded the area. I removed it, The average number of NESIS events with a December event is over 4!

 

 

Screen_Shot_2020_12_29_at_6_33_12_PM(1).png

 

On average, we are owed 3 more 

 

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2 minutes ago, PB GFI said:

Joe D`aleo.

 

 

Years with major events in December in the NESIS list since 1956 has winters with 2 to 5 major events. JB had an early version in a video that had a mislabel for 1960/61 (1950/61). He caught it. The winter of 1990/91 was not memorable for snows on the east coast but was on the expanded NCDC version of NESIS that expanded the area. I removed it, The average number of NESIS events with a December event is over 4!

 

 

Screen_Shot_2020_12_29_at_6_33_12_PM(1).png

 

On average, we are owed 3 more 

 

2000-2001 would have had one more if March 5 wasn't such a fiasco.

2013-14 could have had a couple more in March if that PV on steroids didn't sit over Lake Placid.

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40 minutes ago, PB GFI said:

@brooklynwx99, yeh it happens pretty quick

 

1610625600-qF7H7CvRP7k.png

 

 

The net drainage of cold air just comes right through the ridge position 1610625600-Gx9LXz1PSYY.png

 

1610625600-GeIZYZhsF6I.png

As many have said, with such powerful HL

blocking, we don’t need a perfect pacific, but that depiction is damn near close to it, and like you said, it evolves very quickly. 

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21 minutes ago, Nick Psomaras said:

 

 

Now THAT is the kind of cold air transport that yields the big time east coast snowstorms...To hell with the Pacific influx, give me that NW flow

 

 

Screen Shot 2020-12-30 at 3.11.41 PM.png

Won't need to worry about warm air when that NW flow begins and dislodges the arctic airmass... getting on the horizon now

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9 minutes ago, WinterGreenie said:

Can someone tell me what a KU is? 

It's like a Saffir-Simpson scale but for Northeast snowstorms...It was developed by 2 meteorologist, Paul Kocin and Louis Uccellini (hence KU). It takes into account the effects a storm has on the economy and transportation throughout the major cities and the rest of the NE as well. Categories include Notable, Significant, Major, Crippling, and Extreme...All corresponding to a different number of people effected 

 

Screen Shot 2020-12-30 at 4.23.11 PM.png

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11 minutes ago, Nick Psomaras said:

It's like a Saffir-Simpson scale but for Northeast snowstorms...It was developed by 2 meteorologist, Paul Kocin and Louis Uccellini (hence KU). It takes into account the effects a storm has on the economy and transportation throughout the major cities and the rest of the NE as well. Categories include Notable, Significant, Major, Crippling, and Extreme...All corresponding to a different number of people effected 

 

Screen Shot 2020-12-30 at 4.23.11 PM.png

 

For reference here is the Dec. 2020 storm...It only ranked 43rd on the NESIS scale (significant) because even though the max snow totals ranged from 2-3 feet, they did not fall over the major cities (much higher population). However, if those 20-40" totals were from DC to Portland then that storm would've easily placed in the top 5 (crippling-extreme)

 

 

Screen Shot 2020-12-30 at 4.28.24 PM.png

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I'm sure most of us already know this, but the pattern change 2 weeks out that we're looking at that is expected to make east coast snow far more likely isn't some phantom pattern change that always stays 2 weeks out (and before you know it it's March). In fact, the modeled pattern change is exactly what we should expect given the stratosphere. Luckily, Mike Ventrice also shared the composite 500mb pattern 2 weeks after a Siberian SSW. Given that it appears to be beginning now, the change to something similar to that composite should occur in around 2 weeks. So, unlike phantom patterns that have traumatized snow lovers in the past, this one is definitely based in some reality. Let's take a look at that composite. The image is a bit fuzzy but you can still clearly see the pattern.

 

Image

 

In the composite, the -NAO continues, unsurprisingly given the coupling between the stratosphere and troposphere. This would retain the tendency for lows to have a latitude cap, and would slow them down allowing for longer-lived snow events. @Webberweather shared a loop with us earlier showing a 50/50 low basically stuck in place, and with a -NAO you can do that and have "sticky" conditions in the Atlantic where patterns like that that would favor colder air in the region can become more persistent. 

 

Another big thing - there's also a bit of a -EPO here, which displaces that Aleutian troughing and instead causes there to be more troughing/low pressure in the subtropics. And with any block in place, this slows down the flow keeping that negative anomaly offshore. What does this lead to? Well, with any -ve height anomaly you tend to get a ridge kicked up downstream. If you have the slower flow there, then a west coast ridge can allow for decent PNA spikes. Over NW Canada, there is a good amount of troughing there in the composite which would mean a cold source region. Follow the flow, and it digs down to deliver that cold air into the CONUS. 

 

Of course you're not going to get something like this happen verbatim as every event is a little bit different, but even something similar to this would cause a far more favorable pattern to set up and a likely more active period during the deepest parts of winter.

 

However, there's one failure mode. Just because there's an SSW doesn't mean you'll magically have a more favorable pattern take hold. For example, take the beginning of 2019. Siberian SSW, similar to this one, at a similar time of year. There's some significant differences here, particularly the Atlantic ridge. This allowed for more southerly flow and warm air up the east coast, and I don't recall it being very much of a snowy period. What happened?

 

Composite Plot

 

time_pres_HGT_ANOM_JFM_NH_2019.png

 

Well the +z anomalies never really downwelled or coupled with the troposphere until the beginning of February (and only briefly), despite the length of the event, which likely caused the weak reaction in 500mb heights. So this is one area where things can go awry. 

 

geos_nh-namindex_20201230.png

 

gfs_nh-namindex_20201230.png

 

It's a little early to tell for sure, but there are signs that there will at least be some at the surface, and more coupling with the -NAO than 2019.

 

In all, don't let the past "2 weeks from now" pattern changes that busted cloud your thinking here; this one is far more real.

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On 12/28/2020 at 8:15 AM, PB GFI said:

Here's how I see this evolving over the next 30 days.

 

The blocking you see over Scandinavia  will begin to retrograde and force any positives over Greenland back South towards Hudson Bay. It gets there by the date constantly mentioned here Jan 10th.

 

So the very 1st effects you will see will be that cutters begin to get sheared out around this time.

 

Now I think there is one more cutter that gets far enough north and drags us into the 50s for a few days and the period finish well AN but once that block is established that's over.

As we get closer to the 10th and those cutters begin to get shredded they will look for a warm source and that's the Atlantic.

What typically happens is you will get some of the centers to redevelop off the east coast. 

The 1st ones to feel the effects of this would probably be New England.

 

It will not be overpoweringly cold during this period but in any of these scenarios you would need a 50/50 to snow because it is January not November.

 

Now I like the pattern after the 10th , so before then I am only looking at a single SW on the backside of a cutter to work, and if it doesn't I promise it's no big deal because this was always centered around Jan 10th and all points after. 

 

After the 10th the center of those heights are working their way back towards Baffin Island and now you're in business.

I mean golden. 

You will knock the heights out of Alaska and send all those PACIFIC lobes under the block.

That will begin to send the EPO to neutral to negative and turn the PNA positive.

So the 2nd phase after the 10th through the 20th you will begin to see more systems attempt to take a more W / E route.

Now the cold will begin to show up.

 

By the time we get to the 20th - 30th as the pressures rise in Alaska a better - EPO takes hold and if you can hold the block through HB , you will be looking at coastals.

By this time you have a chance for the vortex to develop underneath.

 

That block is going to overpower the pattern for a while and it will actually force the fast Pac to eject its lows down through Central and Southern California and stop the flow into British Columbia.

 

So the block still has to get set 1st.

So until then you wait to see if the MJO stays muted and look for a piece of the vortex in the longer range. 

 

@Sundog I didn`t like the first 10 days, I don`t think that`s a real block. But the 8th SW gets shredded and that`s the 1st sign the block is getting to HB. 

 

My Jan forecast found in here was Jan 1- 10 plus 8 Jan 11- 15 N to slightly below Jan 15- Feb 15 as the core.

 

We are not wasting a block before the 10th because it`s not a real one.

The real block starts on the 10th and stays in place for a while as SW after SW potentially moves under. The PNA spikes and that`s how we snow.

 

I promise you didn`t waste was never a real block.

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