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Met Winter 20-21: Pattern Drivers & Evolution


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30 minutes ago, bird said:

Mikey V has a great presentation on the stratospheric warming it’s great for novices like myself It’s a long presentation so either grab a cuppa coffee or a captain Morgan and Coke your choice

MJVentrice (@MJVentrice) Tweeted:
Here's the Youtube video of today's weather discussion pertaining to our stratospheric warming event in the forecast... plus some Winter weather banter

https://t.co/ujNUYvMIdT https://twitter.com/MJVentrice/status/1344439880747802625?s=20

Agreed.  I watched this on twitch earlier today.  Great stuff!  I like how he explained it.  It wasn't just for weather nerds.  

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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

Posted Images

Great post, @OHweather! - The one thing I noticed w/regard to the +EAMT lingering on the ensembles is that the HP signal is still consistent in the vicinity of Mongolia. Even though the pressure does drop off into the late medium/long range, I wonder how much of that is attributable to the ensemble mean muting the signal at that point vs a true drop-off? It's still a rather robust signal, but I lean towards your interpretation & believe the reduction in torque is real. Even still, the retrogression of the -NAO block from the continued NATL wave breaks seem to be doing a good job on their own in thumbing the ridging west. So it almost seems like a potential good PAC outcome either way... Deal a blow via the block pushing far enough west... Or the +EAMT attenuates->we get the diminution of the PAC jet extension->see a retraction in conjunction of the block->+PNA/minor -EPO? I kind of like where we stand with regard to potential PAC improvements either way, but figured I'd toss that out there.

 

gfs-ens_mslpaNormMean_ea_10.png

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48 minutes ago, bird said:

Mikey V has a great presentation on the stratospheric warming it’s great for novices like myself It’s a long presentation so either grab a cuppa coffee or a captain Morgan and Coke your choice

MJVentrice (@MJVentrice) Tweeted:
Here's the Youtube video of today's weather discussion pertaining to our stratospheric warming event in the forecast... plus some Winter weather banter

https://t.co/ujNUYvMIdT https://twitter.com/MJVentrice/status/1344439880747802625?s=20

I'm 48 minutes in.   I recommend watching this if you want to learn about SSWE(s) and their effects on global weather patterns, specifically in Eurasia and North America.  

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6 minutes ago, OHweather said:

Yeah, I feel the +MT weakens in the 7-10 day range, but definitely can't say it's gone yet...so, any improvements to the Pacific jet probably won't lead to a sudden, long-lasting -EPO by the middle of January, and I think we don't see significant cold through at least the middle of the month either. The EPS and GEFS both developing negative SLP anomalies over eastern Asia 300+ hours...that's too far out to lock in, but THAT would give us a real chance to pull back the Pacific jet and pop a tall ridge near the West Coast or into Alaska. Even if the ridge axis is a bit far west, it still causes enough cross polar flow to really get some Arctic air involved, and as long as the NAO stays negative that would eventually work into the eastern U.S. 

 

So, I think there's good confidence in Pacific improvements starting around January 10th, but not immediately a huge EPO drop and not immediately that cold...but, could be enough to get winter storm threats. After mid-January, IF the mountain torque goes neutral or even negative, then there may be a more prolonged window of western ridging that starts shifting more cold into North America. That's more uncertain, but would fit the timing of the real cold getting into the U.S. 2-3 weeks after a strat warming event, which seems to be a common lag (with Europe often getting it first, as they will in this instance too). 

Terrific post! 

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53 minutes ago, OHweather said:

Yeah, I feel the +MT weakens in the 7-10 day range, but definitely can't say it's gone yet...so, any improvements to the Pacific jet probably won't lead to a sudden, long-lasting -EPO by the middle of January, and I think we don't see significant cold through at least the middle of the month either. The EPS and GEFS both developing negative SLP anomalies over eastern Asia 300+ hours...that's too far out to lock in, but THAT would give us a real chance to pull back the Pacific jet and pop a tall ridge near the West Coast or into Alaska. Even if the ridge axis is a bit far west, it still causes enough cross polar flow to really get some Arctic air involved, and as long as the NAO stays negative that would eventually work into the eastern U.S. 

 

So, I think there's good confidence in Pacific improvements starting around January 10th, but not immediately a huge EPO drop and not immediately that cold...but, could be enough to get winter storm threats. After mid-January, IF the mountain torque goes neutral or even negative, then there may be a more prolonged window of western ridging that starts shifting more cold into North America. That's more uncertain, but would fit the timing of the real cold getting into the U.S. 2-3 weeks after a strat warming event, which seems to be a common lag (with Europe often getting it first, as they will in this instance too). 

Thx Jim! 

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5 hours ago, PB GFI said:

I have always been Pacific driven in my downstream ideas, however this is the only year when something so massive retrograded so forcefully coupled with a SSW lining up from 50 mbs all the way to 500 mbs that  for me it pushed the pattern in the N Hemisphere.

 

I am sure when that block goes I will be Pacific driven again. 

One would argue that the Pacific matters because it can speed things up.

 

The Atlantic/Arctic matters because it can slow things down.

 

You need both. Never have understood the bizarre Pacific vs Atlantic wars in here.

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You can see the blocks influence is now felt by the 8th.

 

The 8ths SW will no longer cut through the lakes as it is now completely shunted underneath on both the GFS / CMC. 

 

Why is that important ? One because I thought that would happen so I like seeing it and two having the influence of the block not delayed and in fact early bodes well for the evolution of how this unfolds 

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5 minutes ago, PB GFI said:

You can see the blocks influence is now felt by the 8th.

 

The 8ths SW will no longer cut through the lakes as it is now completely shunted underneath on both the GFS / CMC. 

 

Why is that important ? One because I thought that would happen so I like seeing it and two having the influence of the block not delayed and in fact early bodes well for the evolution of how this unfolds 

Increases the chance of seeing a accumulating snow event before mid month. Nice

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On 12/30/2020 at 9:27 PM, OHweather said:

There continue to be consistent ensemble hints that the strong +EAMT...giving us the strong Pacific jet and mild pattern in the near-term but weakening the polar vortex at the same time, will start weakening in 7-10 days:

 

ecmwf-ens_mslpa_global_fh0-240.gif

 

Note how the pressures start trending lower over eastern Asia...that's the mountain torque weakening. In response the Pacific jet begins pulling back. Also, the longwave trough over the northern Pacific starts retrograding. These all lead to a pattern where heights can start rising on the West Coast and into Alaska in the 10-15 day range, which both the 12z GEFS and EPS do:

 

GEFS 500 5-15.png

 

EPS 500 5-15.png

With a strong, west-based -NAO, this improvement on the West Coast, while not immediately cold, would increase the risk for winter storms in the central and eastern U.S...there are already modest ensemble hints of systems January 10-11 and 12-14 over the eastern U.S. They may still have to deal with a marginal airmass, but given the blocking and some Pacific improvement will have a shot to snow if either occurs...with the pattern continuing to improve with time after that. 

 

Note how today's 12z EPS, the Pacific jet continues backing off in the 10-15 as the longwave trough continues to retrograde towards Asia. This would even give room for ridging into Alaska if it continued through the middle of January!

 

14-km EPS Global Globe 200 hPa Wind (1).gif

 

I'm still not sure if another round of +EAMT or tropical forcing result in another period of strong Pacific jet later in January...but for now, we have a lot of reason to think improvement begins after the 10th, with a window through at least the 20th or so of some ridging on the West Coast, potentially getting into Alaska temporarily along with a continued west-based -NAO. With a rather active pattern, this will likely yield snow potential, possibly significant, for good portions of the central and eastern U.S. 

 

It's hard to say how much fun we'll have at this range, but it shouldn't be too boring. 

 

Great post, the weakening of the EAMT begins in the day 7 to 10 period and by that time that monster positive is already all the way into W Canadian and Alaskan launching the negatives out towards the dateline.

Again when I look at the 250mb jet streak the initial slowing of the jet is near the west coast of Canada as the jet just folds in response to the giant positive just completely bending the mean air flow.

 

Again even if both are happening simultaneously and we benefit that's great, I am just glad the target date of the 10th showed up like I hoped and thought it would. 

 

One reason is because there will be so many Pacific vorts in the flow forced under a mid January block with what I believe will be just sufficient enough mid level flow.

Another is that the net drainage of air ever very early before the pattern takes hold between Jan 10 to Jan 15 will be out of a snow packed Eastern Canada and down into New England.

 

So what's 2 sigma AN 2m air in Bangor in mid January, if transported fast enough down east of the Mountains that is just sufficiently cold for me with maxes off to my S and E.

 

After that my goal is hold that block between Jan 15 thru Feb 15.

And I think the 1st bought of true Arctic air shows up by the 20th.

 

My goal for this period is that it will evolve into a period we talk about in here the way we speak about 02/03, 09/10, 10/11, 13/14 etc.

 

I think it gets to that.

 

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1 hour ago, Snowy Hibbo said:

One would argue that the Pacific matters because it can speed things up.

 

The Atlantic/Arctic matters because it can slow things down.

 

You need both. Never have understood the bizarre Pacific vs Atlantic wars in here.

 

I think it stems from people's view of their own great personal snowstorms they have stuck in their heads, so those anomalies tend to stick out.

 

The one thing is that being on the coastal plain at 41N is that LLC air has always seemed to work here the  best,  so having that EPO / PNA couplet forcing HP arriving across the top of LP with the net seepage to the base of the trough has been vital.

 

Having a good Atlantic block buckling the jet and having a 50/50 is also vital. 

So I agree, you need both chicken and the broth to make chicken soup. 

 

We continue to find transient features over the decades here that although present in the means come together so that yes both are important.

 

Now me personally as a trader I would use the Pacific more because I was always looking for downstream anomalies 

 

Except this year I just could not ignore what I believe is the bully of the Northern Hemisphere during my peak cold climo.

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The guidance continues with the arrival date of the trough around Jan 10.

Here is the 5 day mean for the 11 - 16.

 

Screenshot_20201231-015851_Chrome.jpg.5140d9022c8cbdf0b2044815d62192fc.jpg

 

How fast does it get cold ?

The GEFS says as soon as that trough digs in.

Screenshot_20201231-015915_Chrome.jpg.57f055b7e630ca525c84261d4afadaf0.jpg

 

So it's important to wait until the 10th before we begin to look for snow.

The 1st 10 days have been outlined and are going according to plan so we aren't " wasting " anything, the block is not where it needs to be until the 10th and beyond. 

 

 Here are some of the individual days inside this period, you can see how quickly the model wants to cool the mid layers across the CONUS.

 

Screenshot_20201231-015930_Chrome.jpg.c2278bd66899f302ff674f0912a4ecaa.jpg

 

Screenshot_20201231-015943_Chrome.jpg.9e7ae27be6d4949145b29f11cd72d879.jpg

 

Screenshot_20201231-015954_Chrome.jpg.04aaffde1f1f4813ce2c45cd9ca5ed96.jpg

 

Screenshot_20201231-020000_Chrome.jpg.47777bd9a794fdf8ce88a22926afaa44.jpg

 

Notice how the negative is out of Alaska by the 10th.

 

A sidenote the backside vort for the 4th was always going to be a bonus,  the 2 shredded cutters before the 10th were not supposed to be snow inside a 10 day plus 8 forecast. They were just suppose to flow underneath to show you the block was retrograding.

 

Step 2 should be that you find a way to snow before the 15th. 

The GEFS 4 inch mean is very close to NYC now. 

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Still that -PNA cutter pattern out to 10 days on the GEFS, but we're getting some signals after day 7 or so that suggest the beginnings of the changeover.

 

Watch the pattern generally retrograde once ridging takes over the pole. No surprise there given more easterly flow. This does 2 things:

 

- shifts the -z500 away from Alaska/EPO domain and towards Siberia, turning the EPO more towards neutral

- takes the ridging west out of central Russia, and takes Mongolia out of the big upper convergence.

 

2077326998_patternchangeclues.gif.82aed7d6cf0d5726176c1537487e18f0.gif

 

The first doesn't quite have an effect yet due to the Pacific firehose, but the second point will mean the Pacific jet stops going crazy and may finally allow for some +PNA.

 

Look at Mongolia where we've had those high high pressures. Once the northerly 500mb flow leaves the area and becomes more zonal the big +MSLPA blob moves away. This would give us a respite to the +EAMT, allowing for the Pac jet to retract, and for us to see less Pacific air entering North America. We're not quite there yet in the 10 day, but we're setting the stage for the change by then

pattern change mslp.gif

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