Model discussion highlights

[Cold Winter Night]

34 minutes ago, bluearmy said:

since then, we’ve had two runs of the 15 day eps which are run at sig higher resolution and I believe more vertical levels than the 46.  Both these runs have stepped away markedly from a tech reversal and whilst still keener than the gefs, way down on where they had been. 

EC46 has the same number of vertical levels (137) as the 15-day ensemble and Operational.
The spatial resolution (longitude-latitude) of EC46 is 36 km vs 9 km for the 15-day ensemble EPS.
However, above the tropopause, in the more stable stratosphere, far from topographic/orographic shenanigans lower down, for the bigger picture in 2-4 weeks for which the EC46 is designed, it is doubtful if the higher spatial resolution adds much extra (it could theoretically even insert greater error).

EC46 on the other hand has the bigger ensemble with 101 vs 51 members.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985059

[Mike Poole]

There’s an awful lot going on at the moment, at different timescales and in different places.  Here’s how I see things:

There is a high probability of a significant cold spell starting early in the new year.  There is also a reasonable chance that it may last (with one or two wobbles) into the spring.  But we have some hurdles to get over first.

So how have we got here and what’s in our favour?  Take a step back from the UK for a moment, and this winter has one stand out feature, a weak polar vortex, in the troposphere that is one that can’t find its usual home, in the stratosphere, that is one that has already suffered a Canadian warming, is now under attack, and on the ropes.  We have an easterly QBO, and we have El Niño, which favours cold in the second half of winter.  We add to that (finally) a more amplified MJO in Phase 1, and rising AAM to put the atmosphere in tune with that El Niño.  

There is a timetable as to how this is likely to unfold:

• Forget the next 9 days, there is nothing to see here!
• About 10 days from now, model runs indicate that high latitude blocking will take hold in the European sector.  How exactly that happens depends on luck - where a ridge punches through, we’ve seen all the outcomes on the various op runs recently.  Provided that happens, could be Greenland (more likely, think GFS 12z), could be Scandi, possibly with retrogression (less likely, think yesterdays pub run).
• At about the same time, a warming event in the stratosphere is taking place, it may or may not be a technical SSW, but it will ensure a weak vortex for the foreseeable.  The models are backing off from a split SSW that looked to me a couple of days ago like it could lock in a -NAO until spring; this backing off may mean it happens later - perhaps when we need reinforcements, I’ll come back to that - or it could mean it is less powerful.  There’s also the possibility that a less than convincing split leaves a remnant of the strat vortex near Greenland where we don’t want it.  None of that is likely to markedly impact the next 3 weeks though, for good or bad.
• The AAM surge may continue, the MJO will move away from favourable phases in time, this might be the optimal time to benefit from a SSW, we can’t be choosers in this, but what I am highlighting here is the possibility that things might work constructively if we can get this winter evolution off the ground in the first place.

Finally, I leave you with the ECM extended clusters, at day 15 you have a choice of cold, cold, cold or cold:

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985366

[WYorksWeather]

Think we need to go back to first principles a bit when watching the model output. Firstly, the background chance of a prolonged cold spell with a significant amount of snow in any given winter month is generally quite low, possibly even very low for some of the least favoured areas further south. Hence, background signals and tendencies for an increased chance need to be taken in that context, by both cold ra+mpers and cold pessimists (or mild lovers).

An increased background chance of a cold spell as the Met and others are suggesting for the second week of January onwards still doesn't mean it's guaranteed, so don't count your chickens if you're looking forward to it. On the flipside, even if it doesn't happen, it emphatically does not mean that the background signals are wrong, just that the odds didn't fall in our favour despite the increased chance.

The next point is that the ensemble forecasts are nearly always going to be more useful than any given deterministic operational run, unless you just like looking at pretty charts (which is an acceptable way to look at the models, I'm not policing what people post here!). The other point is that there are so many models, runs, and charts, that you'll nearly always be able to find a 'game over for Winter' chart or a '1 in 100-year snow bomb' chart.

With all that in mind, going to have a look at a few things.  Going to cover background drivers first.

SSW chances

Here's the ECM mean zonal wind chart for today.

And here's the last week.

 

The notable thing is that after getting closer and closer to predicting a major SSW, followed by the vortex remaining seriously disorganised afterwards, we finally saw the mean showing a reversal yesterday. The vortex does become very weak, as has been suggested for some time, but the mean is now in the 5-10 m/s range. The pattern afterwards has also shifted, with the vortex recovering fairly quickly to just below its climatological mean by February. Of course, these charts will continue to change, but the odds of a major SSW in the next two weeks have certainly diminished, but there may be some effects even if we don't actually get a reversal.

Sea surface temperatures

As others have said, well worth mentioning that sea surface temperatures will continue falling in the weeks ahead, despite the fact that we're now past the shortest day. There is a huge amount of inertia in this system. Of note is that most of the waters to our east will drop from the 8-10C range to the 6-8C range over the next two weeks or so.

  

In anomaly terms, not a huge amount of change except a cold spot emerging to the south-west of Iceland. And of course, the sea ice advance is visible on both charts. Still positive that we've lost some of the crazy anomalies around our immediate coasts. I would continue to note, however, that as we have seen at times recently, any south-westerly influence is likely to be notably mild, even compared to what we would normally expect from that wind direction.

GEFS and ECM ensembles

So, with all that in mind, onto the NWP output. Both ECM and GFS show something quite promising by the time we get to the second week of January. 

Promising for the north as well.

Here's my overall take - background factors aren't all that favourable for the next week or so - up to that point you're probably on a hiding to nothing chasing cold away from the most favoured spots (note the large amounts of snow for some parts of Scotland today, as an example!).

But, as we enter the second week of January and go beyond, the chance of cold does increase relative to average, and is starting to be picked up at longer range in the ensembles. All of this is regardless of what the individual OP runs decide to do - as we can see there are some very mild runs in here, alongside some very cold ones, so best not to spend too much time on an emotional rollercoaster. There will always be a mild outlier chart or a cold outlier chart each day.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985401

[Ventnor Viking]

I haven't seen that much cold uppers over the arctic for a long time and over such a wide area as shown on the GFS 12z. This also ties in with the arctic sea ice gains as shown below. Its finally making its way back in to the 1980-2010 range, which it hasn't been in for quite sometime.  What this does mean is that if the SSW takes place then there is plenty of cold air to displace

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985157

[Eagle Eye]

As we go into the next few weeks we should begin to see the change towards a fairly good - VE NAM.  This isn't entirely supported by the composites but the general wave setup is and w/the rise in AAM due to the large recent +VE EAMT, the wave response appears to have been shifted. 

The significant +VE heights that developed towards Ural in response and caused the Strat wave, will have shifted the jet response enough for it to have a fairly significant change. The tropical forcing inputs remains the same, however, the energy is scattered differently. 

Importantly, the American pattern has held up w/higher AAM than a typical Phase 8 response and the Phase 8 remaining quite weak. As we go towards a response to Phase 1, it's important to note that the +VE EAMT has caused essentially a flip of the setup as you can see w/the Pacific ridge. So the wave response is pretty much flipped fully. 

The +VE EAMT is pretty much running things at the moment as the event has forced the wave response to push the - VE NAM development. Of course a - VE NAM is just a setup, it doesn't guarantee cold but given this Winter so far, I think a lot of us will just be happy to see it return. 

As we go towards a Phase 3 response, we lose AAM due to the frictional torque response. So I suspect some of the pattern will retrograde but then the MJO pattern we move into is a - VE NAM with a sliding mid-Atlantic ridge pushing into Greenland on the composite. Potentially signs that it's a Greenland-Scandi dipole which is a very supportive pattern for the undercutting low from the east potential. So the Trop is still in a supportive potential as we go towards the response likely responding into the first couple weeks of January. 

This in my mind points to a what should a be at least 1 cold spell by mid January. Best supportive pattern we've had so far this Winter. 

Even w/out a reversal, the vortex looks to stay weak for the next bit. Going through the mid January pattern, meaning that outside influences are unlikely to be strong and if the eddy driven jet can remain strong then a lower Strat - VE NAM development could downwell in and of itself and strengthen the setup. The Strat influence should be positive unless it's a displacement type SSW w/the main vortex lobe over us. If that were the situation I'd much rather a weak vortex w/a - VE NAM. +VE zonal anomalies are unlikely to be pushing their way down and interfering w/the already in place Trop setup. 

These composites need to be taken w/a pinch of salt especially with the tropical forcing being wave based and it's only a really basic setup that these can give you. The global dynamic composite models like this cannot take base setup into account and the distribution of AAM is still based off zonal distribution and the Rossby packet in initial setup as the RWT is modified by the change in AAM in trerms of strength of wave propogation which effects the size etc of the waves. 

Looking at the Strat and it's mostly the same as it has been. Whilst the modelling has been going away from a SSW, the stretch on the vortex and the baroclinic tilting w/height is still SSW esque and it's quite a hard forecast for the models because it's multiple waves of warmings that have battled down the vortex and now comes the point where we have no idea whether it'll be displaced or split. Ill be honest, I don't think I could hazard a guess at this point.

There are still signs towards the end of the runs recently that there's a subtle Wave-2 creeping I and maybe Wave-1 splits into a Wave-2 to force a Wave-3, it's hard to tell. Perhaps backing up what I said yesterday that this could well be a case of delayment and/or the models arent handling the possibility of an initial Wave-1 maybe being strong enough to split rather than displace the vortex and so we see a large temperature gradient forced but it kind of just moves the vortex and weakens over time rather than the atmosphere trying to find a thermodynamic balance w/the Strat Rossby wave that's been created. 

If a SSW were to occur then that's a good sign, they reinforce the surface-mid Trop pattern initially. The Trop-Strat-Trop connection forces the downwards circulation of velocities to structure like the base pattern that forces the SSW. We're talking here about a transient initial setup around the Arctic area, the mid-latitude response won't necessarily be the same. The Arctic imprint should remain the same but mast longer than the transient pattern that formed it and the mid-latitudes will have a furthered wave pattern. 

For now, it's not ready steady go nor is it stop looking at the Stratosphere. It is however a very important time for the Trop and Strat for future cold prospects especially when we're going into January and up until around mid-January when the MJO will have moved on, likely to Phases 4 and 5 for later January response. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985480

[Met4Cast]

Just now, Gowon said:

I hate it when people say that.

No run should be discounted.

The GFS is well known for "bowling ball lows" in the extended part of the run. When you've been watching models long enough you start to get a feel for when the model is going off on one. 

Having said that - At these ranges we should be focussing on the ensembles anyway instead of individual det runs. Broad theme is good on the 18z, high pressure building northwards. Anything beyond that is noise.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985535

[johnholmes]

Well the NOAA 500 MB anomaly charts have now shown a similar pattern for the last two outputs, that is a 500 MB ridge building to the W/NW of the UK.

Will do a run down tomorrow morning

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985537

[Ruzzi]

1 hour ago, TillyS said:

This is as zonal as I can ever recall for this time of year. We are in the midst of an Atlantic battering  .

 

47 minutes ago, TillyS said:

Thanks for your realism

 

The absence of any useful northerly blocking is incredible tbh. There’s usually something around Greenland, Iceland, Svalbard, Scandinavia at this time of year.

Come on! You really can't be serious with this? Pessimism must be high to conjur up statements like this but unfortunately they just aren't true. 

For balance of presenting fact as opposed to dramatic headline grabbers .... here is yesterday's verified chart: 

 

 

Now compare that with a genuine pessimistic chart for this time of year, aka 2004: 

 

You can see the cause for pessimism diminishes somewhat.... and further to that, we have a more amplified MJO, phase 8/1, a pending significant weakening of the strat, and a host of other favourable teleconections which I won't go in to analyse given the great job that others do so well to explain to everyone and have been giving a running commentary on. Point being, there is plenty reason for optimism beyond the 7-10 range, its not your 2004 picture painted above. 

To keep the balance of realism for maybe newer members (as I suspect the seasoned will know that you threw a few statements out with no rationale, or indeed, maybe for the controversy) ....

Here are 5 of the last 8 years for this same time of year: 

 

And here is the 3 remaining years of the last 8 years which have a slightly better viewing albeit, still don't show up the "abundant end of December northern blocking" 

 

Model fatigue can be a funny thing, but when you go back to looking for the basics, eg a roaring PV over Greenland, strong azores high or iberian high with a strongly positive NAO, or indeed strongly positive AO, a cold strat with zero warmings, an MJO in the COD and so on.... if you can tick all those boxes with no change in the forecast, only then will your statements hold credence. 

I envy the newer members this winter more than probably any before because the sheer amount of false factless based information being branded about this winter makes for a difficult read and gives a very poor foundation for learning. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985691

[bluearmy]

4 minutes ago, Hotspur62 said:

Just a quick one hopefully someone can answer.If models are showing a possible SSW around 4-7 January isn’t there a time lag of about 10-14 days before we feel effects of this if we are lucky enough to feel the effects coldwise(about 60% chance?).Would that mean we would be looking at 15th January before any chance of cold weather?Remember something of 2018 BFTE but can’t exactly remember all details.Do we still have a chance of colder weather before then with the teleconnections etc?I am more confused than ever now after 12 years of being on this forum as this season seems to be so many different views on things I am confused.com!

The models have trended away from a ssw in the technical sense where it reverses at 60N 

however, they are still reversing the flow high up at latitudes above 70N - it’s a sig warming but not an actual ssw 

so we can expect a generally neg AO signature for some time to come because that reversal wave will continue to drift down into the trop - usually there are three or four repeats with 14/17 days in between (approx) 

your question about lag times is relevant - usually we would expect a wave to downwell 10/14 days later after the reversal first appears at the top of the strat. But on this occasion we already have a neg AO  in the trop so any downwelling wave would simply reinforce  that.
There looks to be a rapid downwelling likely in this instance so no wait and our issue is getting that wave far enough south once it arrives in the trop. Hence the general consensus that 60N is important for the reversal to get down to. 

We do have a potential issue in some  modelling where the reversal wave will push down the residual positive flow ahead of it into the trop and you’ll see a temporary increase in zonal flow at 500 hpa. I’m afraid we just have to wait a while to see how that will actually play out 

if you run through this view of the gfs op then you will see how and where the blues (neg zonal flow) are currently in the atmosphere and how they are expected to evolve. Remember that the lower down you go the less reliable the forecast becomes. In theory, higher up in the strat is supposed to be less chaotic - suspect that isn’t the case over the next couple weeks! 

Vertical cross u-mean + EPflux

WWW.WEATHERISCOOL.COM

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985720

[Met4Cast]

Strongest signal yet from the EPS for cold weather beyond week 1 of January.

Again, following det runs when the spread is this vast is a pointless exercise, 06z GFS det already on the warmer end of the pack by the 2nd Janaury. No worries or concerns from me at this stage.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985825

[IDO]

As expected, the ECM op at D10 is moot as the op run is an outlier early on in the run via at least two metrics:

One for the bin highlighting the uncertainty with taking op runs seriously beyond D7-8.

The De Bilt shows that the mini-wave in the Atlantic on the D10 mean will have more significant colder benefits to the east of the UK:

 

Let us see where the 12z takes us.

 

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985714

[johnholmes]

Thursday 28 December 2023

Just over a week since I did an update on the anomaly charts. Nothing much changed for a few days but over the last 3 NOAA has changed now showing a fairly marked 500 mb ridge with a small +ve height anomaly . The anomaly first showed sw of the UK and has moved wnw over the past 72 hours. It has also created a very slight trough east of the UK. On the 8-14 chart the height anomaly is now shown at 150DM, a reasonable value with the contour ridging directed towards Iceland and Greenland. West of this is the usual trough over ne Canada with a ridge west of this north of Alaska. Not a classic set up for the UK/European areas but better than no signal for any meridional flow in this area.

Turning to the ECMWF for a similar time scale to the 6-10 NOAA. It keeps the 500 flow in a general WNW flow with minor ripples running through it. Over Europe it does show  a fairly flattish trough in day 1-2 but sharpens it somewhat by day 5 (7 Jan). The 850 MB temperature shows little sign of any colder air for the UK with the far north almost in the -5 C at times with most of the rest of the UK in the zero to -2 to -3C region.

So a watching game to see if it will follow the NOAA and, indeed, if the NOAA charts intensify the ridge-trough set up for the UK and Europe region.

Climate Prediction Center - 8-14 Day 500mb Outlook

WWW.CPC.NCEP.NOAA.GOV

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985886

[Met4Cast]

We currently have the GEFS which does have a signal for cold but with a huge amount of spread it's very difficult to guage much of anything from the model at the moment. The det runs are all over the place and not really worth analysing. 

The EPS is far more convincing with the biggest cluster well below the mean into week 2 of January. The colder solutions broadly fit with the expected development and current state of play re: GSDM/MJO. A reversal to -ve AAM anomalies near the equator should help to sufficiently weaken Iberian heights so I don't fully buy into runs producing stronger Iberian heights at the moment. Poleward propagating +AAM anomalies support increasing HLB and a southerly tracking jet. 

NWP modelling is evidently struggling at the moment.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4985857

[Catacol]

It is time to take a step back and wait for model chaos to disperse. I have shown twice in the last two days how modelling gets to the 6/7 day mark and then doesn’t have a scooby… and I think until we see what happens post Hogmanay storm (that is certainly coming) the rest is like a basket of odd socks.

I’ll trust the drivers as I try to do all the time because they tend to deliver more often than not - though once again it is worth acknowledging we play a probability game when trying to forecast the future and nothing is ever certain. And those drivers, semi predictable though they are, are also apt to change.

I think many of the big guns out there are waiting to see what happens with the next 2 or 3 rounds of strat modelling. Quiet has descended and the nature of the strat profile is going to have a huge say in how the majority of the rest of winter unfolds. In the near term we are continuing to see a signal for height rises as we move beyond New Year but timing is unclear and the extent to which we get a block rather than a wedge - and how the Atlantic interacts with this is still up in the air. But a zonal profile for January this is not despite the claims of some earlier today.

On the positive side of things trawling for opinion is a useful exercise. Cohen last night has come out and thrown a tentative hat into the ring for a split vortex SSW despite the uncertainty in the modelling. Webb has rather distanced himself for now from the SSW discussion but gone for persistent -NAO through Jan and Feb no matter the outcome of strat developments. And he has been quite bullish on this trajectory for a while. And the third interesting bit of browsing is this dataset. This is the shape of Jan and Feb in years where a displacement SSW occurred when ENSO was an El Niño,

It makes for good viewing and matches the direction of travel currently of EPS anomalies.

A hell of a lot changes in two weeks of weather watching. I have no doubt that, once we get to Thursday 11 Jan we will be looking at a very different picture and my money is on a blocked and cold pattern, either in place or soon to be in place. Enjoy the ride…

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986053

[Cold Winter Night]

About that SSW that "disappeared"/"dropped"/"was taken from us,"

Here's EC46 mean zonal wind at 10 hPa from one week ago, December 21:

Slowest ensemble mean: +/- 5 m/s, about January 6.
- Around January 3, all members are below 30m/s, the strongest reversal outliers hit -30 m/s.
- Late January, the ensemble mean returns to about 15 m/s.

Now the latest one, from yesterday:

Slowest ensemble mean: +/- 5 m/s, about January 6. Yes, that's the same values as a week ago.

What has changed is the spread that has reduced (logically at that closer date).
- Around January 4, all members below 15 m/s, the strongest reversal outliers hit -25 m/s.
- Again, late January, the ensemble mean returns to about 15 m/s.

The output from December 20, 22, 23 and 24 is very much along the same lines.
Then how about the impression that "the SSW" is now gone?

Well, that's especially the 26th, the only day EC46 showed a reversal on the ensemble mean.

In other words, EC46 has consistently not showed a mean reversal for more than a week, with one exception. The one off that should be rejected for that reason.
Not much has changed with regards to a potential SSW.

And that's ignoring the fact that a considerable number of members still go on to reverse at some point in January, and also ignoring the fact that an SSW does not equal a cold spell, nor that the absence of a SSW does not equal the impossibility of a cold spell.

The current blocking signal in the extended is a trop led feature, let's watch that, and keep in mind that the stratospheric slowdown can still very much have its effect lower down.

No need for nervous breakdowns at this point.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986055

[Catacol]

3 hours ago, MJB said:

Which was my poorly made point, we are looking at the poor mans viewing, they have much more to look at . They certainly won't make a knee jerk reaction. If the message is still the same at the weekend, then we are , I think in business.

 

I have to say and I did ask this a few weeks ago, just what is the formula ( combination ) that is needed for the UK to have a Winter?

Just how many background signals have to come together ? 

Hehe - I like that question. Headscratcher.

In a nutshell, as we know, we need a block over Greenland, Iceland, Scandinavia or a combination of these. How do we get it?

The answer perhaps is split into two: either upstream forcings encourage the ridging of the semi permanent sub tropical high (Azores) with the jet dropping in underneath to sustain it or we get a top down forcing caused by vortex disintegration and a resultant -AO that also allows for a block to drop into place. It’s worth remembering - and we see ir clearly in summer when there is no polar vortex - that high pressure does not naturally evacuate these regions. It is forced out by the pressure of the vortex and then the strength/latitude of the jet.

So we look for two developments.

1. Upstream amplification of the jet caused by an injection of momentum. Think of a pole vault. The pole is straight until significant force is applied and this causes it to compress and bend. Same with the jet when momentum is injected - and this creates a wavy long wave pattern. Combine this with a Rossby pattern driven by the MJO in phases 7/8/1 and we tend to get the bend in the right place to drop the jet southwards in our North Atlantic sector and and eddy to form to the north = a block. Once in place, even after a reduction of momentum occurs (because it doesn’t stay enhanced for long under normal circumstances) and the block can be sustained via retrogression and ongoing jet angle. Nino conditions enhance momentum spikes which is why Nino is generally better than Nina for midwinter blocking.

2. Disruption to the vortex. If the vortex is weakened it follows automatically that the conditions for blocking to return to higher latitudes, as occurs in summer, improve. And if the stratospheric vortex is shifted away from the Atlantic sector then so much the better. What weakens the vortex? Warmth. Warm air fired into it slows it down, acting like a spanner in the spokes of a wheel. The dynamics are complex, but warm air bounced off Rockies or Urals plays a part as does ridging driven by tropospheric factors described in 1. above. There is therefore a clear link between tropospheric drivers that promote ridging and stratospheric impacts down the line. This is why many of us are not unhappy to see a Euro high in November if it encourages a trough over Scandy and ridge over the Urals downstream, and why I have personally been celebrating the Nino-driven North Pacific trough. American cold lovers are hating it right now, but it has allowed some significant poleward heat off the Rockies over time.

Right now all is good. We have a bending jet courtesy of high momentum, promoting northerly ridging in our sector. We have a distressed vortex, SSW or not (though SSW would be better.) We have the MJO in the right phases. There are other factors not discussed here, the most important perhaps being the QBO. It is an umbrella feature and it is in the right easterly phase also to encourage an SSW.

Sit back and wait. NWP will come into line. A cold spell in January will happen. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986134

[Cambrian]

Looking at the broader picture, it’s fair to say we’re seeing a decisive pattern change in the nearer range than can currently be as decisively developed in the longer range. 

It’s fascinating how these pattern changes appear a long way off, then seem to so often get submerged only to re-emerge in around the day 7 mark.

This change manifests as a classic jet stream split off the eastern seaboard between day 6 and day 8. 

12z GEM op

  

12z GFS op

  

Weakened by the split, the northern arm undulates north and then south in tandem with the amplification of the Atlantic ridge. The southern arm remains flat and diverges even further south. 

From there, it’s a weaker but still significant northern arm capable of downstream trough development over Europe and a more dominant southern arm ultimately pointed at North Africa. At some point, some very cold air will spill south into the trough. 

The 12z ECM mean brings the cold a good way south around the base of the Scandinavian trough, with a slack northerly flow down into Western Europe. 

  

Where exactly the ridge and trough set up will determine will where and how cold the conditions will get over the different parts of Northern Europe, but with the consistently modelled build in heights well north in the Atlantic by day 8, continuing to build through day 10, it’s clearly looking to get generally much colder at some point, at the T850 level and / or (especially) at the surface, and when it does, there appears to be very little upstream in the Atlantic to shift what might very well become an extensive cold pool.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986434

[Derecho]

Well the latest EC IMO is of interest but those critical of it do have a valid point. Unless heights drop over Iberia then you wouldn't get deep cold over the UK. Clearly in the following frames there would be cold air pushing south on the east side of the Greenland high but if the Iberian high were to remain over us, that would block the path of any cold air going towards the UK.

The output following T240 could go either way but the high pressure over us stretching from Iberia would need link up with the Greenland high on the back of that Icelandic low moving east. Not a bad chart but those who say it is a cold bomb waiting to go off need to relax and calm down a little.

In the closer time frame its good to see the cold building back NE after those terrible synoptics over the Christmas period.

Just need to pump up that mild air in the tropical Atlantic towards Greenland to really tip the scales. Large swathes of Canada as you can see are already much milder then average but the cold air from Greenland is helping ramp up the Atlantic.

Back before Christmas I mentioned that the second week of January or slightly before is when things should start to change if the MJO decides to exert an influence on things. That assumption still looks good at this stage.

The MJO continues to go in a stronger phase 1 then expected. So a good chance of a strong phase 2 as well. The BOM has been getting this hilariously wrong, I wouldn't rely on it...

Overall I'd give the EC a 6 out of 10. Not particularly exciting but something COULD happen from it which is better then any of the output from this largely awful December we have endured.

The EC ensemble mean these days I think gives a more insightful answer also so lets see what that says.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986399

[bluearmy]

52 minutes ago, Mucka said:

The whole ensemble suite and mean is night and day compared to last night

Definitely not to be sniffed at as a 10 day mean, last night any Atlantic blocking chances were a write off

Todays 12z mean anomoly is just a firmed up version of yesterdays ? 
Yesterday 7/14                                          Today 6/13

.  

44 minutes ago, feb1991blizzard said:

eps solid on Atlantic ridge again.

poor in the strat though, vortex gradually moving slap bang over the pole.

It’s not over the pole at any point ?  generally svaalbard/barents and then stretches across to central Greenland before returning back to svaalbard/barents. Not great considering what we were looking at a few days ago but could be worse and the mean at day 15 just begins to pick up a sniff of the warming over Asia that has begun to show on ncep 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986467

[Met4Cast]

With the +FT coming to an end and reversing to -ve we can probably expect a similar route forward for +MT and as such, AAM tendency is now falling again. These events have done their job though by increasing global westerly momentum and producing a strongly +ve AAM. 

These poleward propagating +AAM anomalies are helping to weaken the Iberian high and allow for a more south shifted jet stream, into January MJO phase 1 combined with these rising +AAM anomalies should begin to lead to blocking and indeed, we're now seeing that being modelled within the NWP output suites, here's the 00z mean for example at day 10;

Details as ever remain very elusive with NWP output struggling to find a clear route forward but the broader expected picture is beginning to be advertised now. 8th - 18th January remains the key period for me in terms of potentially blocked/cold patterns developing. 

Going forward the MJO needs monitoring, a more progressive transition east (perhaps helped along by a minor SSW) should allow +FT and +MT to return with the AAM already starting from a higher, likely Nino-esq base state we could see renewed blocking attempts later in January > early February, indeed a couple of EC46 runs have hinted at this possibility too. 

All in all - Whilst details are yet to be ironed out, the broader picture remains conclusive to blocking, cold weather & the potential for high impact winter weather. For me, the GEM & EC are leading the way in terms of expected pattern, our friend GFS will probably get there eventually.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986701

[Catacol]

6/7 Jan is where the fun begins, with a significant atlantic wave break creating the change. This is an MJO/GWO/+EAMT product.

See ECM last night for the 7th:

GEM lunchtime today for the 6th:

GFS lunchtime today also for the 7th:

 

Any further analysis at this stage is a waste of time. A wave break like this is not easily modelled, and how the jet interacts with it to the south, and how the break aligns, will be crucial to any low pressure undercutting through Scandy or indeed from the atlantic. ECM and GEM offer very tasty, if different, mid term solutions to this at 240h, GFS doesn't get the break quite as high. Given both ECM and GEM are outperforming GFS in 120h stats at the moment I'd go with then over the GFS....but time will tell. 

The next few days of model watching are going to be some of the best of the season so far.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987040

[Ruzzi]

17 minutes ago, Harsh Climate said:

It's already happening and in full flow by Sunday!

We are seeing an almost immediate impact due to the Canadian Warming and a few various other factors this is why we may not need a full SSW for Cold/Snow Nirvana Mid January, we might just get away with it but we are relying on pieces of the jigsaw coming together in just the right places.

That chart you're showing is the warming taking place higher up in the strat .... and when you look at the mean windspeed up top for today/tomorrow, they are at the peak. 

The reduction in the zonal winds doesn't bottom out until the 6th of January, again bearing in mind this is up top. There's no way that the great reduction of zonal winds can imprint its effects on the trop on Sunday when it's still 1 week prior to the reduction in windspeed taking place. Following on from that, even when the windspeed reduction takes place up top, there is going to be a respective response time, with even a QTR taking some time to filter down to the trop. You can't have an instantaneous response, it just isn't possible. There needs to be a degree of time for the effects to filter down to the trop. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987631

[Mike Poole]

11 minutes ago, Ruzzi said:

That chart you're showing is the warming taking place higher up in the strat .... and when you look at the mean windspeed up top for today/tomorrow, they are at the peak. 

The reduction in the zonal winds doesn't bottom out until the 6th of January, again bearing in mind this is up top. There's no way that the great reduction of zonal winds can imprint its effects on the trop on Sunday when it's still 1 week prior to the reduction in windspeed taking place. Following on from that, even when the windspeed reduction takes place up top, there is going to be a respective response time, with even a QTR taking some time to filter down to the trop. You can't have an instantaneous response, it just isn't possible. There needs to be a degree of time for the effects to filter down to the trop. 

It is not just about the zonal mean zonal winds number - that is averaged round the globe.  it is about where the winds are, particularly in terms of their effect on trop patterns that are of relevance to our small island.  Bear in mind also, that when the split was in the modelling, it was right through the layers of the atmosphere - and bear in mind also that winds lower down were weaker already from the Canadian warming.

It is a complicated picture, and the point I was making is that to discount effects higher up in the current pattern evolution is an oversimplification.  

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987646

[mulzy]

Just looking at the Operational runs, there is a significant difference by day 6.

ECM, UKMO, and ICON are the most similar - although there are differences between them. UKMO doesn't have the pool of low heights over the Baltics whereas the ECM and ICON do.  The Atlantic profile of the UMO and ECM is very similar.

GEM is very different - the low in the Atlantic is close to phasing with the low close to the British Isles.  In subsequent timeframes, it does phase - I don't think we want that to happen.

The GFS has this spoiler low to the west of the southern tip of Greenland - no other model has this.

Still a lot to resolve but good to see the ECM and UMKO-GM broadly similar at day 6.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987612

[Mike Poole]

Just now, Met4Cast said:

But that’s irrelevant to whether or not we’re seeing a reversal.. 

No it’s not.  The pieces have moved differently to how they were modelled a few days ago, and that is why we are not getting a reversal!

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987625