[Hunt For Cold: Model Discussion - Heading Into Christmas]

EC op is notably more Wave 2 days 8-10 at 10hPa. GFS parallel some days behind on this. 

[Hunt For Cold: Model Discussion - Heading Into Christmas]

Tropical modelling suggests a coherent MJO wave to enter the Maritime Continent during the coming week.

Week 2 forecasts indicate a widespread -ve velocity potential across the central and eastern Pacific, where SSTAs are well above normal and likely to favour enhanced westerlies (weaker Trades). There have been several westerly wind bursts this autumn and early part of the winter and I would expect this to be another one in evolution.

So although the MJO signal is relatively weak, this will transpose onto a an El Nino low frequency base state. Total and relative angular momentum will as a result increase. There's no indication of deposition of westerlies via mountain torques over Asia, so a steady laddering up of angular momentum looks most likely. 

That's a signal for phase 6 evolution in the first week of January.

We should then begin to see westerlies scrubbed from the atmosphere over time returning the GWO towards phases 7 and 8 during week 3 (2nd week January, possibly with the MJO in phase 8).

That could well coincide with the first lagged impacts of the stratospheric warming. With the net ENSO, GWO and MJO signals, I'd suggest mean trough solution across Scandinavia and into Europe with a Greenland type high is the most likely evolution. What do you say CFS ?

[Hunt For Cold: Model Discussion - Heading Into Christmas]

9 minutes ago, bluearmy said:

So it’s a good thing according to that sample of data ....... approx 75% below normal temps for mid feb to end March according to that  

I read that as below average probability signal days 12-42 post warming date. So earl/mid Jan through early Feb ?

[Hunt For Cold: Model Discussion - Heading Into Christmas]

Just a reminder. Stratospheric time scales are extremely slow. Modelled effects of the warming not likely to show until 7th January at the earliest. That's still a few days yet before it gets into GFS range. 

[Hunt For Cold: Model Discussion - Heading Into Christmas]

15 minutes ago, IDO said:

All this will seemingly do (till we get the mid-Jan SSW input) is enforce that UK Block. As I have been thinking, the SSW is going to be make or break for UK cold as the current repeating pattern remains bad for the UK cold picture.

Some conflicting views expressed here on the desirability of high pressure centred UK immediately after a strat warming..

https://twitter.com/WorldClimateSvc/status/1075165825290723333

[Hunt For Cold: Model Discussion - Heading Into Christmas]

10 minutes ago, snowblizzard said:

Unfortunately, it looks to me that we are slipping headlong into a very common and stubborn pattern of Atlantic dominated weather.

Seriously ? Have you looked at the mean height anomalies for the next 5-15 days ? Total opposite of what you're suggesting. 

1 hour ago, booferking said:

MJO looks to be heading for circle of death in last updates would that be right?

Wouldn't read too much into these plots as they don't show the whole picture. Strong region of suppression developing across the Indian Ocean and broadscale favourable environment for westerlies (weakened easterlies) across the central and eastern Pacific. Net increase in angular momentum incoming and atmosphere taking on a very El Nino look to it.

[Stratosphere and Polar Vortex Watch]

EPS lending support, split signature day 11

[Hunt For Cold: Model Discussion - Heading Into Christmas]

Decent signal for a strong mid latitude ridge signal across Christmas week in the EPS and GEFS for NW Europe. This looks plausible on tropical and extratropical signals right now.

Thereafter, EC locks in a -AO out until February and drives the NAO negative mid January onwards. I would be expect movement of mid latitude ridges north and north-westwards over time as the impacts of the stratospheric warming take effect.

[Stratosphere and Polar Vortex Watch]

Struggling to find comparable displacement episodes of this magnitude based off GEFS and EPS..

This one possibly:

[Stratosphere and Polar Vortex Watch]

EPS is top down, but for indicative purposes, looks very similar at 10 hPa to gfs 06z wrt placement of ridge and residual parts of SPV.

[Stratosphere and Polar Vortex Watch]

Suspect Mike Ventrice will be tweeting the eps mean at t360 soon. Very similar to gfs 18z and 06z. Strong mid winter warming looking very likely.

[Meto Uk Further Outlook]

Would concur with this, although NE vector may be more appropriate early Jan.

[Stratosphere and Polar Vortex Watch]

So timing looks like this:

Stratospheric (displacement) warming 27-29th December. First tropospheric response would on that basis be around 10th January, peaking on first wave response 15th-25th January. Second wave early to mid Feb, potentially very interesting if split vortex occurs.

Bear in mind, this doesn't take into account troposheric forcing. MJO cycles suggest mid latitude ridges early January, but GWO lagged impact of westerly inertia being deposited as of 9th December suggests more -NAO variant to this. Either way here, January and early February hold the greatest cold weather potential for this winter (and has been the case for some time).

 

[Stratosphere and Polar Vortex Watch]

Ticking along nicely. Big convective anomaly traversing through the Indian Ocean with clear extratropical impacts.

Best analogues early December 1965 and early January 2004 for a strong displacement type warming,

Day 1: 10 hPa

By day 26 indication of split vortex (and therefore a sustained event):

Tropospheric impacts start day 12 on these analogues (not allowing for fast propagation waves), running through day 30. Day 16 here for reference:

If there were a split vortex, trop impacts would continue well beyond this.

All of this occurring when tropospheric drivers signalling low pressure in the Atlantic and pressure rise over Greenland for January. The key issues will be: getting cold air embedded over Europe during the initial stages, and not to much high pressure over North Africa and Iberia and sufficient upper level flow through central Europe.

Interestingly, 1984/5 continues to resonate here. MJO evolution that year suggests another opportunity for a strat event during February.

[The hunt for cold - Model discussion (late November)]

12z GEFS 10 hPa mean is a heartbeat away from a technical strat warming event.....(closer than 00z)

[The hunt for cold - Model discussion (late November)]

1 hour ago, Nick F said:

I see, the VP200 does indeed suggest increasing -ve anomaly depth propagating east, perhaps a good reason not to rely on the RMM plots alone. So do you think this more coherent signal of MJO propagation through eastern hemisphere may lead to a temporary relaxation of the block to the NE and resurgence in the Atlantic zonal flow towards the UK? Or do think we can maintain blocking with energy going underneath?

Actually thinking here that there will be two, possibly discrete episodes of ridging to our NE depending on the extent of Atlantic incursion on the back of this. The first being reasonably modelled but probably too progressive in its break down / eastward displacement and a second phase around the 21st - 23rd December. 

[The hunt for cold - Model discussion (late November)]

17 minutes ago, Nick F said:

The antecedents for the formation of next week’s Scandinavian high certainly contributed by in a large part by extra-tropical forcing of a coherent MJO wave moving through the tropical East Pacific and Atlantic towards Africa (phases 8-1). Yes, the MJO wave continues through the ‘warmer’ phases into the eastern hemisphere on the RMM plots, but the signal looks increasingly muted. Also AAM looking suppressed after its recent positive spike, which suggests westerly momentum will decline upstream for a time from its recent El Nino-esque state – with associated jet retraction over the Pacific, which may allow a meridional flow downstream over N America and into the N Atlantic - with the blocking signal maintained to our NE through mid-December.

Tend to disagree with this statement Nick.

Mike Ventrice's plot shows increasing depth VP centred east Indian Ocean / Maritime Continent week 2:

These tend to under represent anomalies in the week 2 range, so an increase in depth of the -ve VP anomaly is significant (and this increased from yesterday's plot as well). So whilst the RMM values might be weak, the angular momentum tendency signal will be increasing once more. The eastward propagation of this will lead to an increase in total AAM as well weeks 3 -4 through the Pacific.

So whilst the +AAM signal through 30N will decay, new inertia off the tropics will hold up the global AAM budget to elevated Nino levels. The real fun and games start when that signal begins to decay, and should coincide with the MJO in phase 8.

[Stratosphere and Polar Vortex Watch]

Worth noting that the recovery of the upper vortex is not necessarily a bad thing from a downwelling -ve zonal wind anomaly.

I've had a lot of success this year with 1984 polar analogue. A lot of similarities since that late warming / early final warming in February.

We are currently at the end of downwelling signal for the autumn in the troposphere, AO set to bounce around as per late November to mid December 1984.

What followed in 1985:

There was an entrenched weak -ENSO signal that year and east QBO, so need to factor this in (although could argue that weak +ENSO signal and solar are more conducive to rapid tropopsheric response during the second half of the winter).

Note the +ve zonal wind anomaly / colder upper stratospheric temperatures following the (downward propagating) warming late 1994 / early 1985.

Strat warming looking like 26th December IMO.

[The hunt for cold - Model discussion (late November)]

Whilst the gist of the extended GEFS and EPS looks credible (+ve height anomalies centred Hudson Bay), there's any awful lot of white space across the rest of the Northern Hemisphere. Suspect that modelling remains too progressive w/r/t Scandinavian ridge demise, and, possibility that we will retain some weakish +ve heights to our north in the run up to Christmas.

My punt of technical strat warming date would be 26th December.

[The hunt for cold - Model discussion (late November)]

43 minutes ago, coldwinter said:

12z GFS following what GP said earlier, perhaps a little too soon as he said, but it’s definitely cottoned on

Yes, this would be bang on with expectation.

So a less cold (T850) phase after the northerly then easterly next week

[The hunt for cold - Model discussion (late November)]

Although relative angular momentum is very high right now, that is partly a reflection of a persistent +AAM signal in both hemispheres. The greatest +AMM signal in the Northern Hemisphere is that westerly inertia signal deposited 30N. That is essentially past peak and likely to decay over time. 

However, with the tropical VP signal migrating a tropical wave eastward during week 2:

We're likely to see jet extension and +AAM signal renewed week 2. That takes GWO signal back to phase 5 (Scandinavian ridge signal).

Because of the slow speed of this tropical wave, suspect that the GWO signal will remain more in phase 5 than would normally be expected. Hence ridge to our NE likely to be more durable and models likely to be too progressive with this feature.

Looking beyond this, the GWO suggests a phase 6-7 evolution on the cards as the decaying +AAM signal and maturing momentum signal from the tropical wave take effect.

Ridge signal to shift further west and NW. Disregard any model output that does not follow these broad lines.

The final third of December into early January is stacked with cold potential. Think a decaying +AAM signal and MJO returning back to the Indian Ocean. High chance that the GWO will be heading into high amplitude phases 7 and 8 and overlaid by a collapsing polar vortex and very weak polar westerly flow.

This I think is where we are heading into January.

[The hunt for cold - Model discussion (late November)]

EC 45 dayer shows marked drop in AO forecast second week of January to -2SD (mean), commencing in trend 26th December. Wonder what's driving that then ? Consistency with GLOSEA on stratospheric forecast.

NAO forecasts not as negative, but worth pointing out that a Scandinavian block is not well represented within that index. FWIW, thinking would be for the NAO to move in concert with the AO here as pressure will likely be anomalously low in the Atlantic.

[The hunt for cold - Model discussion (late November)]

On 27/11/2018 at 17:57, Glacier Point said:

..and the value of this approach ?

Q: how likely is GFS P t300 ?

A: based on reasoned assumptions, likely.

So, how are we doing here ?

The tropical wave evolution identified has been slower and that shifts any comparative analysis back by 2-3 days. Suspicion that it may continue to be slower than forecasts currently advertise. That places GWO in a likely tilt towards phase 5 around 12th December

For 12th December, GEFS vs original vs GWO composite phase 5:

  

So a continued indication of anomalous ridge development to our NE for this time scale. Regular readers will understand the value of this approach, particularly in view of the volatile model output in the last few days and particularly back end of last week.

Looking ahead, the tropical wave development has week 2 and possibly into week 3 as most favoured for the GWO to be in phases 5 and 6.

For high amplitude phase 6:

That suggests we're looking around a week's to five days worth of blocking ridge to our NE mid month, trending towards ridge transferring to our west and NW in the lead up to Christmas, troughing into Scandinavia. That I think gives us a first proper opportunity to pull some genuinely cold air into Europe, although the for Western Europe I suggest that this continues to mark a skirmishing phase of the winter which would be typical of similar ENSO events.

The real interest lies in the final week of December into January given the stratospheric vortex will be under intense pressure and may well have collapsed. Synthesising the GWO attracted to a phase 6-7 evolution suggests a proper episode of Greenland blocking (and more identifiably from the composites distinctive -ve height anomaly in the North Atlantic) and impacts of the strat warming being overlaid with that pattern.

 

[Stratosphere and Polar Vortex Watch]

Displacement forecast for mid December, but expectation would be for full on warming event to occur towards the end of the month. MJO wave passage through the Indian Ocean will likely delivery the knock out punch.

 

[Stratosphere and Polar Vortex Watch]

FWIW, UK Met model showing a full on warming event for end of the year, as is CFS (which has continued to do so for the last few weeks).

That's not wholly surprising given long lead thoughts of stratospheric behaviour and this plot which, if verified, would be signalling a stratospheric warming within 9 days: