Model Output Discussion 18th Nov.2013-12z onwards.

[R.P. McMurphy]

But with respect you are using GFS charts to back up data supplied by ECM and those temperature profile map readings are as useless as a chocolate fireguard at that range but hey ho everyone to their own.

Gibby,

 

I have to go off some form of data, or no argument exists.

 

The fact remains the charts show cold all the way, no sign of mild anywhere....

 

P.S - I believe you also supplied charts well into FI, which are unreliable, but hey ho... Each to their own

 

P.P.S: From Ian Fergie, whom I would say is more use than a chocolate fireguard!

 

an Fergusson â€@fergieweather13m

Colder spell expected later next week may last somewhat longer than initially suspected but longevity & influence to S UK very uncertain.

 

looks like met are more confident now of a more prolonged cold-spell,for most of the country

Edited November 27, 2013 by Glenn W

[SMU]

With regards to Steve's post, I don't agree that it is poor form to go for a return to mild after any initial Northerly, unless we are looking at a Dec 81 scenario, which we are not,  Gibby and whoever else are going with by far the most likely solution, particularly as we see nothing to suggest HLB in the NWP.

 

Well Ian if we were sitting here & it was Nov 2009 then I would 100% agree, after all there would be no basis to assume anything different

 

However You - & possibly Gibby need to think about the most recent weather over the last 4 years & factor in these 2 thoughts...

 

1) -AO index.

Since 1950 there have been 8 months in total that have averaged out sub -3 on the AO index they are-

Jan 63

Jan 66

Jan 77

Feb 69

Feb 78

Dec 2009

Feb 2010

March 2013

 

Of SIGNIFICANT note is that 3 of them in the last 4 years -

 

& that since 2009 the Winter anomaly has been this

 

 

Even more significant that is the 4 year December Anomaly...

 

 

& for comparison the 1995 & 81 anomaly for reference...

 

 

As you can see the match is VERY good for Greenland-, so as you can see- or maybe you both choose to ignore - either or, forecasting the Form horse isn't as straight forward as you & Gibby make out...-

 

You just choose the mild perspective, where as others like to explore the cold perspective.

 

S

Edited November 27, 2013 by Steve Murr

[Summer Sun]

 

an Fergusson â€@fergieweather13m

Colder spell expected later next week may last somewhat longer than initially suspected but longevity & influence to S UK very uncertain.

 

looks like met are more confident now of a more prolonged cold-spell,for most of the country

 

 

There longer term update yesterday and again today say temperatures would be below normal from later next week through to Christmas so Its not a big surprise to see that tweet.

[terrier]

Can I just clarify the gfs run was a cold outliner not supported by its ensembles so until we get a clear signal on this I will remain sceptical and back gibby on what he is saying we have seen this many times before that as we get nearer the event it all gets shunted further east so I remain cautious at this stage

[Weather Boy]

The GEFS 06z mean is looking very good for a cold, windy, unsettled blast from later next week into the following week with progressively colder arctic air digging south, so frosts become widespread and sharp and showers turn more and more to hail, sleet and snow with significant accumulations across hills and in the north, showers merging into longer spells of wintry precipitation at times. ..I expect further upgrades as the clock ticks down into the reliable timeframe.

 

Why, please, Frosty?

[snowking]

Well before this turns any more into a mild vs cold battle royal (snow shovels vs beach towels) perhaps we should take a look at the evidence in front of us.

 

Det. Runs @ T + 240

 

 

cmanh-0-240.png

gemnh-0-234.png

(Sorry - it would only let me post a certain number of images!)

 

So fairly useful agreement between deterministic runs of some kind of mean Northerly flow by day 10.

 

We can also see this from our ensemble suites:

 

 

You will also notice some support stratospherically for the trough to drop close to the UK from the ECMWF:

 

 

And from the GFS:

 

 

Note though the subtle differences over the Atlantic with far more tendency towards ridging on the GFS.

 

What is important to note though from a stratospheric viewpoint is the tendency for the vortex to move across the pole from the Siberian sector towards Canada. Unfortunately at the moment this path also takes in the Greenland locale, and this tends to scupper chances for any extended periods of Greenland blocking. We go from this at day 3:

 

 

To the day 10 chart shown previously.

 

 

 

With a pretty significant portion of the vortex shown to be close to Greenland, as indicated by the vorticity charts too:

 

 

Now of course there has been some discussion of a disconnect between the troposphere and stratosphere in recent weeks, perhaps a little justifiably, and so if we continued to see the same disconnect between stratospheric and tropospheric modelling we could potentially look at things a little more optimistically, however after a quick consultation with the extended ensemble means:

 

NAEFS

 

From this:

 

 

ECMWF

 

From this:

 

 

To this:

 

 

Whilst the transition occurs a little later we still see the transfer of the main energy of the vortex from Siberia to Canada, and once again part of the journey takes it across Greenland.

 

So in terms of what we can look at factually for now yes there is some good deterministic support for a cold spell once again next weekend, with all the usual potential for wintry showers/snowfall/snowmageddon headlines in the media/tantrums on here and somebody in Inverness ends up with 2cms more snow than somebody in Plymouth, but in terms of expecting something more extensive to develop? Sorry I am a big fan of cold weather in winter (along with warm weather in summer, just call me Frosty jr.) but even looking through optimistic eyes at present I cannot see anything too extended in terms of cold occurring if we are to take the models at face value.

 

Now of course there is the argument for second-guessing the models. This is a tricky debate because on the one hand I fully agree with such a sentiment. But on the flip side of the coin you are then second-guessing a process which is designed to make an educated guess at the weather - essentially we are guessing a guess.

 

It is clear from previous years that the GFS does indeed struggle with it's modelling of anything blocked or split across the Atlantic. However if we are to take a look at the biggest winter-time model frailty of recent years for me it has to be the tendency for all NWP to over-estimate the extent of any height rises around the Greenland area. We saw this last year (both tropospherically and stratospherically) despite the cold conditions when forecast models were continually suggesting that we would see something more substantial developing close to Greenland both early and late in the season (as it was we ended up with a rather decent steve murr sausage - available from all good stores) and we have seen this occur once again in recent weeks.

 

So with all of this taken in to consideration then yes there is the potential for a cold snap towards the end of this week and this is, as BA has been saying, something which should not be of surprise to anyone who has been following the ensemble suites in the extended range in the past 5 days or so. However it has always been suggested that such a spell would once again prove to be a short spell rather than something more extensive and that still looks to be the case as we draw slowly closer to the period (still lots of time for ups and downs between now and then of course). The only hope would be that the likely high pressure area that would then return to the UK following the colder spell ends up a little further North-East as the vortex energy starts to drain away from Scandinavia/Siberia.

 

I think the mild vs cold thing has always been a part of this forum and for me personally always adds to the drama, but it has mostly been not only in good jest but also usually with evidence to back it up. What there seems to have been in recent days is more of a case of accusations being thrown backwards and forwards with very little evidence to back up any arguments (this is of course not true of everyone here with some fantastic posts amongst the bickering). But for me it seems simple, if you have a point to make back it up with evidence. This is how I have been fortunate enough to learn the amount I know about meteorology over the last 15 years, by viewing forums (all the way back to the good old BBC slow-watch days) and reading through the excellent posts made by members that still exist on here even today, and long may such learning methods continue to be applied. I am sure there are some newbies lurking around here who would wish to have the same experience and until we can get back to providing such a posting format that is going to be difficult to achieve.

 

Kind regards to all,

SK

Edited November 27, 2013 by snowking

[Summer Sun]

Can I just clarify the gfs run was a cold outliner not supported by its ensembles so until we get a clear signal on this I will remain sceptical and back gibby on what he is saying we have seen this many times before that as we get nearer the event it all gets shunted further east so I remain cautious at this stage

 

The ensembles do support a cooler spell later next week for a time

 

[phil nw.]

It looks to me that the driving force for the cold spell early December is the vortex displacement towards our side of the pole created by the Alaskan ridge.

That very cold pool to our north will be driven this way by the modeled mean upper trough digging down through Scandianvia as the jet axis turns nw-se across nw Europe and the UK.

http://176.31.229.228/modeles/gens/naefs/run/naefsnh-0-0-240.png?12

http://www.meteociel.fr/modeles/ecmwf/runs/2013112700/EDH101-240.GIF?27-12

 

No High latitude block but In this case only a modest amplified Atlantic high will do the job in getting the cold air south because of the angle of that trough.

 

The ens graph still showing that fall in temps around the 4th Dec

 

certainly various ens have been consistent in the colder trend over the last couple of days.

 

We may well squeeze more than a toppler out of this but we always have to keep expectations of a prolonged cold spell realistic whilst we have a well formed December vortex showing.

 

 

[Frank Trough]

Well Ian if we were sitting here & it was Nov 2009 then I would 100% agree, after all there would be no basis to assume anything different

 

However You - & possibly Gibby need to think about the most recent weather over the last 4 years & factor in these 2 thoughts...

 

1) -AO index.

Since 1950 there have been 8 months in total that have averaged out sub -3 on the AO index they are-

Jan 63

Jan 66

Jan 77

Feb 69

Feb 78

Dec 2009

Feb 2010

March 2013

 

Of SIGNIFICANT note is that 3 of them in the last 4 years -

 

& that since 2009 the Winter anomaly has been this

 

4 year anomaly.png

 

Even more significant that is the 4 year December Anomaly...

 

Dec anomaly.png

 

& for comparison the 1995 & 81 anomaly for reference...

 

DEC 81 & 95.png

 

As you can see the match is VERY good for Greenland-, so as you can see- or maybe you both choose to ignore - either or, forecasting the Form horse isn't as straight forward as you & Gibby make out...-

 

You just choose the mild perspective, where as others like to explore the cold perspective.

 

S

 

it's a bit more than explore!!!! blimey. Trumpet, would be more accurate.

 

As well as those interesting composites (which are very interesting and a little bit exciting) it's probably worth also considering that Decembers 2011 and 2012 were both cyclonic and westerly (2011 was very westerly). Just a thought....

 

now bring on some snow and cold for December!

[Frosty.]

Why, please, Frosty?

Why not? and further to that, have you read the latest met office update? subtle upgrade in the wording since yesterday.  But it is set to turn colder from the north. Later in this period, there is a chance that the colder conditions may spread south across the UK, with more wintry weather and a risk of gales, especially towards the north.

 

It's sounding more like the whole of the uk will see the wintry blast compared to yesterday when it was a northern uk event only...and the further outlook through december is for temperatures to be below the seasonal average with wet and windy weather, especially further north, you can also take it that sleet and snow would occur during that period too since temps are expected to be below average.

Edited November 27, 2013 by chionomaniac
Toned down bolding.

[The Northern Ramper]

But with respect you are using GFS charts to back up data supplied by ECM and those temperature profile map readings are as useless as a chocolate fireguard at that range but hey ho everyone to their own.

Looking for the 12 Z's to continue the cold theme for the end of next week, very promising Met Office update.

Edited November 27, 2013 by A Boy Named Sue

[forecaster]

With a pretty significant portion of the vortex shown to be close to Greenland, as indicated by the vorticity charts too:

 

 

 

 

That's PV on an isentropic surface. I never have an easy time interpreting isentropic output. It's useful for some applications, but primarily academics love it because it's mathematically quite elegant. In the operational context, we view fields on isobaric surfaces most of the time and it's really hard to picture (at least for me, and I'm sure others also) how the 340 Kelvin* isentropic surface behaves (how does it slope, where does it slope?), and hence what relevance potential vorticity (which is a combined field of both a thermodynamic quantity and a dynamic quantity - and hence on it's own pretty hard to interpret!) has when displayed on that surface.

What I'm saying is that I'm not totally convinced the chart shows what you say it's showing. 

 

* I mean, the 340K surface - is it purely stratospheric air, or does the sloping surface pass through the tropopause at some latitudes - if so, which latitudes? This muddies the waters even more.

Edited November 27, 2013 by forecaster

[Duane S.]

AO forecast is sniffing something is in the air...

 

 

No surprise at all to see such a split around the end of the forecast period with 2 members tanking

[Cheshire Freeze]

Is it me or are the only people bringing high latitude blocks into the equation the ones who say that one isn't going to occur??

[JOPRO]

12z Rolling, its my first time on here since the build up to the March cold spell, hopefully I can bring some luck to us coldies!

[Snowy L]

Is it me or are the only people bringing high latitude blocks into the equation the ones who say that one isn't going to occur??

 

 

Spot on

[Ian Brown]

Is it me or are the only people bringing high latitude blocks into the equation the ones who say that one isn't going to occur??

 

Of course we don't need HLB for a decent North/North Westerly blast, but the synoptics being shown are 200 hours away plus and that should lead to extreme caution especially given how quickly these synoptics can fizzle out to the cold blast missing us to the East etc..

[Mucka]

A nice balance of optimism and caution in the forum today which reflects where we are I think. Also glad to see there has been no support for GFS 00z Op which looks like it was a rogue run - hopefully it won't develop that shortwave on the leading flank of the Aleutian ridge to such extremes on future runs either because that really is a total spoiler for any decent amplification later as explained this morning.

 

It has already been explained how we develop our cold snap/spell and even beginners will be familiar with the shape, amplitude, and orientation that we hope our ridge/trough will take in order to maximise any Northerlies potential and while even this initial set up is not a done deal it has been alluded to as to how we could extend the cold beyond this time frame (say around Dec 12th when the pattern looks likely to flatten out as our ridge comes under pressure and topples over the UK cutting off our cold feed..

 

Currently,as has been noted it doesn't look  likely that we will be able to build a Greenland block from the foundations of our Atlantic ridge but the more pronounced (and vertical) this is modelled to be then generally the better our chances will be of extending the cold. This is because it will be a symptom of a deeper trough to our East and less energy being modelled to run over our ridge which in turn opens up the small possibility of a link up between our leaning Atlantic ridge and encroaching Scandi ridge which is likely to be modelled if we can get good amplification and  alignment. (ramp ahead!   ) If that happened then there would be a good chance of a continuation of cold and the door would be open to all kind of Wintry options such as a Scandi high, a retrogressing high (though if strat forecasts are correct then this would be difficult) and even a Murr sausage!

 

Okay I am getting way ahead of myself and I can understand a few rolled eyes given we don't even have our ridge in place yet and I'm crystal ball gazing and hopecasting to some extent but for those learning the models it is an explanation of what some other posters have alluded to when they mention blocking at 60o North being a way to prolong the cold and snow prospects beyond the initial Northerly, a way to achieve a notable cold spell (At least I assume this is what they mean, what do I know? )

Edited November 27, 2013 by Mucka

[R.P. McMurphy]

Of course we don't need HLB for a decent North/North Westerly blast, but the synoptics being shown are 200 hours away plus and that should lead to extreme caution especially given how quickly these synoptics can fizzle out to the cold blast missing us to the East etc..

 

It's showing a very decent spell for next week, and this has been progged for quite some time now, with pretty much all models agreeing. I do think though that if the charts were showing milder weather next week, it would be a "set in stone" thing and "will" happen, but when it's cold and snow, it's "FI" and "unreliable".

 

We can only report on what the actual models show, and not what we would like them to show, whether this be cold or mild or somewhere in between! FI is FI, and it is unreliable as there is not a model out there that can predict the weather with 100% accuracy, but my point above still stands.

Edited November 27, 2013 by Glenn W

[MR EXTREMES]

indeed the vortex is something of a monster this year so for us to get a cold snap is something of a surprise!

 

problem is there is a likely outcome that the cold we will experience will be only a short but sharp cold snap other than a prolonged cold spell until we establish a more sustain pattern.

 

we are also unlikely to se a north easterly from this coming set up and the likely hood of a much watered down version of events with perhaps a more mobile pattern setting up into the second week of December.

 

its also worth mentioning that the southern half south western and south eastern parts most likely wont experience anything that the nw and north will experience. the models have already been a little wild so far this autumn in there prediction so its always best to expect downgrades.

 

we are talking about weather that is being predicted still far into fi and failure to develop the patterns shown is of coarse a very real outcome.

[Ian Brown]

It's showing a very decent spell for next week, and this has been progged for quite some time now, with pretty much all models agreeing. I do think though that if the charts were showing milder weather next week, it would be a "set in stone" thing and "will" happen, but when it's cold and snow, it's "FI" and "unreliable".

 

We can only report on what the actual models show, and not what we would like them to show, whether this be cold or mild or somewhere in between! FI is FI, and it is unreliable as there is not a model out there that can predict the weather with 100% accuracy, but my point above still stands.

 

Glenn, that is because mild synoptics have a huge margin for error. Cold ones, especially easterlies but northerlies also, do not have that margin for error.

[UV-RAY]

Is it me or is anyone else sick to death of reading vortex this or vortex that, yes it's an important player but cold spells can and do develop with a strong PV. Back to the models and having been out all day I've only just caught up with todays goings on and as some rightly point out we've still got 200+ hours to go and we all know that's an awful long time for things to change, but with my squinty eyes all I see is another upgrade for next week for both duration and strength of the projected cold blast. I see no reason why we can't squeeze out at least 5-7 days of cold and potentially wintry synoptics, who knows what can happen thereafter.

[A.J]

Interesting outputs from the GFS/ECM/GEM et all...I wonder if we'll see some WAA or some SW's to scupper the party...maybe even some MLB & HLB....of course a SSW event could raise hopes...maybe even a plume with an MCS could be on the cards...Of course it all depends what the MJO & NAO combined with the AO and the QBO show...then again, a little bit of CAA interacting with the LLJ could change everything...then again I forgot to factor in the DLS & PWAT...

the moral of the story is to always read the instructions as I do believe I've just overdosed on Model Output acronyms...


I'll fetch me coat!!

 

(site team joke....sorry!)

[Snowy L]

Is it me or is anyone else sick to death of reading vortex this or vortex that, yes it's an important player but cold spells can and do develop with a strong PV. Back to the models and having been out all day I've only just caught up with todays goings on and as some rightly point out we've still got 200+ hours to go and we all know that's an awful long time for things to change, but with my squinty eyes all I see is another upgrade for next week for both duration and strength of the projected cold blast. I see no reason why we can't squeeze out at least 5-7 days of cold and potentially wintry synoptics, who knows what can happen thereafter.

 

The reason a cold spell is on is because of a shift in the vortex...

[R.P. McMurphy]

Glenn, that is because mild synoptics have a huge margin for error. Cold ones, especially easterlies but northerlies also, do not have that margin for error.

 

Ian,

 

What is the proof of this? Not discrediting your above statement, but I have never seen the data to prove it.

 

The bottom line is, there is no mild weather shown on any of the charts for anytime soon, so it's confusing to other members to see this debate going on, all the way out to T384.