Model output discussion - winter proper underway

[frosty ground]

2 hours ago, feb1991blizzard said:

Not in the 15d period from now.

EDIT : It might change but not significantly enough for a proper cold spell to suddenly develop.

You  where talking about the extended period.... is this now upto 15 days?

[comet]

So the 18z shows over 24 hours of snow for east/south east England from around t190 to t216. Love the pub run.

[Tim Bland]

It’s been a good evening for coldies! ECM & GFS have most parts of the Uk under a blanket of snow in just over a weeks time !

 

 

[feb1991blizzard]

9 minutes ago, frosty ground said:

You  where talking about the extended period.... is this now upto 15 days?

15 days from the start of the run - ie today +15d  -  anyway I like the GFS 18z - I wouldn't rule out something like that happening, however, that's right at the bottom end of the spread of potential cold solutions IMO - in other words that's the very best we will do - definitely wouldn't rule out you seeing snow, just cant see any protracted severe cold spell - ie - proper Greeny high, Scandi high affecting UK,  etc etc.

[Frosty.]

The GFS 18z gives us some festive weather next week with snow and frosts which would be most welcome!

[Nick F]

25 minutes ago, comet said:

I was of the thinking that by showing the anomaly charts you could see where any forcing is taking place rather than the broad brush of the mean. The wave breaking several days back at the 10mb level is without doubt the reason behind the Alaskan ridge and cold surge that we engulf a number of states in America over the coming days rather than MJO, Glaam , GWO etc.

Certainly there looks to be downwelling through the strat, wave breaking like you say over Alaska, though yet to be convinced that the negative amomalies on our side of the northern hemisphere will work through to the trop. Would like to see a SSW rather than displacement to guarantee a cold pattern in January, but alas no signs yet. We've seen SPV displacements in recent winters but the cold patterns / HLB has failed to materialise.

 

11 minutes ago, Tim Bland said:

It’s been a good evening for coldies! ECM & GFS have most parts of the Uk under a blanket of snow in just over a weeks time 

It would be better if they were likely to verify though! Long way off to think about snow, but certainly looks like there will be some cold air following these lows moving west to east, but how cold and wintry will depend on the spacing between each of a succession of lows likely to roll in on the strong jet stream. A little amplification between systems will certainly help ...

Edited December 19, 2017 by Nick F

[Frosty.]

Just to show how fair and balanced I am..I posted snow charts from the 18z but here's something for sidney too!

[northwestsnow]

18Z GEFS are a significant upgrade on the 12zs.

Plenty of PM incursions- infact  quite a few now drop the 528DAM line south of London..

[feb1991blizzard]

1 minute ago, northwestsnow said:

18Z GEFS are a significant upgrade on the 12zs.

Plenty of PM incursions- infact  quite a few now drop the 528DAM line south of London..

Was just going to post that myself - although in their own right, they aren't extraordinary, compared to recent suites they are very decent indeed.

[northwestsnow]

Just now, feb1991blizzard said:

Was just going to post that myself - although in their own right, they aren't extraordinary, compared to recent suites they are very decent indeed.

Quite right - although with each passing day the trend is downwards- from 00z to 18z the GEFS have pushed the jet further SE - 

The period 27th to the 30th holds potential now for many places exposed to the WNW winds blowing in cold - at times there are -6 - -8 uppers into NW Britain meaning places like yours could get really quite snowy!

I do agree with your sentiments regarding beyond this period tho- it cannot be sustained , evntually heights will build across Europe-UNLESS the trend extends further tomorrow morning and the word 'slider' suddenly becomes relevant.

of course the models may trend back north with the jet tomorrow - but at this stage i think there is potential for some suprise snow events midlands northwards.

[Daniel*]

Not a bad set London GEFS 18z strong agreement for a cool down to commence on Christmas Day itself, could be the sort which starts off more mild. A considerable shift, going for cold for at least 4 days nothing crazy cold but wintry precipitation possible, particularly for the north some elevation and I think you’ll do very well.  

 

 

[Guest]

HOW RARE IS A FULLY SPLIT POLAR VORTEX AND WHAT MIGHT WE EXPECT FROM THEM?

Many of us on here are aware that we have been seeing some very unusual charts appearing in the models with the Polar Vortex coming under attack from one or more directions and some fully split PVs in the mix. A few days ago this was limited to distant FI output only showing up in the far reaches in a few of the GFS runs and some of the GEFS ensemble charts – mostly D11 to D16. These charts came and went but there has been a fairly steady trend towards bringing them into earlier time frames. In the last couple of days, almost all the models have started showing charts with at least the PV coming under stress and a few showing a fully split PV. These are not just FI charts but in the medium (usually more reliable) period from as early as D6 to D10.

Although we have seen fully split PVs occasionally showing up on an odd model run before (mostly in distant FI) in the last few winters these rarely verify. They “seem” to be pretty rare events, leaving us scratching our heads in terms what they might lead to and how the models will cope with them. I have produced a number of posts tracking them, most recently in a long post on page 72. I felt then (and I still do now) that this was an early indication of the broad pattern change to a cold(er) regime as suggested as a good possibility (with the usual caveats and downsides fully explained to us as always) from our teleconnection specialists. In fact @Tamara and @Catacol have provided some encouraging updates yesterday and earlier today.

CAUTION: To that tiny minority who are sceptical of the relative merits of teleconnection science this does not mean that this outcome is set in stone, it’s simply a highly useful (and very often reliable) forecast based on what the current background signals are showing and what drives the model output with the usual lag time.

I spent a few hours today (sad person that I am!) researching split PVs. I looked for articles on the subject as well as for analogues in the re-analysis archives. I wanted to see how rare and unusual they actually are, what can cause them to develop and what impact might they have on the weather, particularly in our vicinity. So not asking for very much! As I always remind readers, I am not an expert and I can only really provide a simple analysis. Hopefully, this might spark some interest and a few of our professionals and highly experienced contributors might be able to elaborate upon (or correct) what I say. FAO Moderators: This subject is highly model related and very topical right now but if there is enough interest, may be a specialist thread could be considered later on.

I started my search in the NetWeather forum and the index of the specialist topics but this (unless I missed something) appeared to be fruitless. It’s seems that the subject of split PVs overlaps several other topics (see later) and might be partially covered under another heading.  

Then I did my usual Google searches and came across a number of interesting papers on the subject. I will list some of the links below for those of you that are interested. Most of these papers contain bibliographies with pointers for further reading with additional links to other sites. There is usually a useful summary or “abstract” with the full paper available. I will be reading some of these papers during the next few weeks. For now, I’ll provide the title and the link and a short summary of what appears to be most relevant or useful bits:

1. “Recent Extreme Arctic Temperatures are due to a Split Polar Vortex” by James Overland (NOAA) August 2016

 http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0320.1

This looks at the continuing trend from the last two winters and takes us right back to one (or more) of the teleconnections! It seems that the near record low Arctic sea ice and the high temperature anomalies there are largely due to a split PV caused by extreme WAA and typically from the east Pacific and Alaska but also from the Russian side.

I can see that this may be a case of which is the cause and which is the effect. As the anomalies are still at extreme levels this winter, one can expect the trend to continue (you can refer to my recent update on Arctic temperatures and sea ice profiles in my last main report (part 4c) for the latest charts with December 12^th output on page 24 of this thread – I’ll do another update before Christmas if I have time. It seems that we might see many more split PVs in future unless we have a reversal of the anomalies. I’ll come back this later.

2. “When Does A Displaced Vortex become A Split Vortex” by WUWT – What’s Up With That, Just the Facts. February 2014

https://wattsupwiththat.com/2014/02/08/when-does-a-displaced-polar-vortex-becomes-a-split-vortex/

This paper, with some excellent diagrams and explanations, covers the stratospheric PV and the relationship and impacts right through from the surface up through the troposphere, stratosphere, mesosphere and thermosphere. This is getting rather beyond my limited understanding and is really one for our stratosphere experts to comment on (there is some reference to this in the strat thread). The paper covers strong and weak PV events from 2012 to 2014 but I see that it has been updated more recently with some winter 2016/17 charts and comments. It shows how quickly cold air can be displaced away from the Arctic towards the middle latitudes due to “Eddy Heat” and “Planetary Waves”. Part of this process seems to be in a similar way to what can happen with a Sudden Stratospheric Warming (SSW) event.

3. “Weak Polar Vortex Could Bring Early UK Winter” by Chorley Weather.com, October 2016.

http://www.chorleyweather.com/weak-polar-vortex-could-bring-an-early-uk-winter/

We know that there were some early forecasts for last winter of a blocked pattern with colder than average temperatures with a very weak PV for late Autumn and a slight early warming in the stratosphere. This paper was published just before that winter but it’s a fascinating learning exercise. In hindsight we know that we did get the very blocked patterns but with little cold due to mostly MLB and very little HLB. For the doubters, Tamara was one of the first to state that we would only see MLB with little chance of any deep cold for much of the winter – she was almost spot on and I’ll challenge anyone to provide “any” evidence to the contrary (remember “blocking” and “cold” do not always go hand in hand).

4. “Weakening Polar Vortex May Yield Longer, Harsher Winters in North America” by Michael Chune (AccuWeather), 2016.  This does provide an easy read article (perhaps over simplified) on the 2013/14 to 2015/16 winters. It focuses more on North America but it does also refer to “pockets of cold” transferring across to EurAsia. It does seem to suggest that western Europe and the UK will not always miss out on these cold opportunities and if they are to become more numerous (with quite regular splitting of the PV) we “may” see more colder winter spells in future than recently, despite (or because of) global warming but largely at the expense of the continuing loss of Arctic sea ice and warming there.

https://www.accuweather.com/en/weather-news/weakening-polar-vortex-may-yield-longer-harsher-winters-in-north-america/70000025

Many of the topics I found are just media hype, such as regular snowmaggedon articles from the Daily Express which must be avoided at all costs! I have struggled to find a really comprehensive study on fully split PVs, although the first two articles do cover more about the causes (which seemingly we can expect much more often) but not so much on the effects (other than the chance of producing a fast release of Arctic cold). I may report back further on this when I have had a chance to read up more but this really does require input and comments from our specialists.

I will now go onto the second part of this post which is much more within my comfort zone, grasp and understanding – looking at re-analysis charts with some split PV events from past winters and comparing them to some of the current model output.

I shall start by looking at the charts for the winters described in the articles above:

Winter 2016/17:

        

Plenty of attempts to disrupt the PV and several near splits but no full split and there were periods with a stronger PV. Running through the charts should at least give you an idea of just how rapidly the PV can strengthen and weaken again with lobes switching locations just as quickly. So, please, no more comments of “it’s a long way back to cold from there” or “winter is over”! These types of charts should always interest coldies, although they often do not deliver the cold to the UK, this winter may well be different (see my concluding comments). 

Winter 2015/16:

        

Some attempts but again no fully split PVs. Some deep cold in the USA but little over here.

Winter 2014/15:

      

Very disputed, weak and almost fully split PV in mid-November (the best example so far) with further attacks later in November and again in December and in January but little, if any, significant cold for the UK.

Winter 2012/13:

     

Now it starts getting interesting. There were numerous examples with the PV under attack during the winter of 2012/13 and we had several periods of significant cold as well as a number of short lived cold snaps before and in between the main events (in January and March/April). The PV was fully split in mid-November and despite repeated attempts to strengthen and recover, it just suffered from further attacks and displacements each time.

Winter 2010/11:

       

On to the winter some many of us remember producing the earliest start to winter (with prolonged cold from the the last third of November) and the coldest December since 1890. Numerous attacks on the PV but again, no full splits, although very close exactly 7 years ago to the day as we moved into the second phase of deep cold following the milder interlude. Even though January 2011 was much milder in the UK, there were more interesting Arctic profiles later on with some deep cold at times on our door step in Europe and in the USA. So, we were pretty close to turning a memorable cold spell (of two halves) into an epic winter. The search for fully split PVs continues. To cut this short (!!!), I shall now just pick (mostly) single charts from some memorable winters with the most split PVs that I could find. If I miss a winter out (like February 1991 or January 1987) that's because the PV was only slightly disrupted. I may well have missed a few others.

        February 1986                    December 1984         leading to January 1985       Early December 1981        Late December 1981         

            

Feb'86 only partly disrupted. The intense cold spell in January 1985 started with a quite strong PV in late December which suddenly came under attack and was briefly split by early January. December 1981's memorable cold spell saw an early split, then a brief milder interlude after an almost countrywide white Christmas but the PV almost split a second time prior to renewed deep cold in the first half of January 1982.

              January 1980                   February 1978                December 1962 (1)             December 1962 (1)             January 1963 (3) 

          

The winter of 1979/80 did managed a fully split PV for several days. February 1978 saw something of a split PV just prior to a 2 week very cold spell culminating in the famous West Country blizzard (where I was trapped in my parent's hotel on Dartmoor for 5 days in 20 foot snow drifts!). Surprisingly many of the frequent cold winters in the 1960s rarely saw more than minor disruption to the PV but the epic 12 week severe winter of 1962/63 did see a number of very disrupted PVs but only split briefly in late January and early February - examples (4) and (5) below.

       January 1963 (4)                February 1963 (5)                January 1947                     February 1947                  November 1890

         

The severe winter of 1946/47 didn't get going until the last third of January 1947 with a rather disrupted PV. The only split came in mid-February 1947. I have not closely examined the earlier part of the 19th century but I shall finish this exercise with the November/December 1890 cold spell, having already referred to it.  Again hardly any disruption but a generally very weak PV, the best I could find was a week before the cold moved in.

In conclusion, fully split PVs are extremely rare even in some of the coldest winters. Partially split, disrupted, displaced and weak PVs are much more common. The trend in very recent winters has been for much more disruption and this appears to be almost entirely due to the warming Arctic and reducded sea ice build-up. If some of the recent and current model output verifies we could see some extremely unusual charts (even more so than on some of those already appearing on some of the GEFS ensembles. Obviously far more research by "those in the know" needs to be conducted but if the Arctic ice and temperature profile "teleconnection" is producing some exceptional strains and stresses on the PV we may well be witnessing an unexpected  change to more frequent episodes of Arctic cold being released towards the middle latitudes (often without assistance from the stratosphere or SSWs). The US and Canada have seen a few extreme PV led deep cold snaps/spells in recent winters but Europe has often missed out. This seems likely to not always being the case. I have not correlated any of this with other teleconnections like, ENSO variables, eQBO, other oscillations like the MJO, solar weather etc, basically because it's beyond my ability to do so.  Given what Tamara and others have been telling us, the re-amplification forces may stand an even better chance of overcoming the seasonal uptick in the strength of the PV and the jet stream. Mastering the teleconnections is definitely the way forward but there are a lot of variables and conflicting factors. For example, it may not only be the true MJO signature that is being masked, other signals might be too but the changes in angular momentum are strongly broadcast and let's hope that this does the trick for triggering the build of heights to our north-east and introducing the cold regime. That's more than enough from me but I hope it develops an interesting debate and also plays itself out in the model output and then verifies. Extraordinarily exciting times ahead!  

 

 

Edited December 20, 2017 by Guest
Correct typos and check charts

[nick sussex]

Good trends today for coldies and given where we were a few days back some renewed optimism that Christmas week might throw up a surprise or two.

I think what we’ve seen over the last few runs supports the issues mentioned by NOAA regarding the MJO.

There is normally a time lag between MJO phase and impact on the NH pattern and what might be playing out is the models slow to catch up and that’s why we’ve gone from foreboding PV blob stuck to the north with a more disorganized one with more se track of the jet.

Let’s hope these improvements continue tomorrow.

[northwestsnow]

1 minute ago, nick sussex said:

Good trends today for coldies and given where we were a few days back some renewed optimism that Christmas week might throw up a surprise or two.

I think what we’ve seen over the last few runs supports the issues mentioned by NOAA regarding the MJO.

There is normally a time lag between MJO phase and impact on the NH pattern and what might be playing out is the models slow to catch up and that’s why we’ve gone from foreboding PV blob stuck to the north with a more disorganized one with more se track of the jet.

Let’s hope these improvements continue tomorrow.

I agree nick- if these increments south of the jet continue i suspect we will be in slider territory - we need further corrections south tho-but for today at least the trend through the day has been good..

[weathermadbarnsleylad]

Fantastic reading as always from you guys thankyou all very much . Ps topic on PV spilt was very interesting good read cheers bb196263.

[weathermadbarnsleylad]

Just now, ShortWaveHell said:

BB1962/63 posts are outstanding for info and from a learning perspective ! Time and effort put into it is just star dust !! 

Yes very very good indeed, can't praise enough.

[knocker]

Today and for the next couple of days the UK is in for a rather benign period of weather with the high pressure centred to the south/south west a slack moist airflow pertains which, with the odd decaying front floating about, essentially means cloudy with the odd spot of drizzle in most places with temps a little above average The exceptions may well be N.Ireland and particularly Scotland which will be brighter albeit with perhaps some showers over the latter tomorrow

Saturday sees the high pressure coming under pressure as the trop vortex displacement gets under way in North America and a strong jet whips across the Atlantic with the rather intense thermal gradient just north of Scotland. Thus the UK in a breezier westerly flow with a front, which is trailing away to the south west, and some rain already impacting Scotland.

Over the next 24 hours the front drifts slowly south as a wave forms on it south west of Ireland whilst the upper trough and the colder air to the north west gets more organized

.

By 06 on Xmas Day the aforementioned wave is over southern Scotland with the front running down through Ireland which could well produce a fair bit of snow in the far north of Scotland which is in the cold air with the rest of the UK in a brisk, showery south westerly with temps a little above average.

Essentially this period marks the transition from a rather benign period of weather to a much more unsettled and varied scenario as has been indicated. Over the rest of Xmas Day the front tracks south east across the country bringing sporadic rain in most areas but perhaps some snow on the hills in the north  The front clears the south east Tuesday morning in time for a deep low, 968mb, and associated fronts to arrive again bring strong winds, rain and the possibility of snow on the high ground in the north.

Over the next 48 hours the low tracks into the North Sea and points east initiating a transitional cooler northerly regime and quite possibly some hefty snow showers in the east during the passage. Best left here but  a reminder that the evolution next week is very knife edgy still and the detail of this is obviously subject to change.

 

 

Edited December 20, 2017 by knocker

[ArHu3]

Ecm 0z rolling out with already big changes @72h compared to yesterday's 0z @72h 

 

 

 

 

Edited December 20, 2017 by ArHu3

[ArHu3]

I can't edit my post but I would like to add we are sure to end up with something very different and new with such big changes early on

 

And at 144h it looks very different from both ukmo and gfs, is ecm on to something, a Greenland high? 

 

Edited December 20, 2017 by ArHu3

[chris55]

ECM raising some eyebrows this morning, genuine Greenland high in the making. With the Alaskan ridge ridge migrating and splitting the PV!

Edited December 20, 2017 by chris55

[Ali1977]

2 minutes ago, chris55 said:

ECM raising some eyebrows this morning, genuine Greenland high in the making.

Agreed, this would be a significant change - from there we could have some real cold, let's see what happens at 168.. no support yet though. 

Edited December 20, 2017 by Ali1977

[Dennis]

Just now, chris55 said:

ECM raising some eyebrows this morning, genuine Greenland high in the making.

a new cold pool is setup --- see the diff -

[Ali1977]

Greeny high tries to hold but looks like the PV's just to strong and is going to shut it down, however the trend of building a greeny high could strengthen , definitely one to watch.

Edited December 20, 2017 by Ali1977

[ArHu3]

3 minutes ago, Ali1977 said:

Green you high tries to hold but looks like the PV's just to strong and is going to shut it down, however the trend of building a greeny high could strengthen , definitely one to watch.

I wonder where this surprise came from, so early in the run. Can anyone see yesterday's ensemble? 

[chris55]

6 minutes ago, Ali1977 said:

Green you high tries to hold but looks like the PV's just to strong and is going to shut it down, however the trend of building a greeny high could strengthen , definitely one to watch.

Yes, Wetterzentrel image rendering was a little deceptive on that 144 chart making the Greenland high look a little more robust than it was.

However still a definite trend for that Alaskan ridge to migrate and hopefully disrupt the Atlantic enough to allow for some cold to spill down from the north.

 

Edited December 20, 2017 by chris55