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It's to do with how many planetary waves are affecting the vortex at any one time. If there is only one wave affecting the vortex then we call that wavenumber 1 and this will displace the vortex. Imagine a balloon blown up - if you put 1 finger of pressure on that balloon then you will distort it into a more banana like shape - akin to a wave 1. For wave 2, put 2 fingers of pressure on the balloon on opposite sides and you will achieve an '8' shape - with a split down the middle. So that is what we are seeing with the strat vortex - here is the classic wave 1 displacement And late in the run we see the pinching effects that wave 2 brings Technically, the waves don't need to be geographically based, but in reality they tend to be Atlantic or Pacific based because of the position of land masses and mountain ranges. Charts courtesy of weatherbell

Lots of uncertainty but signs that the atmospheric circulation is going to mirror stratospheric turmoil and wind itself up for some elastic band rebounding that suggests some interesting and contrasting surface detail fall-out to come for the last third of winter This is one of those occasions where falling torques and atmospheric angular momentum tendency in the first instance and then a consequent sharp rebound of torques and atmospheric angular momentum in the second, might engineer catalyst to snap the NH pattern out of its seasonal rut. Cued by tropical forcing in the Indian Ocean, -ve balance of frictional torque has been leading mountain torque decisively downwards over recent days. A considerable -ve mountain torque has occurred over the Rockies and rapid removal of westerly winds from the atmospheric circulation is occurring. This has signalled a drop in relative angular momentum tendency With (approx) two day lag plot showing the Global Wind Oscillation heading towards low amplitude Phase 1 Taking into account response timelines to -ve mountain torque this advertises an upstream amplification over the Pacific to occur around the 6 day period which will have downstream ramifications in terms of retracting the Azores High westwards and taking the Jetstream southwards across the Atlantic as fuelled by the displaced Canadian lobe of polar vortex. No surprise to see some distinctly unpleasant weather modelled in the next weekend period in the first stages of this process However, the upstream jet, is set to decelerate rapidly as pressure rises over the Pacific and re-build the Aleutian Ridge. In the Atlantic sector, the retrogression of the pattern will tend to angle the Jetstream more NW/SE and, conceivably, enable build of pressure over Scandinavia. With time, as the disrupting trough gets separated from the Canadian lobe of vortex, downstream amplification from the Pacific sector becomes possible at the same time as the wave 2 response is activated by rising pressure over Scandinavia to work on the vortex in tandem with the Aleutian Ridge. It is key at this point that the vortex is sufficiently weakened to allow the bleeding of secondary systems to be cut off from the Canadian lobe - and hence terminate the thermal gradient. So much for the extra tropics. Returning back to the tropics, we have to watch eastward propagation of the MJO towards the Pacific starting to act constructively on the pattern in terms of increasing downstream amplification. With angular momentum, what goes down goes back up again to balance the global windflow budget, and with total angular momentum levels close to parity... then a logical correction to reflect the background ENSO neutral ocean base state can neatly be supplied through eastward moving tropical forcing. What does this mean in possible synoptic terms? It firstly bolsters/re-enforces the inflection point of downstream amplification into the Atlantic as, ultimately, transferred by a decelerating jet from the Pacific - which then encourages a follow-up link up (over the top of cut-off disrupted troughing heading into Europe) to the pressure rises over Scandinavia. Meanwhile, with (hopefully) a split enabled in the vortex, the chances of the jet crossing through the northern arm with spoiling energy to inhibit amplification are reduced (also assisted by tropical signal progressing through Phases 7/8 which would teleconnect to an increasing -NAO signal and help back cold air advection west) This is of course the ultimate prize scenario of what *could* happen through February. Whole timing of all this beyond the point where troughing may start disrupting and reduce the vigorousness of the Atlantic is obviously uncertain this early, and of course much depends on both emerging stratospheric signals and medium to longer term tropical forcing signals both sustaining, and, very preferably upgrading. Its a case of looking to an orbit of the GWO to Phase 2 (shaping the Atlantic ridging and as already given credence by latest GFS 06hrs) and then looking to the stratospheric fall-out of the tropospheric uppercut and westward encroaching Scandinavian ridge as tropical forcing moves eastward to the Pacific and hopefully maximised amplification as atmospheric angular momentum tendency returns the GWO through to Phase 4 But whilst we are familiar with what can go wrong will go wrong, lets consider for a change that this process does, on occasion work the other way too. There is feasibly reasoned GSDM extra tropical and tropical support for both the tropospheric and stratospheric possibilities - and with just the last few weeks of February left of official winter, and whilst we cannot control the elements, frankly there is little to lose in approaching the evolution with a mind-set that is otherwise Late edit: GEM 12z and GFS 12z are also trying to read the longer term script...

Ahh well that is the difficult bit. But what @Singularity and @Tamara allude too. Scandi Block moving westwards with undercutting southerly displaced jet. It could happen with this type of set up, I guess as ever we just need the resultant daughter vortices to be in the right position and strength for that to occur, because we all know that near misses are far easier to achieve than a direct hit. But there is definite interest in this projected set up.....

Evening All nice negative tilt on the WAA heading up through Scandi on the ECM @120 !

What a brilliant read Thankyou and plenty to be postitive about . I remember last week you said this weeks post would be a much smaller post lol you obviously put so much hard work in to your posts . I'm glad it was not a short post . Thanks again .

Well, after 14 years' model-watching, I think It's about time I asked this question: How can a computer model be 'bullish' about a solution? When its prediction fails to materialise, as is often the case, does said model suddenly become 'sheepish'?

0Z ECM attempting to develop the near perfect E,ly. http://old.wetterzentrale.de/pics/Recm1681.gif I say near perfect because the high is descending from the Arctic. Reason I say near perfect is because the lobe of PV doesn't descend with the high i.e Jan 1987. Still if you want a bitterly cold E,ly with very real cold upper temps then the 0Z ECM is the way to go. Only downside is I don't believe it after the winter of let downs so far/.

Emergent theme from today's models is a stalling atlantic picture, heights building over NW Russia, fronts not making any inroads into scandi, inbetween we have alot of warm air advection, ripe evolution to trough disruption, indeed both ECM and GFS showing secondary low developments splintering off from the longwave trough, and simply coming unstuck in situ. Fully expecting the models to show trough disruption occuring in the reliable timeframe over the next few days, and heights ready to attack from the NE.

Stay tuned for later Febr

Nick strong backing this morning from our portal forecast for a very cold snowy spell for the middle period of the month with expected A/C airmass taking charge(60%) expectancy. That's a strong forecast rate from our Central European prospective. So we can expect developments in some other model forecasts soon. In the meanwhile this is our 31st day of sub zero temps and locally some valley stations near by could be on for the coldest January for 30 years after a run of exceptionally low night time minimums.However, its been a very dry month for many and a welcome prospect of some snow next weekend in the Austrian Alps butvwill lose the very low temps of the past month but a very cold and snowy mid month beckons. May be its time for the British Isles to get some proper winter charts by mid Feb?

im going to stick my neck out and say we are in for a big freeze mid February on wards or later part of Feb with heavy snow showers blowing in from east high pressure over Scandinavia low pressure pushed south just my take on things wants going on in the stratosphere SW . Folks may shoot me down but we have all been wrong at some point..

No views to speak of on West Lomond at 9am this morning but the rime on the trig was worth a few snaps The summit was scoured but the east slopes had a good 4 inches of snow. I've seen some beautiful pics of the Cairngorms on Instagram this morning too - lots snow and not a cloud in the sky.

Cracking demonstration of a complete wave 2 split that chart. Wave 2 upwelling splits have often delivered the best results in term of cold since I started monitoring the strat. Hopefully the Canadian daughter vortex won't be too top heavy.

I'm wondering whether we're seeing a closer match to an MJO phase 5 lower amplitude signal. If you compare the GFS 06hrs run at T240hrs and compare it to the MJO lower amplitude you get this: If we see less interference to the MJO signal then we should be seeing that closer match. Last weeks NCEP update mentioned less interference in week 2 and if this does verify then chances for something colder to develop should increase. PS I've edited the MJO to give an easier comparison.

I am tracking the 6ZGFS (out to day 9 so far) and it is definitely heading the same way so perhaps the ECM is not an iffy run. A split here will give the best potential for something really cold later down the line imo. But all ifs and buts.

A lot of negative thoughts re any cold and not sure why. The 500mb flow is going to be a very strong westerly over the coming 2 weeks giving some windy spells especially for the Nw'ern half.. Never particularly mild and with contour heights lower at times than the previous many weeks. This allows for colder incursions from time to time, frost in any temporary ridges overnight and wintry type showers in the colder air spells. Something for most weather enthusiasts, even some thunder possible. No prolonged mild spells and sure enough little sign of any prolonged cold. A mix in fact from day to day. In terms of snow then the northern half of the country is better placed but even in some southern areas sleety type showers are quite probable. Lying snow, then yes, only for the higher ground of the northern half with some good falls likely over the Scottish ski areas.

Some SS's Sunday stroll snaps. Climbed Black Hill just south of Earlston with my 4 girls (Mrs SS, Mini Miss SS, Islay [pictured] & Lily). Snow in the distance on some of the higher border hills. Frosty the morn; lovely sushine as we climbed.

Just thinking briefly about how much the more active phase 5 MJO that ECM is going for might be influencing it's wave-2 projections. The composite for that on the left suggests some support for W-Asia height rises and a more disrupted Atlantic jet pattern so there may be something in that. On the other hand, the usual model biases may be at play; GFS too flat and ECM too amplified, with the reality likely to be somewhere in between. Any improvement on GFS is noteworthy though given that it already looks quite reasonable at 10 hPa by day 15... it may be far away in time but the seeds are sown much earlier on. Regardless of level of impact while in phase 5, continuation to 6 would make for an interesting period if we had a big block to the NE before that time, though all these composites are only loose guides given the potential influence of many other variables. These may only be possibilities, but at least they're nicer than some that we've been having to contemplate in recent times

Love this part of our small country,Love the scenery and views that you guys post. Hopefully be making another visit in the summer for a long weekend . C.S.

Exactly Nick. It is the combination of the wave 1 downwell and wave 2 upwell that will give us the best chance. That is why it really doesn't matter whether there is a technical reversal or not at 10hPa - what is important has and is already occurring - the strat vortex is on the ropes and primed for an uppercut.....

A fine, icy, still morning with a peachy glow at sunrise. Temp -2C earlier. Excellent conditions for an early morning walk. Snow confined to elevation. My doggie is having a week's reprieve before her surgery.

Great to see the trend continue despite the UK missing out at the moment. When the models indicate such a scenario I often start daydreaming of a repeat of Jan 87. The descending high from the Arctic with the associated bitterly cold airmass pushing S is a rare sight and even rarer to hit the UK. For the UK, especially the SE, to experience upper temps between -15 & -20C then this is the only synoptic pattern that will bring these kinds of temps. For the younger members. When you see old video weather forecasts on youtube i.e Jan 87 with max temps of -8C during the day, then this is the kind of set up you need. Finally being looking at some GFS N Hemisphere charts for the past few days and comparing with todays outputs. I am somewhat encouraged that we may see tomorrows outputs suggesting that cold pool may get closer!

Interesting that the models think it's all East to West around Valentine's Day. I love you all.

Again we don't know how things will exactly work out tropospherically, but in my experience once we have that wave 2 upwell modelled across the board at day 10 then we know that the trop output will all have slightly different paths in the modelling but we end up with the same result. Best chance of deep cold this winter coming up imo.

Good trends again from the 12z.. The GEM also looks good.

It wouldn't surprise me if a block to our north east develops more rapidly than what the models are currently showing. The models are clearly sniffing this out but I have a feeling that they are a little behind the game. Don't be surprised if tonight's 12z runs show a scandi high in the 7 day timeframe

There must be something on the nw site re this chris - perhaps the first page of the strat thread? anyway, wave 1 will be driven by one wave (usually from one big ridge ) whereas wave 2 requires two ridges ( see upcoming trop pattern with big n Pacific and scandi ridges into the polar field) these waves can be driven from the trop upwards or the strat down

Here we go again , buckle in folks . We may win the raffle this time . Let's face it everyone else has a prize it's about time we have one too.

Ice here again this morning. Min temp about -1.3c Been a few years since seeing ice leaves on the cars:

I'm old fashioned Jonathan, so to my way=explain what the chart is showing. 'localised...zonality', to me is a meaningless phrase and I would imagine to a lot of new folk trying to get to grips with model outputs quite confusing. However each to our own idea.

Support is support. When models agree on things like this its always good news.

You maybe right and as amateurs we can all "stick our neck out" as we are not accountable. Interesting output and interesting trends. Let's hope we get something, as it will be 10 months until us coldies are back in the game.

Who needs a cigar? I'll settle for that.

An interesting trend developing in both the ECM and GFS 06hrs run but theres one thing getting a high to the e/ne and another for that to get far enough west to effect the UK. That PV lobe in Canada needs to move further nw otherwise its likely to be another close but no cigar outcome. We've had enough of those this winter so need to see westwards corrections if this trend remains.

Some interesting developments on the last two ec runs. Berlin charts reveal that the stretching of the strat vortex as illustrated by singularity results in the main part of the vortex residing over Canada at 150hpa whilst higher up it's over ne scandi. (The vortex stretched across). you will have noticed the intesresting part of the op run this morning and at 50hpa, there is indeed a split beginning to show a good trend towards a possible split in the mid/lower strat. and also note the latest ec op brings a further warming at day 10 into the equation. that does make me wonder if it is a reliable run.

Please God, NO!!!!! I thought I would at least get to next season before the MOD started blabbering on about another possible easterly! (Hope it's gone in the next run!) Anyhoo........Icy here too this morning

18z takes WAA cross-polar flow. Less Scandi blocking and more Svalbard High. Could be some fun and games. We are all dreaming, after all.

Well i think this winter has shown that computer models are a valuable tool for weather forecasting. They dont build "garden paths" to lead us down. They try to make sense of a chaotic force of nature. The stubborn block to our east is a case in point. Its like trying to predict the individual paths of a flock of sheep. There is a point where we can predict movement but after that its speculation. Personally, i'm disappointed in this winter. It had so much "potential".... but i dont think we can blame anyone's forecasts or any model predictions for the lack of snowy weather for the UK so far. Thats just how it goes. Its not over yet so there could be something to look forward to but hey, its not the end of the world. I really think that the next 2 years or so will bring us a winter to remember....

Oh yes indeed steve and all models are playing with the Scandinavia ridge. Things are moving in the right direction

We're seeing a a rather quick downwelling? To the troposphere on GEM most evidently. This thread would be ablaze with activity if it was earlier in the winter, it seems to me the atmosphere is going to be the most inclined towards cold not seen since early 2013. Eye catching synoptic in GFS 12z FI northern blocking galore reminiscent to early winter - back in December when EC monthly and glosea were modelling extensive northern blocking, it got the droves out.....round 2?

Noticeably more interesting Northern Hemisphere charts appearing. This is what we have been lacking recently. We have been hoping for a cold spell relying solely on the jet playing ball rather than having the building blocks in place that would ensure a less risky route to cold, longer too. P8 = LOADED

todays AO and NAO

Guten tag from Garmisch-Partenkirchen in Germany. I'm here for a week's skiing with 6 friends and we've just enjoyed the first full day on the pistes. An absolutely glorious day here. Started off at -6C when we first hit the slopes, but warmed up considerably as the day progressed, so that a couple of beers could be enjoyed sat outside one of the mountain restaurants in the sun this afternoon. The town is currently buzzing as the cancelled Mens Downhill from Wengen was run here Friday. Then yesterday and today there were Womens and Mens Slalom racing. We tried to get into the finishing area and see the racing but there were so many people we gave up. Nevertheless it was great to enjoy the atmosphere and see so many supporters with faces painted in national colours and waving flags. I'm a bit concerned about the rest of the week as it looks like rain here on Monday evening and all day Tuesday. There's always the Zugspitz Glacier - but who wants to be up there in heavy snow? Anway, some pics from today. The view of the Zugspitz mountain from our apartment in town; the view down onto Garmisch from one of the runs on the 'Classic' ski area; looking down on the Slalom course finish area as we travelled over it in a ski lift; and finally some Swiss supporters in traditional clothing and faces painted. I'll try and post some more later in the week.

Bit of snow here this mornin.

Just need to the Azores ridge to do one, what interests me about the appearance of the Scandi, even if it doesn't bring an easterly initially, is the increase in tropospheric wave number 2 forcing it will bring - which will help towards perhaps splitting the strat PV. Split of the PV much better than displacement for long term prospects of HLB and cold/snow prospects as we head through February. Anyway, only one run and we'll see if the EPS z500 mean supports the idea later too.

Bennachie and hills to our West looking as white as I've seen them all winter this morning. Nothing in the village of course. Lovely sunny morning.

And usually, Dave, dislodges a lobe of the vortex with it on its eastern flank which descends into the mid lats with it. generally stays over Russia/Ukraine but if lower heights can be pushed into the med then these can be advected west . Talking of lower heights into w Europe, the eps continue to deliver an envelope which covers most bases.

the ec op shows a split devloping in the mid strat by day 10 which I presume has its roots in the wave 2 type set up illustrated

So Matthew Hugo predicts half of Feb could be wet windy ,no sign of any cold ,right I, m off out of here ,the guy is very knowledgeable but we will have some very cold air to our far north driving this cyclonic wintry spell and if we get lucky we have a good chance of the white stuff ,right I, m back ,don't give up this cyclonic spell could bring us what we desire ,cheers gang .

I was! Pea sized hail now.