Singularity

One thing I've been struck by in terms of the GFS runs today is the removal of the west-based negative NAO. ECM has a sharper trough-ridge-trough sequence across the U.S. which allows the ridge to nose into NE Canada and seemingly force disruption and a total split apart of the trough that GFS allows to remain in-situ over Canada. This being another important piece of the puzzle alongside the N. Atlantic calamity capers. Perhaps we have here a case in which we could really do with ECM falling foul to it's tendency to go a bit far with ridge/trough amplification? That being different to the usual situation in which that produces tantalising charts that prove merely to be cruel teasers?

If the overall trend could continue a little further, ECM and UKMO would have no choice but to start reconsidering alternatives to the blowtorch-mega-thermal-gradient-storm chain as I expect the media would be keen to call it. I wonder just how much influence this tropical storm is having on the output. Perhaps quite a bit; GFS treats it very differently on the 12z; Instead of staying independent of the jet stream, it becomes entangled, adding an extra loop south to that jet which holds up the pattern more than is seen in the case of ECM or UKMO. I must say it's weird seeing a tropical system heading for the Canary Islands, although I expect cooler waters would probably force ex-tropical transition before arrival there. Probably. Worth noting that UKMO looks in a better position to take the trough as a whole east of the UK by day 8 than ECM. So somewhat ironically, it's the American model that's by far the closest to the UK Met's recent mid-range outlook updates. Really interesting how they describe a colder, drier north and milder (but not more than average), wetter (but not that wet) south. This is a matter for (and being well discussed in) another thread, of course.

Given the above points from various contributors, a vortex versus amplified planetary waves period seems the best to hope for. Question is, will there be a decent tropical perturbation to work with? A high-stakes game for those attempting to produce guidance for the season ahead.

Seems to be two distinct routes to improving from the 12z GFS; Route one sees the shortwave low breaking away cleanly and scooting across across the UK fast enough that it's at least in the N. Sea by noon Monday and ideally into Norway, with this quick motion separating it sufficiently from the main trough to the west for that trough to interact more with the next one off the U.S. as opposed to the low that's crossed the UK. This interaction encourages the Atlantic trough to stall, and potentially take on a negative tilt. The ridge may then build N or NW from the UK and link with the HLB, however it's possible that another low could break free from the Atlanitc trough as it takes on the negative tilt (such is the nature of trough disruption) and cut through beforehand as per the 00z ECM. Route two, very nearly achieved by the 12z GEM today, sees the shortwave low moving much more slowly, so much so that it misses the chance to cut east through the ridge, and must instead slide southeast. In this scenario, the main Atlantic trough could feasibly develop an extension on the SE flank that stretches the system out from NW to NE, so producing a negative tilt with a ridge building NW from the UK to link with the HLB. Now this does put us in a strange position, as for one outcome, we need faster eastward progression of the shortwave low (but not trough to its west!), and for the other, we need slower eastward progression. That's going to make anticipating the path of the next few model runs a bit difficult to say the least .

I expect differences in the timing of lows nipping across the UK, and the transient ridges in between, have greatly muted what would otherwise be a mid-Atlantic ridge and Scandinavian trough picture. A little surprised to see the mean polar heights drop so much though. Perhaps a scattering of runs moving the potential deep Pacific-side low near or across the N. Pole? Members with greater ensemble access/time on their hands will know more.

Well, it only took until the third time of asking to separate the troughs, so that's something I guess The stronger Arctic blocking on this run is notable. AO readings must have gone very low? In some ways we've gone fill circle, with the proper cold signal back to the final week of the month, having been severely teased about a possible earlier onset (and given the GEM 12z, this may yet continue depending on the ECM 12z). How... Amusing?

It's amazing how powerful and expansive that block it, and yet how little GFS has it doing for us up to this point. It does seem like that next low really ought to stay back west, but recent runs have refused to weaken the jet sufficiently despite the blocking situation. Shame the Scandi trough is shallower on this run as it might be interesting to have that low nipping across and then engaging with a deep trough close to our east. It really is going to be cut that close, isn't it? Oh the joy of model watching Time to take a dusk-stroll I think.

So that low still managed to push east despite everything. Seems we really need that little tropical/subtropical low to catch up with it on day 4 as per the 00z ECM. With the low pushing east the best we can hope for is that it doesn't open the door to further troughs coming along behind it. It'd have to become disassociated with the trough to its west and then either fill in-situ with a new ridge building from the south, or nip across the UK with a new ridge building in behind. Unconvinced about so much eastward movement though; the low is almost disconnected from the primary jet stream.

+ 87 hours. 06z: 12z: Low by the UK further east, and low by the Azores further NE, interacting with the one near Canada to set up a more negatively-tilted flow up the western flank of the ridge. Positive adjustments to this point with respect to building a stronger ridge and holding the Atlantic back for longer. +102: Differences not as marked as hoped for following earlier changes, but the flow on the western flank of the ridge is still better aligned than the 06z had it. Shift east in pattern may just mean a cold, dry sort of weekend, but this may lead to a better shot at building a ridge to our N/NE early next week... with any luck! Given this is GFS, keeping my expectations on the low side.

I wonder how much of a role the resolution of the model plays in handling situations where the wavelengths may be particularly short, such as Sun-Tue next week? This being when ECM has them very short indeed, facilitating (by my interpretation, at least) the hold-back of the Atlantic trough to the west as it and the trough over N. America become so close together that they directly interact, resulting in disruption of the Atlantic trough and movement of the N. American trough sufficiently far NE to prevent the HLB retrogressing to E. Canada. UKMO might be heading that way as of +144 but I'm not sure - can anyone post the +168 from that obscure site to which my links never seem to bookmark properly? TIA

+180 06z: 12z: Deeper but still negatively tilted trough to west positions the ridge ahead of it more favourably for us on this run compared to the 06z. If the model behaves itself with the trough disruption, it could serve up a run of tasty charts going forward, although still mainly for the north.

https://www.theweatheroutlook.com/twodata/chart.aspx?chart=///www.theweatheroutlook.com/charts/gem/192_mslp500_arc.png?cb=755 GEM takes GFS and ups the ante. Too many deep troughs in the mix to get a clear angle on things; slight day 5-6 variations in timing and path have very large impacts by day 8+. As in - larger than usual.

Trough tilting very negative and disrupting with a secondary low running into the base of the trough by the UK... this is what I put forward a few days ago as the most realistic way forward based on past examples. Not only provides some wintry chances but allows ridging to remain to the NW rather than sink to the W, and the new import to Europe 'props up' that ridge even as it comes under pressure on the NW flank. The better blocking to the NE by day 10 is of course a potential bonus, but it seems a lot of luck will be needed for that to align in a nice way for us to work with. With the polar vortex stirring, this Azores-Scandi type 'hang-back' of cold air looks to be about as good as we can manage from what the GFS 12z offers. Given the changes in the 8-10 day range, though, things could yet end up quite a bit different!

You've not been at the controls of the ICON model have you Mr. Murr?

06z: 12z: Well, GEM just went from horror show to near-nirvana from the 00z to the 12z, so it seems uncertainty is sky-high right now. Makes me less concerned about the UKMO 12z, but less jubilant over the GFS 12z or this GEM 12z for that matter.

Comparing the GFS 06z with 12z across the N. Hemisphere at +228, it looks like a marked step away from much in the way of HLB to the NW, with us instead looking at a combination of a mid-Atlantic ridge and blocking across N. Eurasia. In some ways this looks like an alternative route to what ECM/EPS has been hinting at in terms of some anomalously high heights to our W/NW but not extremely so, and HLB shifted off the pole enough that the AO doesn't so particularly negative. All very interesting, but the inconsistencies of late greatly limit confidence in these interpretations retaining much relevance even a day from now!

Hmm how has UKMO done that? Where's the Arctic blocking or western-Canadian trough to keep the polar vortex from heading toward Greenland? Odd to see the interaction with the low by the Azores brought back having been dropped by ECM a few runs back. Not so keen on that way forward although with the trough west of the UK tilting negative we could see height rises across Iceland for example, as long as the vortex stays west of Greenland. GFS remarkably sharper and faster on this run what with the U.S. trough tracking further west and north. Good jet angle for sustaining the mid-Atlantic ridge and perhaps undercutting it a day or two later which given the sharpness of the trough by the UK and how far south it's digging (Euro high finally vanquished!) could make for some teasing charts.

It's the battle between a tropospheric-stratospheric linked vortex and anomalous ridges - usually kicked up by tropical forcing - pushing toward the high latitudes that tends to be how strong vertical wave flux takes place to produce large stratospheric warming events, which is essentially why a full winter of persistently blocked and cold weather is (AFAIK) unheard of in this country. The best winters see early internal wave breaking events leading to a HLB period with the blocks aligned nicely for the UK (the bit we're really struggling to resolve this year), then the vortex winding up but only briefly thanks to some tropical activity for example having also got going and driven poleward ridges that come up against the vortex, trigger vertical wave flux, and so on. It's true as Catacol posted that - after an often chilly and at times cold Nov - Dec 1962 was very mediocre in terms of the UK's weather until shortly before Xmas, and the major cold didn't set in until Boxing Day, but looking back at the stratospheric analysis, it appears that was actually internally driven, with the Nov weather related to the Canadian warming event. Amazing really that it persisted so long afterwards... and I really wonder how?

The movement in the trough in the W. Atlantic is probably one of, if not the major driver behind the wide variability in height anomalies seen in the ensembles at the moment. GFS 06z has a shallow shortwave low splitting off this and running into the base of the UK trough next Saturday and this seems more realistic to me than the wholesale eastward movement of the trough from the W. Atlantic that then follows. With the jet in such a wavy configuration, eastward propagation ought to be limited, if any... or am I thinking about this wrong? ECM 00z did a better job of holding the trough back, but still looked a bit iffy by day 10. It's essential that this happens, because for one thing it opens the door for the Euro High to vacate northwest, and for another, you otherwise have a huge wedge of tropical maritime air being pushed east where it can interact with the cold air to the north and initiate some cyclogenesis that has a good chance of taking place W or NW of the UK, much to our discontent. I have to say though, as much as the signals have long been for a difficult Atlantic versus HLB battle, recent GFS runs have just been taking the piddle, although having seen this behaviour to some extent even in the run-up to late Nov 2010, it's hardly surprising. Currently, there are no consistent enough trends to make a call one way or the other with respect to the W. Atlantic trough. So it is that even with the immense HLB signal, no reliable trend toward significantly colder conditions in the UK can yet be interpreted.

Sometimes I wonder why I dare hope for it to do something more intelligent In reality I'd expect to see the HLB interfering with the organisation process and causing the troughs to become very flat on the N. flank with secondary lows splitting off and heading between NE and SE depending on how close blocking and deeper cold air has made it to the east. Even with the monster low, this is getting close to a cold continental feed getting in on the act. Just imagine the flatter version of the low with a secondary feature edging under the ridge to advect a bit more of the cold air across. This run has managed with it's dartboard bias to produce the least possible goods from some truly exceptional blocking patterns. Into the naughty corner with you, GFS!

Well how about that - the Canadian segment has become a disorganised mess! Take from that what you will regarding the resolute nature of this blocking event. With the blocking focused to the NE, deep cold has been moved closer to us than at this time on the previous few GFS runs. At this stage the GFS solution is looking more effective for achieving a cold spell than the ECM 00z was, in which light the similarity of the UKMO 12z to this GFS run may be a good sign. Time now to see whether GFS insists on ramming a broad trough across under the big ridge to the W/NW or tries the arguably more realistic feeding across of shortwave lows... or just stops everything getting across, which would also be fine by me.

Hmm so yes as CreweCold has pointed out, changes on the Pacific side have altered the balance of vortex distribution between the Canadian and Siberian sides - but this may have focused the high-lat blocking in a more useful location for us if it can persist long enough, which is still possible if the Canadian vortex segment stays far enough west and away from Greenland. Watching with interest.

What a strange period of model-tracking this is turning out to be. Big high-lat blocking versus a difficult Atlantic has long been the signal, but the former has come under some question today for the first time in about a week. Anyway, the 12z GFS has seen a sizeable step toward the ECM runs of late in that the low off the Eastern Seaboard is slower and allowing a ridge to build ahead rather than behind it. With the trough ahead, however, the model remains relentlessly more progressive in moving the storm close to the UK. UKMO has moved closer to GFS as of day 6 though in that the flow near-N of the UK remains flatter than ECM has been having it, so it's not out of the question that ECM could be the one left with egg on its face. This outcome, with no quick feed from the E. Seaboard trough, spares most of the UK any particularly strong winds or large rain totals, and instead brings a chilly polar maritime feed with some showers, most in the west. Much more appealing than the previous few GFS runs.

Well, ECM is a zillion times more amplified than GFS across the Atlantic as of day 8... https://www.theweatheroutlook.com/twodata/chart.aspx?chart=///www.theweatheroutlook.com/charts/ecm/192_mslp500_arc.png?cb=784 The deep low GFS keeps powering across looks to get stuck west of a Mid-Atlantic ridge on this run, and so it should help the blocking to the N/NW rather than interfere with it

What do we call this - a Greeberian Block? Man, the trop. vortex is hurting on this run, much as it was on the 06z. Trying for the trop. led wave break too. Fantastic stuff but still all in the virtual realm. Hammered-in wariness still standing strong for now. I underestimated GFS' capacity for making a dogs dinner from fine ingredients in the trop. but look - the wave actually appears to have impacts all the way up to 30 hPa on this run. First one to take it that far - trend or fluke? Edit: starts looking stretched as early as day 11 which gives it more chance of being meaningful. Will have to watch ECM output on the Berlin site for any signs of such behaviour. Or look out for BA's handy updates .