stodge

Evening all If I had my choice in this life, I'd take tonight's 12Z GEM OP home to meet my mother. It looked close to winter perfection. To be fair, after last night's wobbling, it all seems more back on track with the large Atlantic LP disrupting and forming a shallow trough east along the Channel and into northern France against the block to the east and north east. Even the UKM is looking like disrupting at T+144. Where does this leave us in detail? Cold, especially the further north you are but, and this may be the fly in the wintry ointment, not as much snow as many on here would like especially for central parts. The east and perhaps later the south look more like to see wintry precipitation though whether I'll see lying snow rather than falling snow in the London Heat Island is very much a matter for another forum. GFS OP promises a rebuild of Scandinavian heights further into FI - very different to Control - and this has been mentioned as a possible further evolution. I'd assume if we didn't have an Atlantic incursion, a renewed run of E'ly air would be colder than the original air mass last weekend - hopefully a bit clearer and drier as well but that may be asking a lot. Plenty to keep us all entertained in the next few days.

I think that's true though I did wonder if convection might occur as it passed over the warmer North Sea.

Evening all I'll be honest - from Tuesday onward next week, colour me confused. Huge number of options on the table, some prolonging the cold, others bringing in the Atlantic and milder conditions. The evolution of the Atlantic LP next week is far from set and that has huge implications - will it weaken and disrupt east through France or will it remain intact and head inexorably NE toward us? I don't know and I doubt anyone else does at this time.

Evening all After last night's ECM wobble and predictions of a "flip to mild", I came to view the 12z output tonight with some trepidation. Have to say, as a fan of cold and (some) snow, it all looks very decent. The models continue to struggle with near-hurricane LP in the Atlantic and how such might evolve but the idea of a west-based negative NAO seems less likely tonight. Out at T+168 and a glance at the 850s on offer GFS, JMA, GEM and ECM respectively. It's nicely cold - I know some would like -20 850s and a storm force E'ly but a -8 in mid December will do very nicely. This won't break any records but will be a shock after a number of mild early winters - it's also more than a 36-48 hour cold snap - the cold looks set for a week or more and perhaps longer for northern and eastern parts with the south and west always more prone to Atlantic incursions and you look at those models running LP east along the Channel and you think the Midlands might do very well in terms of a pre-Yuletide snowfall. As to what happens nearer Christmas and beyond, I've no clue. It wouldn't surprise me to have some milder interludes especially again for the south and west - the question is whether January will see a reversion to type or whether the synoptics and teleconnections for cold can remain in place.

Yes - that ridging holds the east based negative NAO in place and forces Atlantic LP east or south east into Europe - absolutely vital.

Evening all I suspect tonight's 12Z ECM OP will bring on the usual hand-wringing, toy-throwing, "I told you it would soon breakdown" nonsense. It's a classic west-based negative NAO and is and has always been a risk of relying on the Greenland HP for your cold. To be fair, the ECM 12Z on Friday sang a similar song and it was supposedly "game over" then (except it wasn't). On a much more positive note (for cold and snow fans), most of the other 12Z output is very reasonable - GFS OP and JMA lead the way with a couple of superb offerings. GFS Control is more like ECM 12Z OP so it's a recognition of the possibility of the evolution of the scenario which translates into "this isn't what will happen but what might or could happen". The truth is there are plenty of options once you get to FI (there always are, you just need to figure out where FI starts). Tonight's T+192 charts from GFS OP, ECM, JMA and GEM - plenty of differences in the synoptic profile especially (but not exclusively) to our north west and north east. So, what can we say? A cold and increasingly colder week ahead and real chances of snow for many later in the week and over the weekend. From there, a possibility of milder conditions returning with the greatest risk of that happening in the south and west while the north and east are more likely to hold on longest to colder conditions - not exactly rocket science.

Evening all The mood certainly darkened by the ECM 12Z though there was a JMA 12Z earlier in the week which also highlighted an evolution to a West based negative NAO and that's always the risk if you go down the Greenland HP route. It's what did for the December 2010 cold spell in the end and has occurred since. The tendency is for the HP to slip further west towards NE Canada and that lets back the milder air. Of interest, both JMA and GFS this evening offer the teasing prospect of the LP keeping to the south over France - if you like, a halfway house (and would be excellent for some in terms of snow). As for ECM, looking at the Northern Hemisphere view, the key moment is T+144 to T+168: The HP splits in two and the trough forces its way through the gap from the south - one lobe of HP heads west, the other heads SE back into Eurasia. Compare with GFS which keeps the integrity of HP and maintains heights over Scandinavia which stops the trough coming north and holds the LP further south - that's the modelling area I think to which we should be paying most attention.

Evening all It's rare I sit slack-jawed at the model output but tonight is one of those occasions. Last night's GFS OP wobble is firmly forgotten - the question now is not whether it will be cold but how cold and for how long and which areas will be lucky enough to see snow and which will miss out but that will be for the day to day modelling. The first stage will be for the initial overcast conditions to be replaced by some colder, clearer and drier air from the east (may take 4-5 days) then to look for this really cold push from the north east and whether we can get the LP to sit just to the south rather than right over us. Hopefully this will be our most "interesting" December weather since 2010.

Evening all A significant pattern change underway and the first manifestation was my unexpected two-hour downpour over East London last evening as the rain band stopped with no energy to push it further east. Classic "Col" situation - only 3% of UK's energy being supplied by wind currently - as we await the rise of pressure to the east and the onset of our E'ly from tomorrow onward. Plenty of options into next week and the question seems to be whether we keep the trough to the south of us or whether the trough extends across the whole of the British Isles. Looking at some T+144 charts tonight: - GFS OP, ECM, GEM, UKM and JMA respectively I'm struck by the lack of consistency - FI is certainly T+96 or at worst T+120 and I really don't know where we go next week. Ideally, we'd like the LP to stay well to the south initially and keep the ridge strong to the north. ECM creates a classic Greenland HP N'ly but at the other end GFS pulls us back into a mild Atlantic flow. I don't know.

Evening all Interesting to see the last hour - a narrow band of rain has stalled and has intensified in situ over east London and it's now a thoroughly miserable and wet evening.

Evening all More output this evening and I suppose we can now see both the holy grail and the bucket of (fill in the blank) and the recognition there's often a very fine difference between the two. I'm seeing two issues tonight - neither at this stage substantial but both possibilities. First, we'll get the synoptic pattern in place but the air mass just won't be cold enough at least initially - yes, it will be quite a shock after the anomalously mild November but it will be "just below average". The truth is we may have to wait (and hope the pattern holds) before we can access the much colder air far to the east. It's always worth looking at where your E'ly is originating . The LP to the south means the air flow is coming from a more southerly origin than a good old Siberian blast. That may spoil the party at least initially for those hoping for a rapid transition to very wintry conditions. Second, and my concern, is we move too rapidly into a west-based negative NAO. JMA hints a this tonight and we've had a couple of recent experiences of the HP moving very rapidly east to west across the northern hemisphere and quickly putting this in the "wrong" NAO pattern allowing much milder air to return from the SW. I'm not over-keen on a rapid transition to a Greenland-based HP - the longer w can hold the core of heights to the north or north east, the more likely we can tap into the much colder air mass. For those who think I'm being too cautious or negative, I'll just put this up: This is the Holy Grail of winter charts synoptically - it really doesn't get much better than that. IF we all get through the next two weeks and get something even close to that, we'll all be happy (or we should be).

Evening all Some excellent charts for fans of cold - for fans of snow too in some areas. One or two complaining about timing but the process has been marching forward for some days and begins with the disruption and dissolution of the Atlantic trough. From the 12Z suite, ECM, GFS OP, GEM, JMA and UKM at T+120 - not some fantasy island timescale but within the reliable. They may not be identical but they are very similar showing the rising heights to the NE and the retreat of the Atlantic trough far to the west and, for the most part, my fears of too much residual energy in the northern arm of the jet seem to be misplaced. It's the first and important stage of the cool down - get rid of the Atlantic. Now, that doesn't mean immediate cold - we need to build the heights and get in the continental drift and then wait for the really cold air to move across Eurasia toward us. That may take a few days and it's still possible the flow will be more ESE than East or NE and that may impact snow hopes for many but for those wanting cold first, it's more steps forward this evening. The modelling of small and secondary LP at this distance is always prone to error - it's the bigger picture which we need to consider and at the moment and coming into the reliable it looks like a significant pattern change.

Evening all The issue for me is how much strength will remain in the northern arm of the jet - ideally, as per GFS Parallel and a few others, it will be so weak as to be unable to prevent the HP building through Scandinavia towards Iceland/Greenland. That's more an early May than an early December chart in terms of where the Atlantic trough is and the jet being more or less moribund. Some other models keep that little bit of energy in the jet such that HLB becomes MLB - GFS OP being a good example. Indeed, GFS toys with what ECM did last night - pushing the trough almost to Iceland and keeping the HLB evolution from happening as we would like. The jet holds the HP further south closer to the British Isles. That's the evolution to follow in the next week or so.

I'm going to disagree with you both - the problem, as we've seen in recent times with a Greenland HP, is how quickly it moves to a west-based negative NAO and milder air comes in with LP from the south west. Scandinavian HPs can collapse but also withdraw east to Russia/Finland where there's a chance of a new push of heights westward.

Evening all Plenty of comment on tonight's 12Z output as you might expect. The remarkable thing is the extent to which the Atlantic LP is forced back west next week opening the door for heights to build from the east. Parallel at T+114 and T+174. I presume this is the PV spinning back west over Canada and a weak jet. From there, the charts become progressively more "interesting" as we move into December especially in GFS far FI. Still a very long way to go of course and it's all for conjecture and amusement at this time. ECM introduces one of @nick sussex's favourite shortwaves to push the Atlantic trough east and cut off the obvious height linkage between Eurasia and Greenland - might be right, who knows? More baby steps tonight and even JMA gets in on the fun with a Scandi HP and a decent E'ly with -8 850s over SE England.

Evening all Steps in the right direction (sounds like something musical) but baby steps and a lot of detail to be sorted. Hints at blocking to both NE and NW and to be honest most would take either - or perhaps one followed by the other. As a southerner, the idea of an LP over northern France struggling to make inroads against a frigid E'ly in December isn't without appeal - far more than a straight anticyclonic outlook but I accept that scenario doesn't work so well for those further north. As others have said, it's unusual as it's often quite a mile or very mild start to December with the vortex and jet at full throttle and +NAO the order of the day. The GFS OP fragments the vortex while the Parallel move s it further west over Canada allowing heights to rise over Scandinavia and NW Russia and extend the vital ridge west or southwest over NW Europe.

Tonight's ECM gets to what I thought we might see but the question then becomes whether it will be an MLB or HLB? It's fair of course to argue that T+240 changes all the time - yes, it does - but the point about model watching is to spot trends and signals which move into the reliable which is for me T+96 to T+120. What I'm not seeing currently is a raging Atlantic with LP crossing the British Isles into Scandinavia and beyond - indeed, getting an LP across the Meridian has been a struggle so far. The block to the "east" has been and remains substantial but there's currently an early taste of cold for Germany, Poland and Scandinavia with a small LP developing in the cold airflow and that energy extends west and draws ou trough further east and south east for the first part of the week.

We know the route to very cold often starts from very mild so this would be entirely consistent if for example pressure built from the south through the British Isles and then into Scandinavia - it's an evolution we've seen before but relies on the jet being slack enough to allow those heights to build NE into Scandinavia and that's often the problem so instead of an HLB we get an MLB - to be fair, if it sits over the UK and we can get some decent midwinter inversion, you can still get ice days - no snow for those who want it but plenty of frost and fog.

Late afternoon all Looking at both GFS and GFS P from the 12Z I'm reminded of what EC46 has been showing in terms of positive height anomalies to the north west. We're not talking full Greenland HP but an area of heights extending between Greenland and Iceland. That is forcing the Atlantic LP further south riding a more southerly tracking jet and crossing through the British Isles so plenty of rain and strong winds (the latter further south) with snow possible for Scottish mountains and northern English and Welsh hills in any heavy showers. It is, as some have said, a hint of some winters past but at the moment all conjecture.

Late evening all What concerns me about the GFS Parallel and OP runs is the signal for HP to set up just to the south west of the British Isles. The problem is once that happens it hangs around for even longer than Liz Truss and we could lose half the winter to non-descript weather. One or two signals try to push it NE towards Scandinavia but we need a lot of help from the jet and the PV for that to happen.

Evening all The main feature remains the inability of Atlantic LP systems to break past the Meridian - effectively the eastward-travelling LP hit a block and either disrupt or recurve back into the Atlantic. It's the kind of behaviour we see with tropical features as they hit higher-level ridges and are forced either over them or to retreat from them. It's a million miles from a typical zonal spell when you would see LP sweeping through to Russia and beyond - this year they get as far as London and stop. That foundation limits where we can go - periodically, as now, there's just enough energy to extend the trough east or possible south east (under negative alignment) as the heights are forced more to the north or north east - this is the hope of those wanting a colder spell - the heights extend to the north and start to move south dragging the weak jet ever further south. Little sign of that if I'm being honest - indeed, the longer range charts look to this observer to be more of the same with a bit more positive alignment as eventually the Azores HP manages to ridge into Iberia and re-establish the SW'ly flow.

Evening all Considerable changes this evening on the suite of runs. After a long succession of output showing LP struggling to get across the Meridian in the face of a strong block to the east and south east, we are now starting to see negatively-aligned LP pushing ESE into Europe with the core of heights shifting more northeast and north. If so, the unsettled period just commencing will intensify into next week with successive and increasingly active LP moving across the country with the worst of the rain and strong winds to southern areas and do we start to see the first manifestations of colder air getting into the far north? The EC46 pressure anomalies hint at a possible blocking scenario for the middle of next month and it wouldn't be the first time an apparently never-ending balmy autumn has suddenly become an old-fashioned winter. Nonetheless, we're an eternity away from that currently.

Evening all We've seen this before in previous autumns - the struggle of LP to get across the Meridian and into Europe. The LP coming into Scandinavia are coming down from the Pole rather than from the west leaving Atlantic LP to disrupt close to the British Isles with heights to both SE and SW. The net effect is to keep S'ly sourced air flows so nothing too cold but increasing chances of rain as the LP disrupt over us.

Evening all GFS OP continues to weave its non-autumnal magic. Tonight sees northern blocking in extremis - charts that do a post-SSW April proud. I'm told this is manifest of the MJO moving which is a change from waht seems a long period of COD. Tonight's gem (not GEM) is from T+276 on the GFS 12Z OP - blocking to the north west, blocking to the north east and the poor old cut off LP has nowhere else to go but the British Isles. By T+360, it's gone beyond a joke - doubtful it will ever get that far of course: All together now, IF ONLY IT WERE JANUARY....

Evening all Plenty of options on the table still for a week or so hence. There's a strong signal for what could be called a reversion to normality with HP to the south, LP to the north and W'ly winds - whether the troughs and ridges are slightly more negatively aligned on some models than others is a matter of conjecture. Not for the first time recently, ECM is on its own promoting more height rises to the north. The 850 profiles from ECM and GFS OP reveal the rationale for the synoptic divergence at T+240.