Cambrian

The build-up to a more zonal flow looks good on the 12z GFS operational run, the injection of velocity appearing over Newfoundland at day 4, pushes a small trough east across the Atlantic to give us our brief northerly by day 5 and then does away with the trough altogether by day 7, the jet stream running straight east all the way through to western Russia. Day 4 / Day 5 / Day 7 Well now, what do we have here? Rarely seen this winter, and for several months previous, from the perspective of the UK and Ireland at least, this is a full on jet stream on the blow at 168h - 200mph winds at height, running almost straight west to east with us in its headlamps. At least for a short while. However, as has been a feature for much of this year whenever a full-on zonal jet looks to appear, the tendency is for it to fairly quickly flip back to something more meridional. So for the jet stream, it’s a case of - welcome back old friend, good to see you, now can you possibly do us a favour, think twice about all this west to east stuff, and dive southeast into Spain by day 10, extending a trough down south from the Norwegian Sea, allowing the formation of a nice little low pressure to our south and maybe even a slow-moving band of snow over central parts? Thank you very much, that’s very good of you.

Another way of looking at the rather stark cooling down heading our way in the first few days of the new year - the zero degrees isotherm charts from the 12z GFS operational run. I love these freezing level charts, certainly not shy of exploring the colour palette! At 60h, 2000m to 3000m for most of the UK and Ireland, with the exception of the northern isles, and even higher than 3000m in the southeast. (Incidentally, this is my contribution to the pattern matching debate about whether certain months of the year being mild determines the path of the following winter - here it is, if the end of one year is extremely mild, then the beginning of the next one will be extremely mild too.) A day later, at 84h, the freezing level range is 1500-2500m, and a day after that 1000-1500m about does it. By first thing on the 4th of January, at 132h, some Arctic-sourced air covers Scotland, the far north of England and County Antrim with the freezing level there below 500m. There is a band of rain, sleet and snow at the boundary of the cold air moving south. This front moves south by the morning and slowly fizzles out. By the beginning of the 5th day of the new year, the freezing level is between the surface and 100m for most of the UK and Ireland. A 3000m drop in freezing level for some over the four day period.

This is a two-stage transition to a cold snap from the 0z UKMO operational run, which is going to be enjoyable to follow, if indeed it’s sniffed out something more notable that the other models haven’t. Stage 1 - clearing out the very mild tropical maritime air, at 96h, +10 uppers across southern areas to see in the New Year, but they don’t last long, closer to +5 uppers at 120h Stage 2 - bringing down the air from the Arctic at 144h, subzero uppers for most, -5 and below for the northern half of Scotland. By 168h, the transition is complete, with -6 to -10 uppers for the UK and Ireland, a proper cold blast. At the surface, a few degrees either side of freezing first thing next Tuesday with streams of showers for some areas. Though the UKMO cold snap looks brief, there is a little bit of scope for a day or so more after 168h if that low in the central Atlantic gets held back a bit or held further south by a lengthier build or stay of high pressure between Greenland and Iceland, and especially if the low off Norway further develops the North Sea troughing.

“Butthats”? I know that a few of us are a bit worse for wear by now, but I’m almost certain they’re supposed to be worn on our heads. Somewhat colder (less mild) air arriving between 120h and 144h according to 12z UKMO, pushes the mildest air away as we move on from New Years Day, leaving more of a polar maritime northwesterly feed for just a little while. 120h 144h

It’s about time for some rainfall charts (and a bit of snow too!) - accumulated total precipitation for the 10 days straddling the end of 2021 and the beginning of 2022 from the 0z operational runs. These are broadly similar in terms of wet west, drier east, but the totals are considerably different between the models. The ECM gives 100-150mm for western parts of Ireland, Scotland, Wales and England, while the GEM computes over 200mm for parts of Wales. The GFS gives notably lower totals, focusing up to 100mm on parts of County Kerry, Cumbria and Dumfries and Galloway. In all three, there is a profound rain shadow effect for Eastern Scotland, Northeast England and the Midlands. ECM / GEM / GFS The higher totals with the GEM appear mainly due to the way it deepens a depression approaching from the southwest around day 8, down to lower than 975mb. The GFS forms a complex depression like GEM but holds it in the mid-Atlantic, continuing to pump up the Iberian high towards the south of the UK and Ireland, with a range of 1010 to 1025 mb, The 6z GFS op is very similar for the same time. The ECM takes the complex low further north a bit earlier, keeping the developments near us for day 8 shallower, around 995mb, in a slacker slightly chillier airmass. So for a few days into the new year, the current options are anything from really quite wet and windy (GEM), to pleasantly mild and dry (GFS), to rather wet and quite chilly with the ECM, midday temperatures on day 8 struggling in low single figures further north, with some snow on the hills.

Some interesting charts this morning. All the action is over the Arctic by day 8, just a couple of days into the new year, with a push of WAA up over the Bering Sea into the polar region. The 6z GFS operational run shows this nicely, with the anomaly chart showing a strong signal at 192h. There is a 1060mb+ high over Northern Alaska and a 1055mb+ high over Greenland. The main PV intensity is on the Eurasian side. The 0z ECM op at 192h is similarly forward with developing the high on the Alaskan side, but more reserved with the build over Greenland and more ambivalent regarding the PV distribution too. The 0z GEM takes the most direct approach, bringing us into a northerly by day 8, with some fresh snow into northern parts day 9. Again the bulk of the PV is on the Eurasian side, extending over to our north and there is the same 1055mb+ high pressures over Northern Alaska and Greenland but with an extension of heights down through the Atlantic to our west, giving us the northerly. And look where the wider spread is at 192h in the 6z GEFS ensembles, over the mid-Atlantic. This area is worth keeping an eye on over the next few days. With the PV primed for something at least akin to a split - given not with the ECM, though the UKMO at day 6 also shows high pressure right over the top Bering Sea to Greenland… - it’s not going to take much for the build to nose down into the Atlantic, and in time push the low pressure over us and then to our east to bring in some colder weather from the north. As pointed out above, the balance of energy around that block is where the current route of this winter rests. So though I can appreciate the wholesale evidence of the current runs for a sustained mild spell, for me at the moment there is too much doubt over the maintenance of the Atlantic trough so I’m leaning more towards the potential of mild for us into the new year granted, but possibly not for very long on the balance of the things up over the Arctic. I’ll be looking for GFS / GEM consistency and to the ECM over the next few days for some more support and if it’s not forthcoming then mild and wet it is for the foreseeable!

Hey, this was the scene at our place while watching the model outputs a couple of nights ago. Of course we accept that none of the models are ever truly correct, all are by definition wrong to a varying degree, it’s dealing with the differing extent of wrongness of each model run that is the trick. However, the recent period just goes to show how the inherent variability and implicit unreliability of the models can be further exacerbated in a prolonged meridional period where they are trying to handle slow, finely balanced movements of increasingly contrasting air masses, as the high latitudes of the northern hemisphere rapidly cool down into winter proper. This is a particularly salient feature in this year of extended meridional pattern. Any model of any complex system is fallible, challenging real world problems are modelled using fallible algorithms developed by brilliant though fallible people and receive brilliant though fallible interpretation, like here. It all makes for some considerable excitement at times though! We shouldn’t be afraid of screwing up. Looking ahead to the last week of the year and beyond, the 12z GEFS spreads shows reasonable ensemble consistency, rising pressure between 1008-1035mb by the turn of the year, confident of the Iberian build, shown on the 144h mean chart, with a small spread to our south. Thereafter, as usual, the spread widens, with by day 10 anything from 980mb to 1038mb modelled, with a westerly feed indicated at 240h, supplied by low pressure in the middle of the North Atlantic shown on the mean chart, but with considerable spread along the entire fetch from the west all the way from the eastern seaboard, with wider spread to our immediate west. Low confidence too over Siberia and very low over the Bay of Alaska. There is plenty of scope for this to get flipped all over the place. It’s not just the geographical extent of spread, it’s where it is in relation to us. Outside of family and work, joining this community is one of the best things I’ve done, and finally drawing up the courage in 2021 to regularly post here has helped me overcome what was a paralysing reticence to openly communicate ideas with others in a field for which I have no expertise and to learn from them through collective discussion. Work is one thing, but there we are forced to play out a role to which we become accustomed and practiced - here we can be more ourselves. There are some people on here who are truly exceptional individuals in their knowledge, support and camaraderie, the last facet being by far the most important and surprising to me, though I appreciate we all have to endure the odd bit of twittery, the friendship and solidarity is something I never expected from an online forum discussing weather model outputs. It’s been an enormous pleasure, focusing on the charts and their communication, and this has helped me finally settle my nerves after losing my Mam and Dad over the last few years. My posts next year are going to be EVEN better! Nadolig Llawen i chi gyd, Merry Christmas to you all. ??

Me, Mrs Cambrian and the Princelings are almost done with another year of keeping the mountain passes free of evil spirits and looking forward more than ever to some great Christmas food and cheer, and for me at least, a bloody good glass of Cognac. (Another….) Aren’t we a handsome bunch? Some extra excitement around this year, there’s been talk amongst the upland elves of lowering uppers and evaporative cooling come Christmas afternoon and into Boxing Day / St. Stephen’s Day, I reckon Grandelf has been blabbing again, because being a true models thread trooper, I’ve said nothing! So I thought I’d take a whistle stop tour at 12z around the islands on my magic Multi Op sleigh to check it out, stopping off at some places dear to me, a veritable A to E of great places - Athlone, Builth Wells, Camberley, Dundee and Enniskillen For each, I’m showing two of the Multi Op sleigh readouts, temperature and accumulated precipitation. Usual caveats and limitations and it will all be completely different tomorrow! Athlone, beautifully misspent afternoons on the River Shannon, some great friends there. Getting colder by St. Stephen’s Day with some chances of precipitation, so a slim chance of snow as it stands, just about cold enough for falling snow early in the day. Builth Wells, charming little market town in mid Wales, home to the best agricultural show on the planet. Turning properly cold by Christmas Day afternoon and into Boxing Day, good chance of precipitation too. Similar profile for many places across the English Midlands as well. Camberley, the lovely, leafy Surrey town, some great pubs and some of the most pleasant random chats I’ve had anywhere. Some rain around, but looking to stay just a tad mild at the moment for snow over the Christmas weekend, could well change though. Dundee, home of the RRS Discovery, I spent some great times there as a research student in the 90’s, brilliant city. Looks cold and dry next weekend, chance of some snow perhaps on Christmas Eve. Enniskillen, always stop there on my summer tour on the way through to or coming back from the west, very kind people there. Certainly getting cold enough second half of Saturday and into Sunday, some chances of precipitation in the same timeframe too. Some good snow chances for many next weekend then, though it will be a big maybe for us all until we see it falling out of the sky, but excitingly there looks to be potential for even better chances between Christmas and New Year. Enjoy the run up. ??

My first (and probably only) gratuitous Christmas snow ramp. But if it’s still looking good for coming in at T144h and less it’s not going to count as ramping! An exceptionally snowy northern hemisphere at 240h for Christmas Day from the 12z GEM operational run. Good to see snow cover over large parts of Europe and the UK, but my favourite bit is Hudson Bay, Canada - it’s largely frozen over with up to 10cm of snow settled on top of the ice. And my favourite 500hPa charts of the day from the same run, again for 240h. The first one is just so out there. And the anomaly chart’s a real beaut for snow too. Just ten days to go and a fascinating set of models and great posts today - Santa hats all round guys??

Much more likely a barrel of booms than a barrel of boobs around here today

The solid story of the next 10-15 days is a very welcome one of drying out, the total accumulated precipitation on the 6z GFS operational run is pretty much the same at day 3 as it is at day 15. 72h / 360h Though not strictly meteorological charts, though sometimes important for fog, Pete @Ed Stone , this is reflected by the GFS soil moisture charts at 6h, 120h, and 240h, by which time the high pressure has done its job and doesn’t change much thereafter, the drying out as complete as it can get with such little daylight around the solstice. 6h / 120h / 240h - the pink colours show saturated ground, thankfully receding by day 10 Of course, the less solid story the models and us bunch are grappling with is the evolution of the high, particularly post-day 8 and where it’s going. For what it’s worth, I get the feeling that the GFS operational is over-complicating the evolution with consequent inter-run variability that’s in danger of missing the most straightforward evolution. The last few control runs, on the other hand, like the 6z control run here, have consistently shown the high at day 10 very near the UK and Ireland ridging north towards Iceland. Lovely crisp weather for the shortest day. By the end of the run, at day 16, a couple of days after Christmas, the high retrogresses west to the mid-Atlantic, elongated northwards, which brings us into a very promising looking northerly (note the polar high, tentative Aleutian ridge, split PV profile), which could well be where we’re most likely to get our next good snow from, i.e. UK high migrating west rather than north, with troughing down its eastern flank. Whatever happens, it’s the continuation of a fascinatingly different year of weather patterns and model outputs, with a good chance of at least seasonally chilly weather and perhaps even a wintry twist towards its end.

Yes, @Battleground Snow, it is a shame the GEM op stops at 240h. However, the control looks “sort of” similar at 240h, at least in terms of central location of the high And takes the high to our north at 300h, establishing a continental feed And by 360h, a very interesting setup with the high near Iceland and a low coming up into the southwest just in time for Christmas And look at all that stress on the PV

The 12z GFS mean gives us less to hang our coats on than usual at day 8, with a very wide spread among the panel members at that time, particularly to our northwest. Within the envelope, solutions as varied as pert 21, with a lobe of the PV dropping down to northern Scotland, all the way through to pert 30 with the Azores high linking up with a Scandinavian high through northern Britain and Ireland. Knowing the tendency of the GFS to default to mobile westerly at the first throwing of the pattern, i.e. Arwen and Barra, the degree of uncertainty is quite telling. After a wild one this week, it wouldn’t surprise me if it settles down quite smartly by mid-month with established surface cold to play around with for several days in the run up to Christmas.

Thank you, @PiscesStar. The post was a reflection on the last few runs of the GEM model showing further episodes of the same broad pattern that we have been looking at off and on for several weeks now, that of an undulating meridional jet stream pitched at the UK and Ireland along a NW-SE trajectory from Greenland. This keeps us in a slowly mobile pattern and rather chilly air flow from the northwest, with some occasional shots of colder air more directly from the north that can bring more general chances for wintry weather, like this weekend. It has to be said that it’s not the only pattern being seen in the models and there are some indications of potential for a change to having high pressure to our north or over Scandinavia after the next week or so which could bring us into a more sustained cold flow from the east. The 0z GEM operational run keeps this NW-SE pattern again this morning. Here’s an example in the 72-96h timeframe. The meridional pattern undulations in the Atlantic jet stream at 72h lead to the arrival of a low pressure system in the North Sea, where the jet stream buckles. If travelling in the stream, from NW to SE, the lows are forming at the left exits of the jet, where it takes a turn to the left. By 96h, the jet stream dives more directly south, bringing us into a more direct northerly with some colder upper air, perhaps cold enough for some snow in northern parts and on the higher ground. It’s similar to how the jet stream was over us this cold Sunday morning. There are further repeats of this general pattern later in the run, though each buckling is slightly different in amplification and in how far south it is, which means that some of the northerly or northwesterly blasts are colder and more sustained than others, some will have more precipitation wrapped up in the circulation of the resulting lows, and these details are getting firmed up at the moment all the way through to only 2-3 days before the lows arrive.

For the second run on the trot, 4 shots on the 12z GEM operational run, here at 6h, 96h, 168h and 204h. Occasional ridges and buckles in the NW-SE aligned jet remains the pattern for the foreseeable future.

That middle one, the 60h chart for midnight Friday into Saturday is a great snow fax chart for any November - the 528 thickness line is all the way through to the south over France and the Low Countries, taking in the whole of the UK and Ireland. And that really does look like a little pool of 510 thickness taking in part of the curl of the occluded front over Wales.

It doesn’t seem like @Griff is on, so I thought I’d better post the 12z GEM op pattern, which is for a chilly outlook for the next 10 days. The swifty at the weekend potential snow event at 66h A refill by a week today at 168h And maybe a quick little half just before closing time at 228h It’s a persistent northwesterly pattern, no PV over Canada, fitting with the EPS clusters up to 240h from, @Allseasons-si, above, the touch of PV over Greenland now and again is only serving to occasionally direct low pressure southeast to Iceland then on to southern Scandinavia, tucking us back into the northwesterly. For us, a range between WNW and NE winds depending on fluctuations in shape of the mid-Atlantic ridge and the precise tracking of low pressure systems down its eastern flank, still plenty of potential for corrections almost up to the last minute like we’ve seen and are still seeing for this weekend’s low. As @Mike Poole points out, some reasonable snow chances, though nothing long lasting, mixed in with with gales and rain and drier days, certainly not mild, during the next 10 days that join the end of autumn with the beginning of winter.

Looking forward to day 9 to 10, after good prospects for some wintry weather next weekend, the 12z operational runs - the ECM and UKMO are broadly similar at 168h, they both set up a shortwave to our northwest, the GFS not picking it up yet, at least at 12z, though it does pick out another one a couple of days later. 168h ECM / UKMO / GFS at 12z Though I’ve just seen that the 18z GFS op has picked it up, at 168h, albeit six hours later with the stagger, but it doesn’t develop it subsequently in the same way as the ECM. The ECM has better pointing of the low perhaps, and runs it southeast just to the north of Scotland into the North Sea by day 9. The low sets down anchor there for day 10 with an Iceland wedge guarding the door. ECM Day 9 / Day 10 / Day 10 uppers Now take away prior expectation, and that ECM chart is a good chart for the first day of December. It looks from the kinks that there’s a wide cold front coming down into the north of Scotland by day 9, with plenty of showers likely with that track on day 10. Reasonable enough uppers, so some potential there for some lying snow for higher areas close enough to the sea to catch the showers, more general cover in Scotland north of the central belt. And you never know with that kind of pattern and long sea track, local developments are always possible. It’s what might happen afterwards that interests me most though. Looking at that day 10 chart, the Azores high is drifting south if anything, it’s certainly not going east and the pressure is dropping over Iberia, and more, the Iceland wedge might well hold long enough for that little rumple to the northeast of Iceland to run south and develop into a proper feature to prolong the northerly pattern further. It’s all a bit precarious, well we’re 10 days out for a start, and also as it relies on a shortwave prolonging the northerly pattern. It does go to show though the potential for these pesky little lows to sometimes work in the favour of keeping us in the colder pattern or at least give us another northerly shot after a couple of days close to the high. It looks like it’s the broad setup for a fair while, so it will be very much worth keeping an eye out for these smaller features too.

So here we are, into the last ten days of meteorological autumn. Where on earth did 2021 go? New thread just to answer that one please. Rapidly approaching winter, we’ve been testing ourselves with seasonal forecasts that just don’t add up, a core PV clearly enticed away from the dark side, a becalmed Atlantic (an inheritance from the end of last winter and the standout feature of 2021), to the point of the mid-Atlantic high and / or an extension / migration of it over towards Greenland (or sometimes pushing out east towards the UK and Ireland or just to the south of us) almost being a given, but with the lack of real biting heights into Greenland to push it over the line and deliver a rare kind of early winter pattern that we could find memorable. No real signs of a Scandinavian high either. At other times, for many weeks earlier in the Autumn we have experienced extended periods of observing a diving Atlantic trough, leaving us to count the days that we are either side of it and to grapple with the failed notion of identifying a regular periodicity on which to hang our coats. Not so much “I’ll get my coat”, more like “where the heck is my coat?” The models of the last few days have flirted with everything from a 4 day cold snap to a 14 day snow on snowfest, particularly for upland parts, with some flitting frissons of excitement for the more populated areas. But I can’t help getting the feeling that what we’re seeing being modelled is only variations on a core theme, inter-run mathematical variability on the same central pattern that hasn’t changed, it’s still A) 2-3 weeks mild and relatively dry (out of the trough), e.g. 6z GFS op at 0h - a slow, slow mainly blocked pattern, fluctuating between no meaningful Atlantic jet 6z GFS op at 84h involving protracted periods under high pressure…. 48h / again at 282h and the occasional appearance of a bit more of a concerted effort with typically a NW-SE aligned jet stream dipping down from southern Greenland to Ireland and the UK. 6z GFS op at 156h Eventually leading to B) maybe up to 1 week cold and wet (in the trough), e.g. 6z GFS op 168h through to 6z GFS op 264h, though you fancy the GFS is a bit rushed in moving it on. This is the same broad pattern we’ve had for most of the year. Some of the ridges and troughs at times might be a bit sharper than others, more V shaped rather than broad U shaped, e.g. 192h with a more direct northerly ensuing, but these variations seemingly follow ia normal distribution rather than a trend one way or the other, depending we’re told on a pulsatory fluctuation of warming / cooling events in the Indian Ocean and the downstream resonance, or lack of, in the Eastern Pacific. It’s meridional but it’s never looked like getting stuck, whatever the hemispheric wave pattern, it’s slow but mobile. As a result, for us, the excitement of a few more days under the cold part of the trough is more than balanced out by the delay in waiting for it to arrive, and further quelled by the realisation of its ultimate brevity. So if it carries on like it is, and there is currently no major forcing to suggest otherwise, then we’ll see plenty of snow over Scotland but also get some exciting marginal snowfall events further south at times, particularly on the higher ground 192h though not exclusively so 231h but the potentially wintry spells will only last a few days at a time, before we come under the next high. This could be good in time though, as we get into winter, if lows drop down, do their stuff, the high comes in from the west for a bit, it turns frosty, we get a homegrown cold pool, high sinks south, next low in from the northwest which might dig a bit further south at times, then we could be looking at more general snow events, with a good chance that the high will build back in from the west around the top of the Europe-bound low, bit of a northeasterly for a while as it comes in to trap in the cold air for a few days of general lying snow. It’s a pattern with plenty of possibilities and one that if endures could bring us more snow events and more days with lying snow than most winters while still ending up with around average temperatures overall. It’s been a truly fascinating prelude to winter and I’m really looking forward to seeing how it all pans out.

On the 18z GFS op run , one of the critical elements here is the role of the southern arm of the jet stream over Southern Europe. It catches the funnelled energy of the fragmented V shaped trough over Northern Europe to form a Genoa low at 168h. The low and fragment of jet then move east clearing the heights out of Southeast Europe by 204h Allowing the unobstructed sinking south and broadening of the trough with the UK and Ireland sat at its base at 240h, and opens the gate to that chunk of vortex and a wide wall of cold air to make us a visit. Awesome.

The 0z UKMO has better amplification into Greenland at 144h than the 6z GFS operational run for the same time, is slightly slower with the southward advance of the cold air, but would keep us in a truer northerly setup for longer. UKMO / GFS By 192h, the 06z GFS brings us back to the idea of a steep NW-SE trajectory on the jet stream directly down from Greenland and a deep, diving trough through Scandinavia into Europe that we’ve been talking about for a few weeks.

12z GFS op at 180h, almost in the reliable for the broad stroke idea at least - settling snow over the hills of Scotland and Northern England with a frost to follow later in the night, at 186h

Wow, -4 uppers into the north of Scotland at day 10 ! (Got my sarcastic comment of the day out of the way with before the kids get home, feeling better now).

Before the next ECM run becomes available in a few hours, time to reflect on what was another very good run from the ECM at 0z. Though the operational run put the trough down through Ireland at day 10, the progression from there is only going to take it eastward. Between 216h and 240h, as well as the southward dive, there is also some eastward movement. It looks like it’s going to drift over the UK into the North Sea from there during the following day. Add to that the eastward correction we normally see closer to the time and that’s in a good place for a northerly in time. 216 / 240 Indeed, the mean at day 10 already has it further east, the “averaged” trough coming down over Scandinavia with a low in the North Sea. The amplification up into Greenland on the +240h op is very impressive and again huge amplification for a day 10 mean, which are often as flat as a pancake, with a strong southward plunge taking the trough all the way down to Northern Spain, it doesn’t look like it’s finished moving south either. Even though it’s day 10, so usual huge caution, if it sets up anything like that, that pattern isn’t going away anytime soon. On the mean, there’s 1020mb+ pressure all the way across the pole from the Azores, up through Greenland, over the top to the Bering Sea. At this stage, the two parts of the PV are acting quite independently of one another, with a striking symmetry in what looks like a 4 wave pattern. The Hudson Bay low only serves to help pump the high up into Greenland, before it begins to fill, then another blast of cold air down its western flank likely sets up a repeat.

Another day of some brilliant atmospheric charts and great posts, so before the pub run melts the server, a much duller post with some basic surface temperature charts. The 12z ECM operational was a great run. Cold surface temperatures beginning to appear in the 10 day timeframe - indeed, some night frost for many creeping into the reliable at 162h, nights getting well below freezing for most by day 10. The daytime temperatures are eye-catching too. In single figures for all at midday a week today, low single figures for the high ground. Temperatures by day 10, in a word, struggling - subzero maxima for good parts of Scotland and the hills of Northern England. A bit less cold further south and west, but bracing all the same.