LomondSnowstorm

A cooling trend through this week, and while it should generally be drier than of late the GFS 12Z throws out some interesting potential for Saturday. Maxima very much on the cold side: A front, albeit quite a weak one, stalling over Scotland: Uppers easily good enough for snow: After that it's a bit up in the air, with some sort of battleground with Scotland in between troughs, with winds likely anywhere from SE to westerly, but our latitude means that with a generally slacker flow temperatures are likely to remain below average: Scotland could be one of the few places in Europe below average for the coming week; although there's no way that chart's going to remain as mild for most of Europe into Week 2: I'll do a full model report once they're all in but the GFS is pretty encouraging and by the end of low res is poised for the undercut:

Shaping up to be perhaps the first stella FI (providing low res doesn't find a way to screw it up). As CC said high risk but high reward stuff, without either the following low or the one after that carving out more amplification and without the lowering of heights into Europe it could easily go the other way but this time it just about looks to work out: Has a real look of last January about it with height rises just enough sufficient to deflect the jet on a NW-SE trajectory, at least as a starting point:

Some improvements on this run, including signs of that Northern Italian low which Carinthian mentioned earlier setting up and a negatively tilted trough, along with a slight westwards correction. The problem perhaps with an overly 'negative' trough rather than vertical is that the pattern is a bit too far north for us at the moment so we'd probably need another bite (since the 'second' low phases stopping that opportunity for Atlantic amplification on this run) but upstream it looks like we could get there with the next low, which is pretty well oriented at +144:

I'm assuming the people who so were so righteously defending the GFS will finally jump off the 'zonality express' and at least consider the potential on offer now that it's (predictably) moved very strongly towards the more amplified solution over the last 36 hours? In all seriousness, a finely balanced setup, and it will be interesting to see whether the Fax charts are modified (I suspect they will, but how much by?). It's entirely possible, and quite likely, that tonight's UKMO op is simply one of a smallish cluster of less amplified runs (it'd be surprising if we didn't see the odd 'rogue' run given these are deterministic models dealing with complex setups at fairly long timescales) and that by tomorrow it'll be back on board with the initial amplification. However, we certainly can't take that for granted given how finely balanced the setup is. If we can get past that point we're still not done with the drama by any means as we still need some kind of 'trigger' low to usher in the easterly flow, and with heights over Europe higher than we'd like that won't necessarily be an easy process. I suspect we'll here more about 'Shannon Entropy' in the coming days (certainly if we do get the initial amplification) with troughing attempting to disrupt southeastwards and the potential for the British Isles to be caught in a battleground between undercutting troughs and the ridge to our northeast. Certainly the most interesting period of model watching this winter, although that's akin to saying that it's the best Man Utd performance of the season

Looks like we're heading for another GFS/ECM squaring off. In this instance it's not quite as massively dependent on Greenland heights but rather a Euro-Scandi transition (most likely, although it also requires that upstream amplification for sufficient WAA to 'pump' the high up). Taking the GEM 12Z op as an example of roughly what we're looking for, we can take the starting point as +48 when the initial mid Atlantic ridge is carved out by that massive US coldwave, the leading edge of which is causing some pretty chilly temperatures at the Packers game I'm watching at the moment. As I said a few days earlier, this was quite crucial to future developments, and although the ridge in itself doesn't bring in any cold it does initiate the change in the pattern. The fact that the GFS was quite clearly wrong on this feature (something which its own modellers suspected a few days ago) should perhaps give an indication as to which way the next dilemma is likely to head: Anyway, roll it on three days later and, while the lows have made it pretty close to fully phasing, there are lighter blues appearing between Greenland and Svalbard, with signs of a split jet north of the UK and a cross polar flow: The 'BUS Siberian Express' would require more WAA, with the trough really digging in further south just off the eastern seaboard and stalling, something like this: Of course, with the jet significantly stronger such a setup is very difficult to achieve straight away, which is why we'll need a few bites of the cherry to get there. Anyway, the main difference between the above and the GFS is that the GFS spills the Atlantic troughing further east, shunting the polar flow and subsequently the increased heights further northeast: With the GEM's slightly more significant blocking the trough stalls in the Atlantic and disrupts southwards, once again becoming sharp and vertical and giving another dose of WAA: The next stage, and the one that's been a bit harder to achieve on the modelling (other than the ECM ens which is a bit better for this) is a euro trough - we need some kind of trigger low, either a flat southerly jet from the west or a shortwave dropping in from the northeast, to extend the easterly flow our way and the further west the heights setup the easier it is for the cold to make it. The GEM get very close to perfection with this but the disrupting low comes in a bit too far east, and by the end we're on the very edge of the cold pool: Because we aren't quite taking the direct route there are still a good number of obstacles to overcome, including shortwaves in the wrong place (i.e. East Greenland or where we'd like our blocking to set up), shortwaves not forming in the right place (i.e. just to our south/southwest), the PV heading back east into Greenland and shunting the whole pattern further east or even a northeastwards ridging of the Azores. Almost all of these combined scuppered a seemingly stuck on easterly in December 2012, and in this instance any one could scupper this particular attempt at an easterly. However, as long as we get to some kind of blocking setup, even some sort of UK high, there will likely be further opportunities down the road, particularly if we can get a block which is far enough north to deflect the jet southwards. In summary, we're slowly but surely making progress, and while it may be another day or two before we see some proper FI stonkers those kinds of outcomes are certainly in the mix at the moment. The GFS almost always flattens the pattern more than any other model and even in instances where it comes closest it usually has to make some kind of adjustment upstream in the process. The modelling consensus appears to be broadly with the more amplified outcome and the GFS does appear at least to be making moves towards the modelling consensus run by run. In summary, trending very definitely less windy, quite probably drier and likely colder, with the depth, timing and longevity of any major cold spell still up for grabs. Maybe a 30% chance of hitting the beasterly jackpot within 10 days (increasing to 50+% if you include setups which give a shortlived chance of snowfall based on either a battleground setup or a weakish southeasterly) with the longer term potential looking very interesting as well, accounting for the usual shroud of zonal FI fog which often lingers over the models, particularly the GFS. The MT is pretty fun at the moment, with the usual GFS fan club coming out to play(as Nick pointed out those guys are few and far between across the pond where the ECM is considered as something close to the gold standard of medium range modelling) along with the mildies, uber pessimistic coldies, overly optimistic coldies and even a few people who actually look at the models before posting

Cheers for those, they hadn't updated since the 1st so we were waiting with baited breath for them. Looks like they've all largely stuck to their guns, with the ECM wanting to kill the convection in the Western Pacific but with the UKMO and GFS wanting to ramp it up. Unfortunately the ECM has the best skill in forecasting the MJO but all of these forecasts are a bit temperamental to say the least so it's rather up for grabs at the moment. A few of the models want to move the MJO backwards into phase 6 or even phase 4, which is quite amusing, and the BOM is displaying almost 0 skill whatsoever: We might as well use TEITS' 'easterly seagull' method from a few years back

I think as with anything in science it's always helpful to understand why something's happening - if you can find a plausible driver for the synoptics modelled (or in this case multiple drivers) alongside good model agreement then confidence in that outcome occuring is obviously much greater than just with the latter alone. It still looks like possibly quite a long haul to deep cold (if indeed we ever get there) most likely 10-15+ days out, but I'd like to think that those of us who are pointing to the increased number of colder and more blocked runs in the ensembles are doing so out of a bit more than just optimism (although there's always going to be at least a bit of that when it comes to snow and netweather).

The ECM op gives a more gradual progression to proper cold, with a cold continental high extending over us. I don't mind this evolution to be honest as it gives us a break from the endless stream of lows, would give some nighttime frosts and also dries the ground out, which is good for everyone. I feel a switch to a proper easterly in the next 10 days is still a bit of an outside bet, but that there are some decent initial signs after that to around the 20th-25th, when both the stratospheric warming (at the moment it looks to be minor although possibly still producing at least a temporary split) and perhaps the MJO alongside AAM become that bit more favourable. Until then, I think we could still see some short sharp snaps but the building blocks are there for later in the month at least, regardless of the exact evolution it takes us to get there.

It always looked pretty transient in fairness but on yesterday's NMM it hung around the central belt past sunset which pushed temperatures down a bit. I was a wee bit perplexed that the MO did make as much of a deal out of this one as they did, considering we've had a few similar ones where low ground snow was possible but not exactly likely, but they must've seen something in their modelling, possibly based around that NAE 6Z chart which showed a rapid localised decrease in the freezing level associated with the precipitation. Evidently, that didn't come off so all we had was a bit of sleet to higher levels: Would've been helpful to see the Euro4 modelling for the precipitation but sadly this doesn't appear to be available anymore for some reason, and instead all it shows is the surface pressure: Gales return briefly tomorrow up the Irish Sea into the Firth of Clyde and across the Minch:

I think the key was the word 'slack' - if we'd had even a modicum of surface cold ahead of the front it could easily have been a very snowy setup. As it is we were reliant on latent cooling from heavy precipitation to drag the freezing level down and that's a pretty difficult ask, especially since the front appears to have been a bit weaker than initially modelled and also because it's the middle of the day after a not all that cold night.

DPs look a bit too high initially across the board for snow from the approaching band but perhaps in the eastern borders they'll be pushed down once the precipitation hits. Charterhall's sitting at 2.3/0.8C so getting pretty close to snow levels. With a bit of intensity and the current 'stillness' we might just be able to drag surface temperatures down enough for snow for inland areas with even fairly modest elevation. Also, this: http://forum.netweather.tv/topic/78936-model-output-discussion-1st-january-2014-06z-onwards/?p=2884354

Having looked at today's modelling, it's clear to me that a change is a coming to the NH pattern. Whether we'll get a proper cold spell out of it or not is hard to say (my bet would be yes but not quite yet, and we'll have a few near misses or perhaps even 24/48 hour hits before we get to that stage) Key trends across the board are: increased heights across the Arctic Increased amplification upstream (i.e. slower progression of US storms off the eastern seaboard) Increasing signs of cold pooling starting to emerge across Western Russia and into Scandinavia Decreased jet strength and signs of an Atlantic which is finally running out of steam Easterlies emerging to the northeast with some models giving a full on cross polar flow All of these are positive signals for European cold, and so tonight's ECM op should come as no surprise: The 00Z NAEFS remains a bit of a headache by highlighting perhaps the only things stopping a deep cold spell at the moment - a Euro high: Without these relenting, we won't see a deep cold spell, although Scotland could easily benefit from a fairly snowy battleground setup either from the east or the west as troughing slows west of the meridian and disrupts over us (see the GFS 6Z): The models continue to slow the upstream low down (we saw this occur last January as well) increasing the window for the initial amplification in the Atlantic which leads to the ECM easterly. I'll compare tonight's ECM +72 to the GFS +120 (it's annotated but probably not all that legibly): First, the GFS +120 from New Year's Day, which is a bit of a no hoper for cold up to D10: The problems with is are as follows: too far east with the US low positively tilted already phasing with the low in the Atlantic For all of these reasons, this run, in spite of decent overall NH trends such as the Arctic high and even a cross polar flow, gave a basically zonal pattern: Still promising for the bigger picture, but not so much for us in the short term. Now, tonight's ECM at +72: A very promising pattern, with a split flow evident and the low negatively tilted. In summary, there's a lot of interesting output in the models at the moment, and although it isn't yet overwhelmingly cold, the building blocks for at least a transient easterly and a generally below average outlook temperature-wise are in place.

Bit of snow potential tomorrow courtesy of a subsidiary low and consequent wrap around occlusion: Looks to be very localised though to central eastern Scotland (at least as far as low ground potential is concerned) with 950s seemingly reflecting some major latent cooling associated with heavy precipitation combined with a slack flow: Some reasonably heavy precipitation, so around 2-4 inches for high ground, less so for lower ground as it appears to be almost entirely intensity dependent so may well start off as rain: The GLOSEA4 model is playing up which means we can't get its take on it, other than that the thicknesses at both 850 and 500 level are both too high where the occulsion is, only falling behind the front when things should start to clear northeastwards: Still time for changes to the northern extent of the heavy precipitation (which was previously just up to the Forth) and it should be noted that the NMM shows it as rain initially before turning to snow for low ground on its back edge by tomorrow night. I haven't really looked at the models in much depth this year so far due to being laid low with this dreaded throaty cold/flu/manflu thing that I've had since Christmas (and in fairness which wasn't exactly helped by the Hogmanay festivities) but I'll endeavour to do a summary post after the 12Zs come out.

Yep, although I suppose the 'spinning plates' analogy does lend itself to the idea that the colder the strat the more spectacular the fall. It could be a 2012 like event where we didn't exactly see the best tropospheric reaction or it could be like a 2009 where there was a strong initial reaction but with a slightly disappointing longer term downwelling, or it could be like last winter where hemispherically at least it was a really very prolonged and spectacular reaction which set us up for an exceptionally cold March - it's too early to give more than an educated guess on that front IMO, but I rather like my at the time massive longshot 25th January beasterly prediction at this stage

Just out of interest, a wee comparison between the 'textbook' pre and post SSW tropospheric height pattern and the current predicted 8-14 day pattern: and the +168 ECM ens to give a clearer picture of the Eastern Hemisphere: Key features (shown by the dotted marks) on the split SSW (if it goes full blown SSW it will be a split rather than a displacement) precursors are the Kamchatka Ridge, Aleutian high and eastern US trough, and to a lesser extent the UK trough. Roll the idealised model onto 0-30 days and then 30-60 days and you can see why this is such an exciting development and also why charts like the ECM control are likely to become more and more commonplace in FI in the coming weeks. Exciting times... Cheers to both of you for the link by the way

I think that's almost perfect to be honest, slightly depends on the upper cold pool I guess but our mean heights in winter are so low that even a little bit above average is still pretty low in an easterly type setup. This is December 1st 2010 in the same format for example: And the actual: It's not quite as good as the above on second glance at the air pressure charts but it'd still be quite a decent wee easterly even at that. Even slightly lower heights with a similarly weak central pressured high (i.e. everything shifted a bit north) would give potential for shortwave activity developing with slack pressure and a good convective setup if the flow is stronger. Anyway, all speculative yet, would be good just to get to a stage where we can discuss charts like that as a realistic possibility again. Also, as Chiono pointed out on the MT we don't have the geopotential 10hpa charts (I'm sure they're out there somewhere though, if you're on Lorenzo link me up to them as it would be enlightening for the 18Z) but this looks pretty close to a full on split SSW for the 15th: edit: no need, appears to be pretty comprehensively covered on the strat thread as ever (also stupidly forgot that SSW is defined by reverse of winds at 30hpa rather than 10hpa )

Typical - I change my LRF a couple of days ago to reflect the probability that there won't be an SSW before the end of January and then this happens: Stonking run though, really exciting stuff for the next part of winter, especially looking at where the core of the warmth is...

Vortex almost gone (or at least what's left of it is on holiday in California )

On the plus side, there's an absolutely mega upgrade on the 10hpa strat warming:

Quite agree, I'm sure I found the same thing last year - getting the height composites to verify for the NH and even getting broadly the correct synoptic pattern is significantly easier than getting the temperature or precipiation forecasts close to any reasonable measure. If I'd just taken the analogue years at face value though I would've come up with a below averaage anomaly, at least using the NOAA air temperature dataset (next year I'll take the Scottish mean for the composite years and see what it comes out with). Breaking the analogues down to individual months might also help, although in general most studies tend to make classifications based on DJF as a whole, and generally conditions in terms of the QBO or ENSO don't change a huge amount through the winter. Anyway, cheers for the feedback! edit: did a quick excel job to give the temperature for my analogue years for each month, Scottish mean is as follows:December: 3.15C (obviously about 2C too low) January: 2.28C February: 1.94C Certainly a signal there for a 'descending' winter given the average for the 1960-2013 period (the set of years I used to select my analogues from since anything prior doesn't have enough data for ENSO, QBO etc.) is: December 2.86C (a good 0.4C lower than the '10-59 mean) January:2.5C February:2.61C Given this is derived from a set of 10 years (with five repeated as an attempt at weighting towards 'stronger' composite years) you would expect a smaller deviation from the mean than you would generally expect from any given month, but it's still a milder month than those analogue years would suggest.

As Steve suggested - a more interesting chart for +216 with yet more hints of amplification upstream. That combined with the emergence of the Arctic High in the modelling and it's simply wrong to say that either there's no 'northern blocking' or that there's no medium term potential - clearly there is both, just that currently the former isn't likely to come good for our little bit of land just yet.

Uppers on the 12Z GEFS are pretty good for hill snow, with the mean only very briefly hitting 0C in the short term and then remaining pretty consistently around -2C for the rest of the shortish term: It's not anything to write home about as far as low ground snowfall goes, but with the mean trough tracking ever further southwards I doubt it'll be too long before we see some snowy setups present themselves on the operational runs. Ian F (the excellent MO/Beeb forecaster from the southwest of England) is currently looking into where the 'significant cold' comment is coming from, given that the ECM 32 dayer was generally quite westerly dominated - it could well be that their seasonal GLOSEA4 model is sniffing out an SSW by the end of January and the possibility of an instant tropospheric response a la last winter or early February 2009.

Hi BUS, certainly is possible (the pattern you mean I'm assuming is the one at the end of FI?): With the core of the blocking centred across Russia/Scandinavia the NAO can be neutral or even slightly positive and still see us in a cold and blocked (as opposed to January 1984 style cold zonality where the NAO can be as positive as you like) setup. The above could eventually lead to a negative NAO as the trough slowly sinks into the continent (although it rather illustrates Steve Murr's point on the MT that we'd need a split flow to bring the WAA to sustain the block and the CAA from the Arctic to actually bring the cold uppers, though the latter is provided on this run by the Asian Mountain Torque event further east) but if the core of the blocking sets up further east we don't *need* a strongly negative NAO to get a decent cold spell. For a long lasting one which is particularly good for Scotland then you really do need a proper Greenie high and subsequent stonkingly negative NAO but a positive NAO doesn't necessarily preclude snow and cold. 9th February 1991 - NAO of +0.4, decent beasterly: December 1981 - NAO for the middle of the month was generally positive (+0.6 for the chart posted) but with very cold and quite snowy, perhaps more especially for southern Britain with -25.2C recorded at Shawbury on this day: To be fair, the spell was initiated by a negative NAO and northerly which dropped a PV chunk down to our latitude which largely sustained the cold spell for the next few weeks (think 09/10 without the semi-permanent Greenland High) but it does show again that a negative NAO isn't always necessary for cold.

There's certainly something in this, and while our local climate is affected by an absolute smorgasbord of different factors often acting in different directions, there are studies which show that (as others have said) low Arctic sea ice extent, by increasing heights to our north and displacing cold air into the mid latitudes: http://www.tellusa.net/index.php/tellusa/article/view/11595 In the longer term of course the idea of the North Atlantic Drift slowing, and perhaps even stopping, via melting Greenland ice is an interesting one - certainly the Atlantic SST tripole, which correlates with a negative NAO and was a key factor in the cold of December 2010 according to the Met Office, suggests that colder temperatures in the mid Atlantic do lead to more blocked winters: On the other hand, one would expect that a weakened jetstream in general would lead to increased summer blocking as well, which certainly hasn't been the case in recent summers other than the last one. This is partly due to the fact that the jet has generally been tracking further south in recent years, which has meant that, instead of being displaced well to the north as is usual in summer, it has been barreling straight into Scotland or even further south. I have my own hypothesis, a rather far out one which is quite probably a load of nonsense, that a lot of our regional climate variability including the LIA and Medieval Warm period can be explained by the phenomena you touched on in your question i.e. increased global temperatures having a disproportionate effect on Greenland's ice caps which then cause a major shift in the NW European climate, in winter in particular, although solar activity also appears to have a pretty major effect (along with major volcanic events). Either way, both low solar activity and decreasing NAD temperatures/strength are going to be predominant factors in the coming decades, and so I would expect colder winters to feature quite strongly in the coming few years - a very interesting time to be a climate scientist or indeed a weather forecaster I'm sure. As for the specific point about December, this is the only month which has so far bucked the trend in having an 81-10 average lower than the 71-00 average. I'm not sure the reason for this but, as December is often the most 'zonal' month with the Polar Vortex building towards its peak, it makes sense that both a southward movement and an increased tendency for High Latitude blocking would be most keenly felt in the month where the jet tends to rule the roost.

I think you're right - hemispherically, the building blocks of a decent cold spell are there, with the AO finally forecast to enter negative territory and the MJO poised to enter into the -NAO friendly phases. Will this come together to bring in a decent cold spell by mid January? Difficult to say at this stage, but given we've spent the entirety of December in a +AO phase we're undeniably in a more favourable position going into January than we have been for most of this month. Whether that actually translates to cold and snow on the ground is largely dependent on smaller scale developments closer to the time i.e. disrupting troughs, increased amplification of the Azores ridge etc. and I'm not all that concerned about the lack of goodies in FI at this stage because we're all (with perhaps the exception of Crewe 'coldest winter since the Younger Dryas' Cold ) aware that this is likely to be a slow burner, if it is indeed a burner at all.