OldMetMan

I've been studying latest model output quite closely and, although there is now a stronger consensus between them with Atlantic systems taking over, something still doesn't quite add up as far as Thursday and beyond are concerned. The latest 500mb hemispheric chart has some noteworthy features which make me still doubt the models: In particular, the amplifying trough over the W US, as I mentioned yesterday, does not seem to be given realistic development consequences by the models for further downstream. There needs to be more significant ridging ahead of it I think which of course would have an effect on jet direction. Also, the amplitude of that US trough would be such as to affect downstream feature amplitudes. In other words, the net effect would be to build a more significant Rex block with the upper high developing over the Norwegian Sea, the European trough retrogressing somewhat, perhaps absorbing the upper low off Newfoundland and, this would force the US southern jet even further S than the models predict. So, instead of the developing HP to our NW merely sinking S and decaying, we would see as a consequence stronger westward pressure rises out of Europe into the N Atlantic. With this configuration, we would have a weakish polar front with, as I also mentioned yesterday, stronger cyclonic activity well S of the UK. Yes I know, a lot of "ifs" and "maybes". Clearly, the models themselves are grappling with these issues, hence the dithering in today's output. We are a mere 3-4 days away from this predicted change in flow so one would hope that by Monday, it should be nailed down. Tomorrow's output should be most interesting!

Thanks Steve, what incredible temps! I also recall surface temps over the Low Countries, Denmark and Germany being somewhere between -10 and -20 on 31st/1st!

Yes, but what was particularly significant in 1978 was the intensity of cold over Europe. I wish I still had the thickness charts for back then, I seem to remember the values being well below 510dm over the S of England, which is extraordinary .

Thank you Snowmad. Yes, exactly, it is such a good example of how this battle can happen. It's been a while since anything similar happened.

It was exceptionally cold air involved on that occasion with a remarkable temperature gradient across the frontal boundary. An occluding LP went right up the Channel and temps to the S of the front over the Channel Isles were +10C! I shall never forget that one!

Some preliminary thoughts ahead of the later model runs. The see-sawing that is taking place within the models currently does, I think, represent one of the most difficult weather trends to forecast in winters when it happens - the battle between mild zonal conditions and cold blocked ones - and the tussle that takes place in the atmosphere, and how the models depict this, can be frustrating and also fascinating to watch. Plus I do believe that in general the models have a pretty hard time with them. As far as what we might be facing in this current situation, although the outcome for later this week is, I think, still highly uncertain, as far as I can see we are likely to see this mild v cold battle for some time yet, maybe well into January. It has the "look" of previous such battles, some more severe than others and many more where the cold never quite made it. But this time around, I sense we might be seeing the real deal at some point. The advance indicators of such a spell can sometimes be seen some time in advance, such as a low-latitude jet and a build up of very cold air to the E and/or NE. At present, while we do have a cold pool to our E plus a substantial HP over N Russia, I think it is not that cold and on that basis it seems to me that Atlantic systems will be more able to make more NE progress than otherwise. However, we are in the depths of winter and there have already been indicators of unusual cold over Europe (the massive snowfall that paralysed areas of Russia not that many days ago) so, from the model perspective, I think the GFS in particular is going to continue with the mild/cold alternating for a while BUT....ultimately I feel there will be a significant cold breakthrough - quite possibly very cold - and out of the models that seem most likely to see this happening first I would say the GFS, the GEM or the UKMet will be the candidates. Meanwhile, to see a perfect historical example of this process at its best, take a look at the last week of December 1978 in the archives, which culminated in the most ferocious blizzard I can remember across the south with day time maxs of -3C. The chart below says it all - a cold lover's dream!

Hi Rybis, sorry I missed this one earlier! I think I will most likely put my trust in the Met FAX charts when we are nearer the time but I doubt even they will be all that accurate. It's a very tricky forecast to have to make but, as far as plain computer output is concerned, I would most likely put more faith in the GFS and also the GEM as being nearest the mark - notwithstanding my reservations from yesterday. OMM

Hi Rybis, I did think it would be possible because of the uncertainty over the strength and direction of the Atlantic southern jet. GFS appears to be mostly out on its own so far today with the UK Met inclining the same way. Having said that, what's the betting it will flip back to it's previous ideas in subsequent runs today?! By the way, I would still like to know whatever happened to the model thread we were using before this one yesterday which seems to have mysteriously vanished. I see that the mods have been archiving so could you please throw some light on it mods? Thanks.

The models continue to tantalise us with the prospect of cold easterlies, and each day brings yet more variations on that theme. Two things in particular are getting my attention today: First, the position of both polar and sub-tropical jets across the Atlantic is pretty far south, with a particularly strong one across the Mediterranean - indicative of cyclonic activity continuing in the area: The jet over Canada/USA is showing signs of strengthening in association with a broad upper low across Canada and seems likely to push NE eventually. Meanwhile, the LP that nearly became a sub-tropical LP is not playing much of a role in the current synoptic development, but according to some of the models, seems likely to be followed by a 2nd one, pushing NW. Second, the models are showing very high pressure developing over N Russia - common enough this time of year - but the cooling in that part of the world is continuing apace (judging by the way the Russians were caught out by very heavy snow over the weekend, when they are supposedly well geared up to deal with, so it must be unusual). So, as I see it, it could play out two ways - one, the HP will extend SW and W towards us with LP to our W pushing a upper ridge towards Greenland or two, HP will build W all the way to Greenland with LP activity well to the S of it's normal track. We have seen this scenario before many times, but for it to stick, I think we need to see more noticeable signs of strong blocking developing which is only slight at the moment - but there nevertheless. Personally, I think the risk of easterlies in the next 2 weeks is high - mostly because the jet is far south - but we'll see how the models vary in the next few days.

Sorry, don't always look at the ECM! Just looked at the tropical Atlantic satellite which has a disorganised area of convection in that general area, but the NHC would quickly pick it up if there was any real likelihood of it happening, even though the season will have ended by then. The GFS is certainly making much of it. I can see the tabloid headlines now "After the floods, now it's a hurricane heading for us". I get very tired of their irresponsible, ridiculous weather headlines!

Yes, or sub-tropical, I posted about it earlier, it would be extraordinarily rare and, as I said, I cannot imagine how the models could conjure that up this time of year. Need to keep an eye on the National Hurricane Centre and see if they pick up on it.

Yes, I am just watching to GFS 18Z unfold and so far its cold theme seems to be present. I noticed one other very curious thing that has appeared on various runs over several days, namely what appears to be a sub-tropical depression forming mid-Atlantic: It is shown above as having a warm core so it must be tropical in nature and some of the runs it has appeared in have pushed it NE in our direction as a major extra-tropical low. That would be remarkable indeed for that to happen early December but if it verifies, it could throw a spanner in the works as far as blocking goes. But for the life of me I cannot understand where the models are getting this from!!

Curious differences in model output today and they seem to be dividing into 2 camps - one promoting yet more mobility, the other setting up a blocking pattern. The divergence beyond T120 is even more marked than normal so what indicators are there at this stage that might suggest which is more likely? As usual, I look at 500mb and jet flow as I have found them very useful in assessing trends. At 500mb, we are developing quite a stable looking wave pattern: The large upper vortex over NE Canada doesn't look likely to go anywhere any time soon, with an upstream upper low in the NE Pacific gradually edging E. Meanwhile, once again, downstream there is still the ever-present upper trough close to the UK - I cannot believe how much this has happened this year - which has a very high amplitude, going way S to the N of Africa and I think this may be a clue as to the likely longer term trends. Meanwhile, the hemispheric jet has a strange configuration: What I find particularly noticeable is the 2 jets over the US which appear to be diverging and I think the outlook depends very much on which stream becomes dominant and I think this might explain the 2 different model outcomes. On the one hand, the mobility model output seem to be favouring the northerly stream, with its associated LP activity heading across the Atlantic near our latitude, which, as I mentioned the other day would effectively discourage blocking. On the other hand, a stronger southern stream would keep any LP activity well S and allow for more blocking, with the corresponding northern stream becoming weaker and maintaining most N Atlantic LP activity off the E US/Canada coast. The high amplitude trough near us I mentioned earlier would seem to me likely to disrupt gradually allowing heights to rise to the N with corresponding pressure rises from N Russia gradually across to Greenland, in other words the setting up a Rex block close to our longitude. The GFS keeps trying to play out this scenario but is dithering over timing and depth of cold. All things considered, I still favour the blocking outlook. It may take a while - the coldness in the next few days being a precursor to the main event as it were - and we must expect some more LPs crossing SE over us as the blocking sets itself up. The models will continue to waver for some time yet no doubt but I shall be watching the GEM and GFS in particular to see how they change.

Thanks s4lancia, much appreciated. I find the fact that the GFS has kept to the same story in the long term quite striking, when quite often it will revert to a default zonal flow. But a couple more days should clarify things.

Will it become cold or not? The GFS seems to be pretty much out on its own today, although the GEM has some similarities. In fact, the GFS is setting up a real cold block in the longer term. For me, the jet configuration has some interesting features that support the GFS: In particular, note the very strong jet out of Mexico at a low latitude, merging with its northerly stream which suggests an increasing amplification of a upper trough off the US easy coast, with subsequent short-waves running E, again well S across the Atlantic.This I think would amplify the mid- Atlantic ridge and downstream trough across the UK and W Europe. This pattern is present already to a degree: I think the other models, including the ECM seem to be favouring the US jet blasting its way NE, destroying any blocking, which doesn't seem logical given the current 500mb configuration. It will be interesting to see if the GFS sticks to the same story in subsequent runs. .

Growing consensus over the upcoming cold spell but some model variance over the details. The GFS (so far) is the most aggressive in terms of the depth and duration of cold but the GEM this morning was along similar lines. Looking at the 500mb current pattern, the building blocks appear to me to be in place. The European block is re-establishing itself over N Russia. The LP complex we will be left with after the weekend will help maintain that block and pull in the colder air from the NE. Meanwhile, upstream there is a developing upper trough over the central and E US and Canada which supports the model development of a large and deep LP over NE Canada which in turn will help build pressure in mid-Atlantic. The feature that has really attracted my attention is the growing cold pool being developed to our NE with 498dm thickness values being shown by the GFS next week. But the $64000 question as far as the UK is concerned is exactly how cold the NE'ly will be. Forecast thickness values show only 528 to 532dm values by mid-week - not that cold but capable of giving snow along the E coast and high ground - but if, as the GFS suggests, the Atlantic LP and trough disrupts allowing a broad HP to extend well W to the N of us, then progressively colder air will get drawn into our area. I think that once thickness values drop to at or below 522dm, then snow is possible most anywhere. To give this some context, values during the coldest part of December 2010 got as low as 516dm. Pretty darn cold! This is a very interesting scenario developing. In the meantime, the small LP developing to our S needs watching. Model treatment of this are varying quite a lot but it has some serious potential for very strong gradients on its south side, across the S of the country, not to mention copious amounts of rain which we really don't need right now. See what tomorrow's output says about this.

I've been watching the longer term trends for cold conditions come and go in the model output in recent days, not just the GFS which seems to change from one run to the next, but today both the GEM and JMA are showing interesting possibilities: Here's today's 12Z JMA: Such a chart would be exciting indeed if it were Jan/Feb but it's a bit too soon. Nevertheless this type of easterly often changes to a N or NE flow as the main HP transfers to our N and NW. At 500mb, once again there are signs of increasing amplification of the flow over the US and Canada. The large vortex off W Canada seems likely to sink SE whilst further E to the NW of the UK, a weakish block exists in the sort of position that would favour what the JMA is saying. One other area of interest is the fact that W European blocking has finally collapsed, again this suggests to me that blocking will re-develop further W. The strongest jet energy across the Atlantic is on quite a low latitude plus the sub-tropical jet across N Africa is also unusually strong. These factors which seem to indicate most of the LP activity in days to come will be well S of the UK, once the upcoming stormy spell this week dies down and the Atlantic vortex disrupts. I think we will have some interesting weather to look forward to!

So many mixed signals from the models in recent days it's hard to see any real trend developing. As I see it, one of the main issues for the models is what to do with the European HP and the corresponding upper blocking pattern. We have seen, and will continue to see, according to the model output, LPs running into the block and the upper troughs disrupting. The persistence of this upper low pattern over or very close to the UK is remarkable - but it has to change sometime. Looking at the 500mb flow, the US flow has flattened quite a lot recently, with the jet a fair way south. Either side of that, we still have a more disjointed jet pattern - not much to suggest any major amplification of the flow but neither anything that hints at a more definite trend in position and location of the strongest jet flow and how it will extend further E towards the UK. Having said that, I am intrigued by the elongating upper trough just off the US west coast, which could conceivably set up some downstream amplification, as this trough sinks SE and heights build over the east US and W Atlantic. Meanwhile, over the rest of the Atlantic and W Europe, I expect we will see rather a messy 500mb flow with a mean Omega block, gradually moving W. The GFS keeps hinting in various runs of a cold air breakthrough. Also, the ECM, out of all the models has been most consistent in sliding LP SE and building HP to the NE and looks the most convincing scenario, based on past experience. All in all, I still believe that a more stable block will developing in the coming weeks - how it will orientate itself is clearly crucial - but I would put the upper HP to our N or NE, with consequent E or SE surface flow over our area. A lot depends of how the US flow sorts itself out but give it a few more days to see if anything more significant or consistent appears across the models.

Some typical November weather likely as per the models - at least until mid-term, with a succession of LPs running from slightly N of W. However, the longer term has some interesting possibilities. The 00h GFS showed a significant cold plunge at the end of the forecast period - clearly not to be taken too literally - but some of the other output, including the GEM, show the possibility also. The jet flow is changing gradually. There is still the strongest and most sustained jet over the W Pacific but further downstream we now have some more vigorous activity over the N USA with 2 streams combining and further downstream still, a strengthening WNW jet heading our way, What I find significant about this arrangement is that, although the WNW jet seems likely to sustain mobile conditions for a while here, the alignment of the US jet would seem to be amplifying SE with the corresponding 500mb low going with it, resulting in rising pressure, and some blocking, developing off the E US and Canada. Meanwhile, the tendency for the persistent upper low over the UK to recur is continuing to be shown by the models and it seems a logical development, given what's likely to happen upstream. The persistence of the mobile pattern we have had so much of this year was largely due to the strong SW jet heading from the US to the UK. But, in spite of what I have said above, there is still quite some disjointedness to the jet, plus a fair degree of meridionality all the way from the Pacific which in my view would tend to favour blocking - this I base on what I have seen happen before. I strongly suspect that within a couple of weeks there will be a major blocking setup with a strong upper ridge over the W Atlantic and a deep and broad upper trough covering much of the central and eastern Atlantic, including the UK, as hinted at by the GFS and GEM. I haven't checked out the 18Z GFS yet but it will be interesting to see if it brings this possibility back. This raises the possibility of the W Atlantic HP extending towards Greenland and most of the LP energy on the forward side of the downstream LP heading into W and S Europe, which could bring a NE fetch our way. Yes, a lot of ifs and buts here but I have to say there are some similarities between this November and that of 2010. I should add that on that occasion, the NE pattern developed very quickly in a matter of a few days. I keep repeating this point but I think we are seriously overdue for some significant blocking that persists longer than a week or so. The repeat performance of an upper low and cold pool at our longitude gives me more confidence that this may indeed happen - and quite soon.

I hadn't seen the end of the GFS 12Z run when I posted, so it seems the GFS favours the HP build. Notice also the cold air starting to be drawn down over N Russia. See how this trend develops this week.

Although there are some fairly consistent model trends in the near to mid-term at present, namely a succession of deep LPs moving SE and settling over or near the UK, the longer term trends are somewhat difficult to read at this stage. We currently have a somewhat static wave train at 500mb, with 4 major vortices, including one near the UK, roughly 90 deg apart. The jet configuration had made some changes but the strongest jet still remains over China and the W Pacific. Although I wouldn't call the overall flow particularly blocked, the pattern is not really showing any immediate signs of turning zonal, so I would agree with much of the model output as regards persistent LP near the UK, as the upper low remains close by, A couple of things that have caught my eye however. First, most of the jet energy on our side of the hemisphere is pretty well south, with a rather disjointed polar jet at our latitude. The other thing, for the longer term, is that I notice both the GEM and GFS are building HP over Russia and into Scandinavia - not unusual at this time of year - but it does suggest to me - based on what I have seen happen in the past - that the last LP in the train coming from the NW will finally sink away to our SE allowing some build of HP over us, from the E and NE. This, I think, would fit in well with a stronger southern jet and would coincide with a weakening polar jet, and would tend to reestablish the Omega block from earlier in the week. I don't have much to back that development up at this stage - as I said I have seen this kind of evolution in the past. I think later runs might clarify but, in the meantime, expect plenty of rain and, at times, strong winds - and some cold nights!

Fascinating possibilities shown in the model output. I have been studying them for a few days, trying to gauge the trends. In particular, the marked level of meridionality, not just in our area, but hemisphere-wide, is the most striking feature. With a strong Omega block to our NW south of Greenland, and another in the NW Pacific W of Alaska, these suggest to me a stable pattern that is likely to persist and this is how the models are playing it right now. Naturally, of particular interest is where the UK will lie relative to the major upper troughs and ridges, and I think the models are pitching it pretty well, with a major upper vortex developing over the UK in a few days' time. Thereafter, there is some variation between output, but I'm inclined to agree with the GFS, with a renewed plunge of cold air in the medium- to long-term. Of particular note I think is the strength of HP shown over Greenland in the outlook, especially on the GFS, and, on certain runs, not just the GFS, the odd sign of HP building over N Russia and Scandinavia. Over the N Hemisphere as a whole, the jet is showing a very contorted pattern: The strongest jet energy is over China and the W Pacific and, while this remains so, I think it emphasises the likelihood of blocking continuing. But I think, as always, what happens over Continental America will be very important. at present, Hurricane Sandy is strengthening the current pattern pushing all its energy up the US east coast so I think we need to watch closely developments once it dissipates. Also, I am going to keep an eye on high latitude HP development to our north. Cold air, as you would expect this time of year, is growing in extent and depth but it will be interesting to see where it's collecting, especially N of the UK. Overall, the pattern, to me, is showing signs of continued cold. This weekend's burst of cold air so early in highly unusual. Interesting times ahead!

Today's model output is showing some interesting changes in the mid to long-term. There is now a reasonable consistency up to 5 days but thereafter, the signals are mixed. Two things of interest that I think will have a bearing on the longer term trend here. First, the NCEP discussion from this morning: "THE BIGGEST TREND IN THE MODELS SINCE YESTERDAY IS TOWARD A DEEP CYCLONE OVER THE GREAT LAKES MID FORECAST. THIS SPRAWLING LOW IS THE SURFACE REFLECTION OF A SHARPLY AMPLIFYING UPPER TROUGH, WHICH WILL HELP TO DRAW ARCTIC AIR INTO THE NORTH CENTRAL STATES. THE BAROCLINICITY OF THE EVENT WILL MARK A PATTERN CHANGE FROM THE CURRENT ONE, WHERE MOST OF THE NATION IS AWASH IN PACIFIC AIR. THE MAGNITUDE OF THE AMPLIFICATION AND THE TILT OF THE UPPER TROUGH WILL HAVE GREAT BEARING ON THE FATE OF THE TROPICAL LOW OVER THE CARIBBEAN." So, we have a significant development over the US which does, I think, suggest a much higher likelihood of longer term blocking plus we now look likely to see some late season tropical activity acting as something of a wild-card. The NHC haven't called this tropical system yet but are giving it a high-probability of becoming a tropical storm in the next 24-48 hours. What this might do, assuming it does develop, is to follow the path of most of the storms this year and push a lot of very warm air up the US east coast, which in turn could significantly affect what happens downstream. 1) It might push a warm upper ridge well north towards Greenland and sharpen the downstream trough over us, or, 2) It could influence the jet and increase baroclinicity so much as to bulldoze its way through any blocking pattern and return us to zonality once again. Secondly, I think the main model differences in the mid- to long-term is how each model is dealing with any polar front features running SE towards us that I referred to yesterday. Both the ECM and GFS and making more of this than previously, and the UKMet at T72 seem to be suggesting the same. All this will clearly affect the extent and duration of the cold air influx. In spite of the above, on balance I still think we are headed for a significant blocking setup, with unusually cold air arriving later in the week, I say this mostly on account of what is now happening in the US and Canada, but notwithstanding what any tropical storm effect there might be to disrupt this. Whatever the actual outcome, it should be an interesting week ahead and I maintain that it will have some bearing on the coming winter in terms of repeating blocking patterns later on.

Today's model output is continuing to show a fascinating scenario developing. Plus there are increasing similarities between the longer term model forecasts, including the NOGAPS which is normally slow, I think, in picking up major blocking situations. In particular, the GFS is showing increasing blocking, added to which we now have a significant switch in jet location and strength. The persistent very meridional jet associated with the upper trough near and to the S of the UK is finally weakening as is the sub-tropical now running away E into the Mediterranean. Meanwhile, a strengthening polar jet is forecast to develop over and to the SW of Greenland, then running SE towards us. The present configuration would seem to bear that development out. Further upstream, with signs of a more persistent, slow moving upper low over the E USA/Canada, it all looks as if it has the makings for most of our weather coming from the NW or N for some time to come. Polar frontal waves and LPs look as if they will drag increasingly cold air towards us. Yes, it is too early to expect much in the way of wintry precipitation - it's this pattern occurring so early that I find particularly unusual I am aware that we are still talking about the medium- to long-term but I am really struck by the model consistency at that range, plus, as I mentioned yesterday, the upper level flow is taking on a noticeably increased amplitude. After so much zonality this year, as well as the persistent jet near our latitude, I don't think it unreasonable to expect a marked increase in meridional flow, and associated blocking in our area, as we head into winter. One other feature I noticed in the model output, specifically the GFS, is the rapid cooling over Greenland in the coming days plus the intensity of the HP there. Most unusual. I've a feeling that we have much to look forward to this winter!

I've been watching the shifts in the models over the last couple of days and now they seem to be all more or less in line regarding the forecast cold air blast. The interest now will, I think be in how long it will last and just how cold it will get, as well as the exact detail of where the axis of the developing cold trough will be. First, all credit to the GFS for picking up on it first, as well as the GEM. Even the UKMet 120H FAX has it developing now. The upper flow now is starting to look decidedly peculiar. Look at the jet first: A couple of very pronounced "troughs" in the jet flow as is reflected in the 500mb analysis: Both have the look of a major blocking pattern and more stable wave arrangement, such as hasn't been seen for a while. In terms of actual weather here, the models suggest at least a brief pool of 528dm air crossing the country which could just about produce wintry showers along the east coast, maybe even as far south as East Anglia. Given the sea temps at this time of year I wouldn't hold your breath but there could certainly be strong convection, wintry or not. I think this is turning into a very interesting pattern which, as I suggested a few days ago, may well repeat itself into the winter proper. I am sure many of you out there are wishing the situation were happening in January! Patience!