Catacol

To a degree yes, but the uncertainty over next week from about Wednesday onwards once the first low has cleared is much less predictable than is usually the case. The operational runs are truly all over the place. They dont seem to have a clue, and I still think this huge split in the vortex which I commented on yesterday and which looks to be the biggest early winter split since at least 1972 and probably a lot further back than that (I went back through the archives and got bored at 1972 once I had proved to myself just how unusual it is) is making a total mess of them. The 2 halves of the vortex are very widely split and divided by a thick belt of cross polar high pressure in such a way that I feel (no science behind it true enough) that model error range is being massively magnified. We will all know in about a week.

Yes - these averaged anomaly charts are being remarkably consistent for next week and beyond as they have been for days. Somehow makes a bit of a mockery of the operational leaning towards cool zonality, though as mean anomaly charts I am quite aware that they can be potentially deceptive in terms of actual pressure patterns on a given day. The signal must be remaining strong - I continue to smell a change in the air. What did GP call it - a shuffling of the deck?

Agree. Not much really changed today for me - we still need to wait for this chilly few days to pass and the low to slide through, but what happens next after that is intriguing. We have seen all sorts of options for the middle to latter part of next week, and the ensembles at the moment are a bit inconclusive. My run of the day would be the GEFS control which, in 12 days time, has a near 2010 replay scenario underway all over again... :-)

I'm playing catch up this morning... running the ensembles up past 192 and the iceland heights arrive in a big way, ridging up from the south. Note to self - finish the research before posting the thoughts. There isnt quite the clash between models and other factors that I posted above - and that looks good news if cold is what you are after. Referring to timescale above then, it looks like about 10 days time. I wonder whether, for once, the models might be delaying the event a bit though... usually you can add on a few hours/days to developments as the models jump to the pattern too quickly, but I'm still sniffing out a more SE track of these lows than forecast and an earlier build of northern heights as per the ECM a few days ago. We will see.

Agreed - whether it slides or not there is not enough embedded cold for this first low to produce much but sleety rain or rain for the majority. What I find fascinating at the moment is the constant attempt by all the models to try to reform the vortex, and as a result bring back atlantic influence in our sector. This runs totally at odds with the ongoing assault on the strat, forecast temp rises at 30hpa and the analogs posted by GP. It also runs counter to the pressure anomaly pattern posted yet again yesterday over at cpc for the 8 - 14 day period. Something has to give. Either we are going to find that the models simply cannot cope with the state of the vortex at present (take any model you like on meteociel: run it past the high res 72 hour mark and all seem to me to jump from the current messy pressure pattern to a much more zonal appearance very quickly) and have it wrong, or a good number of analogs,teleconnections and heavy weight forecasters are going to have egg on their faces. Add the EC32 to that list, and also the updates of the MetO which, while accepting considerable variation in output, have leaned towards cold. I am still confident that the models are struggling, certainly beyond 120h. The pattern is forecast to be fairly flat over the US, but there is still the strong suggestion of mid atlantic height rises and pressure over greenland hand in hand with an indication of the ridge over california backnig west (cpc prognosis yesterday)... and with that announcement I will stick with the feeling that we might get height rises to the east of Iceland. Timeframe? Good question... certainly 144h plus.

Scanned all the ops after checking the ensembles - none the wiser really for next week. Track of low uncertain; strength of height rises in mid atlantic and possibly over scandy also uncertain; I'm also not sure that the models are doing a very good job of reading the split vortex: they seem to be wanting to reform it out in FI all the time, and that just doesnt seem likely.. I have just trawled back through the archives on Meteociel and stopped at 1972 - there isnt a year anywhere in the archive that matches the severity of the current vortex disruption and others more knowledgeable than me are saying that disruption is going to get worse. With this in mind do I think the models, with a definite zonal bias in FI (120h plus really) can get a proper handle on all this? No. So - agree with a few on here that the energy being fired out of Canada is the spoiler at the moment, but also less than convinced that the pattern will be as flat as is being modelled. I also think that height rises could be sudden and quite sharp in this scenario. Basically I'm off to bed pretty convinced that next week is wide open.

That seems quite steep. A quick look at the CET records shows that the average CET for December is 4.06. Only a handful of Decembers have ever come in at 4 degrees lower than that, including 2010 of course. Does the Met therefore reserve the word "cold" only for a handful of such freezing months?

The weather may be appearing normal on the ground at present, but it is not. The state of the vortex is not normal; the amount of forecasted blocking is not normal; the southerly tracking jet is not normal either - at least in the "modern" era. How is a 70s/80s December "typical"? Do you mean 1981 which was the 9th coldest December on record, or 1974 which still holds the record at CET 8.1 as the warmest December ever. That kind of statement means absolutely nothing because you have not defined what you mean as "typical" at all.

Follow the temps over on here once December starts: http://www.climate-uk.com/ Let's see if we are "slightly colder". How much below average does it have to get before we call it "cold"?

But these ensemble charts from a week ago for this coming week have verified? What was subsequently picked up was the zipper low which has upset everyone so much. Also weather is never a precise science, and the operational runs that showed deep cold coming towards us were typically deceptive. Watch them for enough years and you learn to take operational eye candy with a pinch of salt. Work with ensemble trends and you are never (rarely?) disappointed. Takes all the stress out of it. :-)

Worth being precise here, because 72 hours later (day 10) the ensemble pattern is thus: I think you are therefore misreading the ensemble signal which is showing - as it has consistently shown for days now - a temporary rise in temp and westerly influence for about 48 hours, maybe 72, before defaulting back to this pattern above which is very similar to the coming weekend. Pressure building behind the low that comes through, with I suspect a tendency for any other systems from around about Friday 7th to come on a much more southerly track. This is in line with the waxing and waning of the atlantic ridge put out there by GP a good while ago. To say second half to last third of December is therefore to undercook the evolving situation by a good 7 to 10 days at least. I agree, however, that by that stage cold pooling will be more prevalent, and so any flow we get from the east by the second half of December will be a good bit colder than it would be now.

Red areas means high pressure. Blue bits low pressure. The red zone stretching along the northern side of the uk and down into the atlantic shows a signal for higher than average pressure across the 6 days in question with lower pressure over Europe. This means a leaning towards an north easterly flow in that time span, though the chart itself is a computer averaged result with human input on top (as someone pointed out to me a few days ago at the weekend there is no human input on top, so charts from the working week are probably best...) and being an average over such a large number of days means it can obviously vary a bit in that time span. The slider low for early next week, for example, was picked up in this model a few days ago and was visible by a noticeable shifting north of the average low pressure zone over Europe, reflecting the low pressure expected to zip through early next week. That signal has now receded again, suggesting to me that after the middle of next week any low pressure systems heading in from the atlantic will be heading for the channel/France or maybe even Spain at a push. These NOAA charts are invaluable tools when looking at the overall shape of the pressure patterns. It is an ensemble mean product, meaning they take the average of a range of computer results which tends to provide a much better feel for the future weather patterns than a single computer run such as the GFS 12z freely available on sites like wetter and meteociel. There is also a very useful prognostic text analysis released each day that explains why they have drawn the map as it has been drawn, and which computer simulations they have preferred and why on each particular day. These are climate experts over at the climate prediction centre in the States. So if you are looking for evidence of whether blocking is likely to remain then you have a computer average with human agreement from a range of models pointing to exactly that from day 8 (Thurs 6th) to day 14 (Weds 12th) Avoid getting hung up on a single run from any model. Direct link is http://www.cpc.ncep....10day/500mb.php

Yep - this could be the trigger. Could happen almost at any time next week or beyond - the models are struggling I think to get the speed of any transition right, and that system dropping into Europe with pressure rise behind it is the form horse I think from the transition from cold to colder as cold air is dragged westwards.

This will do you.

At this risk of treading on TEITS's toes, it is pretty obvious. We have a signal for high lat blocking and troughing over Europe, and the op runs are picking up on a series of systems zipping through on a NW to SE trajectory in the short to medium term. Possible/likely result is for one of these to end up over central Europe with the winds pulled into an easterly direction as heights rise to the north and north west. I would suggest that all the data is almost literally screaming for this to happen in the medium term - the fact that the EC32 hinted at it too, and GP posted yesterday in the same vein, is high quality back up context.

The whole crux of this ongoing "thread of gloom" would appear to be on the definition of "prolonged cold." I stick by exactly what I posted yesterday: take out the zipping low at the start of next week and the outlook IS cold. However its not Siberian Cold - posters on here need to get their radar set right, and recognise that ice days at the end of Nov and early december are extremely uncommon. The fact that we have signals suggesting ongoing blocking, with a vortex strong enough even in its split state to through sliding lows at us (as per the EC32 reported on this forum yesterday) is very exciting indeed... and a whole different sort of "cold" to the lobe of vortex on a northerly that dropped over us in Dec 10. Temps will hover in low single figures with frost at night for the foreseeable future... and the chances in the medium term of a slider low triggering an easterly look to me to be very good. Certainly higher than your average December anyway! So - let's have some perspective?

The more complicated the scenario, the more important the mean runs. To plump for one run when the models are struggling is no good at all. When there is general model agreement out to T144 or even beyond (which is rare) then the single op run is worth a look.

Excuse me if this sounds daft - and I cant see a better thread to ask the question in - but can volcanic eruptions affect the stratosphere in any way? I notice today that the previously inactive volcano Tolbachic in Kamchatka has exploded today, sending ash on day 1 33,000 ft up. I guess it may go a fair bit higher if today is only day 1. Any impact or am I showing myself to be a total novice? Genuine question... EDIT: hey - guess what... the volcano erupted in both 1962/63 and 1968/69. Surely that must be a sign... :-)

Yes - missed that low down there. Curious - no comment from me on that one. That aside the fact that the GFS is also hinting at Scandy height rises has me very interested. The ensemble signal for deep low pressure over Europe must make this a possibility - the mean chart below does show high pressure strong from Greenland right across to northern Scandy, so maybe the kind of thin high pressure belt touted by ECM could be a possibility. To have an evolution like this even brushing the edges of possibility this early in the season is a marvellous start to winter.

I agree. For me blocking will be maintained, and my jesting about the ECM op at 216 was actually only partly jest: I think SteveM's enthusiasm on that one may be correct, and I would not be at all surprised to see height rises to the north of the Euro trough at all. GP popping up and implying the possibility too only serves to strengthen my opinion. What a change from last night when everyone was throwing their toys out over a single shot of energy across the atlantic leading to a gentle hiccup in the blocking pattern. This thread makes Punch and Judy look sane...

Though seriously - actually 4 De Bilt ensemble members were colder than the op for Thursday next week. That's not quite the outlier I expected - and up until the day before (Weds 5th) the op is slap bang in the middle of the cluster. Maybe a Scandy development is a realistic option after all. More runs needed.

Wot - ECM at 216 was an outlier? That's a surprise. Dammit.... Feb 91 slips through our fingers again. :-)

I'm not one to get overly excited by a single op run, but the evolution for midweek next week that has started to poke its nose through the current veil of uncertainty does look rather good. I dont share your confidence on the first zipper low causing snow Steve - I dont think we will have enough embedded cold, and modification at this time of year will make it a cold rain event for anyone other than those sat on a hill, but the CAA westwards helped along by the zipper flying through will make 5th December onwards a totally different ball game. Would I be ramping it up a bit too much if I posted this little chart from 1991? There are some striking similarities with the ECM chart for Wednesday next week...

Thanks Stewart. That's the nearest to exasporated frustration I have sensed in your posts probably ever. Some posters on here cant see the wood for the trees...

I honestly dont know what all the fuss is about. Maybe some on here were expecting a flow of deepest siberian air for a week, but the ops never had that anywhere but in FI. The mean charts continue to show a flow predominantly from the north and east dominated by blocking to our NW. This isnt a change - it is consistency. The trough over the UK is modelled now a little further north on the means than was the case, probably the result of the low pressure zipping through in the early part of next week, but GP flagged up days and days ago that this energy had to go somewhere and SE was his call. Voila - SE it is. More consistency. The MetO never updated to "snowmageddon" at any stage in their outlook, and the only alteration in the short term prognosis has been the 24 - 48 return to nearn normal temps that the low next week MIGHT bring. Note the word MIGHT because they have repeated over and over that there is considerable uncertainty about the track of that low, and experience suggests that when blocking and energy come toe to toe the models have a poor record at getting the track right. Living in the SW I have seen constant examples of low pressure systems in the past have their track altered by 50 miles or more even at 24h range. For those interested in real model forecasting go up a few posts and look at GP's mean charts - I wont bother to post them again and add to unnecessary clutter. Prognosis - cold. Not siberian cold, but cold. Snow chance? Certainly present. No embedded cold yet so for this week high ground only. As cold becomes more embedded over time snow chances increase for all parts with a flow from the north east bringing wintry showers, and possible attacks from the SW bringing frontal battleground events. The suggestion from SK above that the EC32 update has slider lows coming in, bringing a SE flow as they pass through certainly has my attention as a SW resident! This was how I saw the forecast last week, and it hasnt changed a jot. Forget the detail (it will only drive you insane) and look for trends. Maybe give your local lamp post a bit of a shammy rubdown... :-)