WhiteFox

Yep. And I hate to say it as it's a tired cliche, but we have more than one bite of the cherry. Experience of living in New York also tells me that these storms tend not to be forecast accurately until quite close to the time. There were a number of occasions where a forecast Nor'easter would switch to a rainmaker as it tracked further inland than originally forecast. These things are a little like hurricanes in that respect; there is a cone of uncertainty which slowly narrows as it comes closer to the forecast period. Until the energy which powers the storm enters the southern conus, after which it tracks up the east coast meeting with energy from further north, it is not usually possible to get an accurate prediction. ECM does not show high building over UK at the end; further disruption looks likely.

Absolutely. Lots of interest in the near future yet to be resolved! Beyond that, background signals stay cold, albeit murky in terms of how it pans out as highlighted by @johnholmes. Also, as @Catacolsays, we can't completely rule out a possible storm if we do get a trough dropping sharply over us. NWP models are playing around with details, but a few have shown a strong storm next week. I know the probability of an incredibly intense system with central pressure approaching 950Mb is low, but we should still be looking out for what happens if/when that trough does dive down over/very close to the UK, albeit with a reduced intensity. The storm that eventually ended last year's first cold spell (@march 2ndish) was largely dismissed as an outlier from around a week out, so sometimes events we see as low probability do occur. We'll have to watch and see if the plunging trough scenario is the one that finally brings the widespread wintry landscape that many on here are waiting for but, background signals remain good and eventually getting even better as the MJO signal moves towards phase 6 in the next 7-10 days or so...

Unsettled! Low temperatures would be modified upwards by the journey over a much warmer ocean and mixed out by interaction with warmer air being dragged in form the south, but actual values would depend on how quickly it moved across, and the direction of travel. It doesn't happen very often, but a few forecast charts have shown such an event recently, and it gave 850 temps of around -6 or so (as a guestimate) which would probably result in rain/sleet/snow for higher ground and further north. As a general summary anyway...

Great post, thank you. The MJO chart is interesting, and could explain part of the uncertainty that has been introduced in recent runs. Until very recently, the MJO forecast was to meander through the COD and then emerge late in phase 5 around the 21st. Now, forecasts are point to emergence in phase 4 or even late phase 3 which is not so favourable for blocking in our region. Phase 4 generally supports lower height anomalies towards Iceland/S Greenland, Phase 5 less so. It's not until we get into phase 6 and particularly 7 and 8 that we see support for blocking to our North. Of course, this may well be overridden by the SSW and consequent effects on the PV. Overall, the background signals are there, and we are still heading to a colder regime from the 21st (perhaps a little earlier), but not the snowmageddan that some are hoping for at this stage! This is no February 2018 event with -15 850s marching from the East; it was always forecast to be a slow-burner initally with troughs disrupting SE on a NW flow with increasing chances of catching some snow if the path was just right. We have to wait a little longer for blocking, and this will not necessarily be picked up on NWP models (particularly the operational) past the 10 day ranges (GFS I'm looking at you). Whichever way you cut it, we are getting colder; just temper the expectations for now! Addit: Found the MJO chart I had from a few days ago. You can see how the forecast has changed (on the GFS at least). Given the GFS signal for a move into phase 4 and 5 we shouldn't be too surprised if it is showing less blocking (even wedges). This is not to say that it is correct of course!

The 0z didn't get it wrong! We are only twelve hours closer to the forecast period and this a variation! At least wait until we are actually closer to the date before calling a forecast bust... As ever with these siutations, we will see variations east and west, with the forecast track narrowing as we get closer to the event. Probabilities of some form of system taking a track around the areas modelled now seems likely, but it'll be around the three day mark before we can really get any idea of the track, and probably a day or two out before we can consider snow/Rain. Etc.

I don't think so shaky. We're looking at an overall pattern change with anomalies showing a fairly sustained flow from North of West as detailed by @johnholmes earlier. One small feature such as a shallow depression will not impact on the overall flow.

I think people are looking at the potential for some snow before then which has always been a lower probability IMHO. Main interest comes after 168.

It's only cherry picking if you select a run that is largely unsupported by the ensemble suite. The charts you have chosen are perfectly plausible as they sit within a decent sized cluster. Go for it!

Given the overall background state I would say at some point there a reasonable probility of it happening. May not happen immediately, but I think once we are into the pattern change, pretty close to the 21st as telegraphed for quite some time, then further possibilities open up. The 500mb anomalies continue to show the NW/SE pattern, with another average heights in mid Atlantic and lower anomalies to our east. Given the background signals of the weakening vortex we could start to see anomalies increasingly push further north. With a cold pool in place over Scandinavia, a snow covered Europe and the AO forecast to go sharply negative, there is a decent probability of tapping into very cold uppers from N/NE at some point after next week IMHO.

Good analysis Nick! A lot of people are concerned about the size of the cold pool firing up the jet and flettening the Atlantic pattern. However, it seems to me the higher amplification actually creates the conditions for blocking to start establishing closer to us. If that trough was not digging so far south over the US then a flatter jet from the US would make it more difficult for us, I think! Still looking good this morning....

On the question of confidence regarding an upcoming cold spell, it's worth looking at the 500mb height anomaly charts. First, the 6-10 day charts shows above average heights off the Eastern seaboard of the US, stretching to mid-Atlantic, with weaker anomaly heading heading towards Greenland. We have below average anomalies to our East and North-East. It would seem to me (although a more trained eye could read it better) that this corresponds to a NW to SE flow. The 8-14 chart maintains the general theme of higher anomalies to the West and lower to the East, although higher anomalies are slightly further west which may indicate that the upper trough to the East has more influence on the UK. Now, I haven't seen them for the past few days, so I'm not sure about consistency, but so long as we see such anomalies forecast, it's not worth getting too worried about details at the surface which will change markedly from run to run.

No more and no less confident than yesterday. Nothing has changed for the medium to longer range outlook IMHO. A few operational runs do not alter the background pattern. I always thought 21st January was the turning point; anything before then was always going to be a bonus. We are still 9-10 days away from what I think will be a slide I to colder conditions. Not a sudden sharp flip to -15 uppers and heavy snow, but a change to increasingly colder conditions with increasing chances of snow. Absolutely no reason to get depressed about flip-flops in deterministic NWP models that struggle post day 7 at the best of times, and even more so when there is an imminent pattern change.

Always appreciated Nick. Newbies yet to experience your musings are in for a treat with the upcoming patterns I suspect! Meanwhile, was looking for a latest MJO, but had trouble finding any other than yesterday. Does anybody know what the CANM is?

Not the place for this discussion, but it is a possible outcome if we like it or not! If we assume that the information being input is perfect (which it is not of course), and the model uses exactly the same algorithms as previous runs (which it does) then it exactly follows previous model guidance. That is not losing the plot, it is coming up with an outcome based on the inputs; the rules of the model have not changed. The difference is likely to be the input data and the difficulty handling the particular dynamics at play in this situation. We're arguing on the same side which is that we cannot make a judgement on a single operational run, and that the outcome is unlikely (albeit an unlikely possibility) due to the rarely modelled background signals...

I know it's a turn of phrase, and this is not targeted at you (some posters seem to believe it!) but models don't "lose the plot"; they predict outcomes based on imperfect input and run it against hundreds of thousands of equations. The fact that the ECM produces a particular outcome for a given input is not unexpected and that is why we have ensembles with slightly tweaked input. Of course, as you correctly say, it's not worth focusing on the single run; as we progress further from the initiation date, the greater the level of chaos becomes, especially in situations where we have large scale background drivers about which we have limited knowledge and information. Surprised nobody has mentioned Shannon Entropy for a while, but it seems to me that there is greater uncertainty (chaos) right now than there would be in a more familiar setup. My focus is still on ensemble means and background drivers for the next few days. I think 21st is the date to watch for the colder pattern, but don't expect any reliable synoptic SLP detail until at least 4-5 days before....

It's not that different from previous day's where the mean was around -3 to -8 towards the end. I did look at that final frame of ECM this morning and thought it was one for @nicksussex! A threading the eye of the needle easterly with lots of shortwaves! Looking at the models this morning it's still clear that looking for detail past seven days is futile. ECM and GEFS means still showing averages around -5 from around the 19th with a bit more scatter than yesterday, admittedly, but signals are all still there, and certainly not worth worrying about an uptick in ensemble means at the 14 day range; we're not even in the colder pattern yet!

Is it not that the MJO was not in a favourable phase for a cold signal, but was overriden by the SSW? The MJO now forecast to move back towards a favourable phase and enable blocking to occur in more favourable position going forward? That's the way I read it. Quite favourable indeed...

Shorter range models, absolutely, but that's expected. However, given all the background signals you'd expect a longer range model to perform better in highlighting forecast anomalies. Then again, I guess the CFS is simply a shirt range model that forecasts out to a ridiculous range?

Yes. Far too much variation between outputs for a model that is supposed to be forecasting at a climatic level.

Look good to me. Mean is sub -5 from 19th. Note that op run still on its own on 23rd, but if 18z continues the theme of possible sub -10 temps then expect some other members to start joining the club.

Spot on. The signals are there, foundations being laid, and now we're starting to see some detail emerging at the tail end of major model runs. There will be variance between models and runs until we get closer to the time, but I think we're on an expressway to cold with most exits leading to what we're looking for. There will always remain a low probability that we take the wrong exit and end up somewhere we don't want to be, but I think it is low risk...

They won't be eliminated until at least 7 days out (even then there will still likely be runs that show a brief warm sector passing through at some point). There are simply too many variables to allow for complete agreement beyond 7 days.

I'd be in full agreement if we were chasing an end of line easterly facing up against an active jet which requires multiple shortwaves to phase in exactly the right way! Such has been the case in many a winter and they mostly end in disappointment. But, the background signals are strongly in our favour. No matter what model you look at, they all show a split TPV devoloping. So, instead of hunting for a shot of cold against a background of a strong jet and organised TPV and a mild underlying setup, we are looking at generally cold background signals favouring an array of cold options. That's not to say there won't be disappointment and milder spells within the period; but in all probability I think you will be searching for how we get the PPN amidst an already cold setup. So, yes, still room for the frantic chasing of spoiler shortwaves, but they won't derail the overall pattern IMHO.

Most roads lead to cold Ed! As good forecaster never gives guarantees... Longer range forecasting gives us probabilities; numerical models are deterministic and based on imperfect information and so are also probabilities. Yes, the probability is high, but there are still a couple of unpaved roads which could lead to something different. Fortunately, I think we are now on the motorway!

Doesn't really matter if it does not have huge support so long as it is within a few degrees of a cluster of runs. We've seen the ensembles trend strongly towards a colder operational solution in the past. Once a colder signal is picked up by the operational, it often leads the way. Having said that, expect more bumps. Experience tells us that the operational won't consistently follow the cold path until a bit closer to the time ..