The past few GEFS (not the op) have had around a 30% chance of -10 850s for London, so that fits. If you look at -8 instead it's closer to 60% - so going by the GEFS, at least, a pretty cold spell around the new year is somewhat likely. 

Incidentally despite all the doom and gloomin, the raw output down here at least remains seasonal. Bearing in mind the 91-20 average high is 8.2 this time of year, the GFS has, over the next few days:

7, 5, 4, 6, 8, 8, 5, 3

There's a good deal of sunshine too, especially as the cold air digs in on Christmas Day. And yes, 4C with 35mph gusts coming in from the sea and a -3 dewpoint will feel absolutely perishing, despite what my personal troll might say! 

The Met Office raw output is similar. Is it the sort of deep cold that brings snow and icicles? Nope, but as I say at least it'll feel seasonal and it is generally colder than average - and that alone is rare enough these days, let alone at this time of year!

The bonus is there's little sign of the Atlantic returning in the next 10 days either, the definite end of this colder spell isn't yet in sight and that, too, is rare these days. GEM is this morning's party pooper, but I'm not going to take too much notice of one run that far out. 

This is pleasing. I was close to throwing the towel in yesterday. Model watching isn’t good for me lol

When the longer range GEFS mean wandered back above -5 yesterday I concluded that a slow return to a westerly type early Jan was more likely than not but this morning there has been quite a shift back in the suite with the mean dropping to -7 (London) which might be the lowest it’s been for a while. 

Given EC analogue and GEM are in the return to default camp this morning I take the new GEFS colder push with only half an eye at the moment but it’s enough to pull me away from the pre-Christmas stuff going on to check the next few runs.

https://www.theweatheroutlook.com/twocommunity/Posts/m1672377-Model-output-breakout-thread 

Christmas is fast approaching but I'm not in a very charitable mood. Take note and discuss the model output. Also, don't try the usual trick of making an off topic post and then adding a line at the end of it referencing the output. 

*IMBY it's a run of 4-7c for most of it, areas further west are often cooler and Scotland (parts of anyway) has a lot of cold days, notably so later on. 

ECM:

The T2M set don't scream mild air in an unsettled pattern, so at the surface chilly is more likely, with a greater chance of average to mild later on but mild still less likely IMO (special mention to the red fella and his ice days and -10 minima!):-

All in all the initial pattern remains as most of the output showed - HP dominated and chilly, the longer term pattern remains uncertain and dependent on whether the HP relocates somewhere favourable or gradually sinks or maybe just wanders about keeping it mostly cool dry and uninspiring unless we get lucky with cloud amounts. Yesterday was a step towards the sinker for me but not convincingly so, today maybe has taken half a step back but I'm definitely fence sitting for a few more runs yet. 

Edit - had this post in draft for ages and hadn't seen Michael's post which pretty much says what I said but much more succinctly 🤣

GFS Op 0z -agrees with FAX (if anything the air arriving for Christmas is even more from the SE than the E). Then after some moving around, the HP moves northwards 1045mb near Iceland Wed 31st with the N-lies more direct and colder for the E coast. Then a brief quiet spell with a cold pool trying to get into the south east before Sun 4th when it sets up S of Greenland opening the gates to a strong Arctic N-ly for Britain, fading by Thu 8th when Scotland gets weak SW-lies but a legacy cold pool with NE-lies lies over SE England. Not much consistency there - quite small changes in the position of the HP make quite large differences in temperature so keep watching!

ECM - like GFS up to Wed 31st but then positions the HP west of Ireland covering Britain and no N-ly influence. However the HP then, as in GFS, does depart for Greenland and bring in N/NE-lies from the Arctic for Sun 4th. Having delivered cold air S-wards, by Wed 7th this is circulating around LP 1000mb in Biscay with cold E-lies for all, not just the SE; any SW-lies are blocked by HP then over Iceland.

GEM - is having none of this shilly-shallying. Once the blocking HP declines S-wards, from Wed 31st, the Atlantic is back in full force Fri 2nd with 940mb Iceland and SW-ly gales for all.

AIFS - London, maxima around 5C, dipping lower towards Tue 6th, and dry; Edinburgh, similar maxima but with more variation up and down, rain on Tue 6th

GEFS - briefly cold on Christmas Day in the S but elsewhere continuing mild (and then in the SE too) until Wed 31st, when sharply colder, mean down to 5C below norm Thu 1st (and several ens members colder still), mean slowly recovering to normby Thu 8th but with wide spread of ens members. Small chances of small amounts of rain in the New Year, about a 2/3 chance this will be snow down the east coast but declining towards the west.

GFS and ECM can't make up their mind except that blocking will continue, so overall dry and cold-ish, detailed forecast likely to change day by day. Or you could back GEM  at least it's easy to understand!

I'm staying at my daughter's for Christmas; reviews will continue but timing will depend on her household schedule.

Now it drying up stage for a week on 0z Nuneaton ensembles and if this HP bring clear skies and frosty nights it will help to lower the ground temperatures and this coming colder spell are not dry as there spikes all the way that starting once the colder air arrive.  Hope to see it lower to below -10c upgrading with further ensembles run  as I only after subzero day maxes, no interest with less cold temperatures 1-4C.

ECM 12z op looks a bit better too as far as the chances of more prolonged cold are concerned, Ally.

Maybe all isn't lost after all, contrary to the view that have been expressed by some over the past couple of days. I mentioned recently that one thing I have noticed about the model output over many years is that the models often struggle with blocking situations to one degree or another, and I think the set-up that is now developing is likely another instance of that. We shall see. 😉

Cold shot moving south over the country from 30/12 before a brief milder period (uppers wise at least) from around 02-03/01 then a reload from the north initially turning north easterly as the ridge moves/topples towards Scandinavia. 

It's a setup that could look very different with a slight change to position/orientation of the HP but continues a theme of generally chilly, chances of colder air with snow as you head towards the end of the year and maybe some reloads.  

Edit - should have said, on the GFS/GEFS at least! Other models may be less enthusiastic. 

Good agreement until around Dec 30th mark....anything after that taken with pinch of salt at this moment of time.....

AIFS ensembles now better (colder) than the ECM which is interesting. 

Perfectly put - you can understand people’s disappointment after the loss of the strong chance of a Christmas dusting, but the models have always struggled and flip-flopped with cold  or blocked weather - in a way they rarely do with Atlantic flow. Maybe that is because blocked patterns are so rare these days - the data is limited and the influences on the shape and orientation of the block make a huge difference, being that we are located on the boundary of 5 air masses. More pert’s on the GFS support the OP than yesterday too. 

UKMO as squish mentions is an improvement on this mornings run, although not as good as GFS IMO. 

One thing just to mention on the end of year Northerly is we are very much on the edge of the cold and it wouldn't take much to make us miss out. (Or I guess a small tweak Westwards and we get more of a direct hit but sods law suggests that's unlikely). 

As John Cleese said in Clockwise, I can take the despair; it's the hope I can't stand...

Op run flattens the ridge @ +252z:

AIF @ 252z - Shows very cold airmass to our east as well:

One of the things I've noticed in recent years, and indeed with the charts being churned out at the moment, is the persistence of heights around the Azores. Even with our Christmas Day easterly, there's still a massive upper ridge over the Azores, and it's that which helps keep the high floating around our vicinity afterwards.

If you look at the last great Christmas easterly, in 1970, you'll see it's quite different around the Azores - there's an upper low there instead:

https://www.wetterzentrale.de/maps/archive/1970/noaa/NOAA_1_1970122518_1.png 

The same is true of many of the noteworthy cold spells of the past, and it's something that's been commented on here before. 

Not that there's anything wrong with a high nearby (even if it ends up shovelling moisture from the Atlantic over us and thus clouds up), but as you say - it does feel like a case of "so near, but so far" when you look at the charts. While I'd love to see one of the ensemble members that shows the European upper trough extending over the UK come to fruition it does seem to be the devil's job to get it to all fall into place. Even as the Christmas cold pool episode shows, once that pool trended south the models have stuck with it!

https://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=1&ech=180 

The Control run - if this isn't frustrating then what is!? Unlike the UK the chart at +288z shows large parts of Europe under the sub -10c@ 850hpa: