Just to recap, it is looking relatively mild for much of the coming week in southern Britain at least. Further north there is a higher chance of cold incursions. The Christmas period itself remains uncertain.

GFS Op - starts with LP running from mid-Atlantic now  to Norway Fri 23rd, SW-lies at first but bringing cooler N-lies behind for Xmas Day (esp for the N); the next LP heads for Spain with quiet weather and rise of pressure for Britain 1035mb Wed 28th before very deep LP 940mb S Iceland Sat 30th dictates Sw-lies then W-lies.

ECM runs the 'next LP' closer to SW Ireland for Xmas Day so weak E-lies rather than N-lies.

GEFS temps up (Mon 19th, Fri 23rd) and down (Wed 21st and after 24th, esp later on for Scotland) but little agreement between ens members after 24th, op and control cool at first, mean near norm. Rain /snow in most places at some time, for Scotland heaviest now and later on, for England heaviest around Mon 19th.

We’ve firmed up now on a much milder spell from Sunday through to approx middle of next week at least. The problem is the European High - how many times have we seen low pressure systems wind up in an Arctic flow to our west, with the UK ending up on the mild side? Many times in my experience. It has scuppered many a cold spell in the past.

There are signs of a cold vs mild battle towards Christmas with lows straddling the UK. No point labouring this too much yet. More runs needed. 

From my perspective, the sooner this milder weather gets in the better to wipe away the most miserable, abysmal cold spell I have ever experienced.

​​​​​​​It has been an unmitigated disaster here, serving only to put me out of work for two weeks as my local radio football commentaries have gone for a Burton, without even a flake of snow to compensate. 

I concur. There is a huge amount yet to be resolved for the period from the 23rd Dec, and most hinging on whether the troughs phase or not. I give GEM a decent amount of respect these days.

It's ahead of GFS at the moment.

I would entirely back this up, as from my memory the majority of GEFS ensemble members have been showing the cold spell ending about now for at least the past ten days.

As for the 00z GEFS it looks as though the scatter of unpredictability starts around the 22nd, though I note with interest that the OP shows a cold snap from the 25th - 28th.  (I tend to give the OP slightly higher credibility because I understand it to be run at a higher resolution than the other ensemble members, so will pick up on smaller features that affect the progression of the forecast.)

Yes the 06z GFS op run highlights that well so far with a string cold airmass to the north battling with a large mild one to the south. Fascinating but potentially very frustrating. Still a lot to be determined for the run up to Christmas and the festive period itself.

Yes, for once, I would agree with you. The height rise towards Greenland on this run looks, shall we say, interesting. 

A good few milder and unsettled days to get through first, but yes, I agree with you that T+192 looks primed for something. 

Another new trend maybe, or just a pebble in the stream?

Cold never far away from the north though (Scotland) especially.,

From the ECM 00z.

https://services.meteored.com/img/models/ecmwf/ECMWF_150_GB_SFC_fr-FR_fr.png 

https://services.meteored.com/img/models/ecmwf/ECMWF_168_GB_SFC_fr-FR_fr.png 

https://services.meteored.com/img/models/ecmwf/ECMWF_192_GB_SFC_fr-FR_fr.png 

https://services.meteored.com/img/models/ecmwf/ECMWF_216_GB_SFC_fr-FR_fr.png 

https://services.meteored.com/img/models/ecmwf/ECMWF_240_GB_SFC_fr-FR_fr.png 

Yes, the Op Runs at a higher resolution, but you're still left with the inevitable errors creeping in. Plus, those short wave features often come and go and get modeled differently as we approach T+0.  Another factor is that short waves can affect the long wave pattern, particularly at the margins, ie they can change the weather we get. For example, in a cold Polar flow a short wave development that runs across the south can turn an innocuous north-westerly into something more potent for the south of it gets beyond the Prime Meridian.

Back to the models and the GFS 06z run has another variation on a buckling jet stream again. A deep LP develops and moves NE along the Eastern Seaboard around T+168 but then stalls over eastern Canada, promoting quite a strong surge of WAA on its eastern flank.  Not sure it's going to withstand the next push from the jet stream but interesting nonetheless.

Particularly the north Midlands I would say with a Boxing Day blizzard!