There was also the 09/10 winter as well; like the following winter it also saw a month-long spell of fairly severe wintry weather.

I recall we were having similar debates to the one in here approximately 12-13 years ago when a number of us were wondering whether sustained spells of notable cold winter weather were pretty much a thing of the past. We got the answer to that in two successive winters at the end of the last decade and the start of this one, and I for one see no reason to think that such spells are history now. Let's not forget either that it was only at the end of the winter before last that we had a major freeze in this country, even if it didn't last that long in duration and it occured at the end of the meteorogical winter season rather than in the middle of it.

Well, I am going to launch a prosecution against this winter under the Trade Descriptions Act as yet again being a winter in name only, without any of the much anticipated proper winter weather, which we are falsely led to believe is expected and eagerly anticipated in what under any imagined scenario is supposed to pass for what is commonly described as something approximating to winter!!!

Indeed. People seem to regard a mild winter as somehow abnormal, whereas actually it's always been more normal than a cold one, at least since the end of the Little Ice Age.

For whatever reason, there seems to be a tendency for cold winters to occur in "clumps". The most recent set of cold winters brought us February 2009, winter 2009/10, December 2010, and January and March 2013. Since then, the winters have been predominantly mild, but we've had as long or longer runs of mild winters in the past, and I don't think that global warming can have changed things enough since 2013 to preclude cold winters in the future. After all, we have February and March 2018 fresh in the memory, which produced the March day with the lowest CET maximum on record for the month, showing that extreme cold is still possible.

...and back to the models...

ECM continuing in positive NAO territory here in the medium term -

And for a short while a couple of days prior to that

https://www.wetterzentrale.de/maps/GFSOPEU18_144_2.png

How did I guess that the 18z Op was a cold outlier compared to it's ENS:

It was not an outlier. That's when there is absolutely no support for the operational run in the ensembles: you can see from the chart that it was never out on it's own, just amongst the coldest options at the end of the run.

The control was also cold at the end but via an easterly, i.e. A completely different evolution to the Op.

A lot of cold is set to be 'bottled up' over the southern regions of the Arctic (if that makes sense) over the next week or so, so a forecast chart like that would not surprise me if this pent up cold was to leak out a bit. 

Some weak movement of cold air westwards from Siberia but the air is well above normal temp so no excitement there. wxmaps.org/pix/temp4

Jetstream looping around in such a way as to encourage HP over UK until Sun 5th then resuming NW to SE over/S of the UK and very strong by Fri 10th so more stormy weather? https://www.netweather.tv/charts-and-data/jetstream - matches the forecast for a dry week followed by a distinctly wet one http://www.wxmaps.org/pix/prec4

GFS maintains HP to the SE of the UK until Thu 2nd Jan, then a mini-breakdown with some cold(-ish!) air coming south but the HP resuming, this time to the SW. Finally, to match the above, a deep and cold depression appears on Fri 10th (960mb over the Borders*) though after an unsettled few days the HP re-asserts itself.

ECM has less of a breakdown after 2nd Jan, but then rebuilds the HP with its centre over Poland

GEFS temps  well ageed on a drop to below seasonal average, even below a degree or two zero in the S, on Jan 4th, recovering to 3-4C above and the mean of the runs declining to near average by the 14th. By this time there are quite a few really cold runs to sustain the cold optimists. Very dry in the S though some activity 10th and after, a bit earlier in Scotland (doesn't seem to have picked up on the depth of the storm on the 10th).

10th Jan seems to be the day to watch but plenty could happen before then..

* Terminology - Scotland has Borders, Wales has Marches; I think our friends in the north might consider 'Scottish' Borders a bit Anglo-centric but correct me if I'm wrong 

P1 GEFS is fun this morning 47 style Snowmegeddon.  

Meanwhile in the mild reality somewhere in Scotland could break the date record today 16.1c to beat.

Not on the TWO chart viewer out to T+180

GFS 00z op was one of the colder runs later on.  

Have a look on wetterzentrale 240h +. 

TWO link is 

https://www.theweatheroutlook.com/twodata/gefs.aspx?run=na&charthour=240&chartname=500mb&chartregion=na-region&p=1&charttag=500hPa%20GPDM

Yes ends in spectacular style all Coldies should take a look.

It has already gone. 16.2C was recorded at Achnagart at 01:00 today. Astonishing. 

Does that count for today or yesterday? I can never remember how the official temperatures are assigned for the overnight period!

IIRC the 9am-9am max gets assigned to the first day of the period but the min gets assigned to the second day, is that right?

thats truly astonishing. 
meanwhile some fun and games in FI, otherwise as you were in the reliable. 

You could be we'll be right. I can't remember either. 

OK,as above, seems indeed that the 10th Jan is the date when things might kick off

Correct me if I'm wrong but sadly that outcome seems to be a real outlier on the ensembles. Nice eye candy though.