I think we all tend to get led up the garden path with the effects of SSW's, most are minor affairs and have little or no impact.  GFS is trending to an interesting period post mid month, the current SSW has nothing to do with it.  But its impact may well be included in the next week or so.  I like your post, they are always very informative and based on facts as current from a consistent source.

By construction, the selected events coincide closely to sudden stratospheric warmings as defined by the World Meteorological Society (WMO). S According to the WMO definition, a stratospheric warming occurs when the latitudinal gradient in 10-hPa zonal-mean temperatures between 85° and 60°N is positive for more than 5 days.

https://journals.ametsoc.org/doi/suppl/10.1175/BAMS-D-13-00173.1/suppl_file/10.1175_bams-d-13-00173.2.pdf 

Yes, it looks like a fairly major event. Having read a few comments by experts in this area this morning I am increasingly confident of seeing some significant blocking and disruption after mid-month.

Just to put that into context, first I'm starting from having limited confidence anyway and second, we still need the blocking in the right place. I don't think anyone should assume we will get a dominant Scandi high; blocking towards Greenland is perhaps more likely. With a large dollop of luck we'll get both....

The defintions in that article seem somewhat contradictory and this is actually highlighted towards the end

“Although studies use different definitions for MWs, there is a general agreement on the poleward temperature increase from 60 N. Some studies are critical about the timing of wind reversal that it must last for 5 days (e.g., Limpasuvan et al. 2004), but no strict time condition is followed by some other”

From what I have read in other sources, what we have by a generally accepted (although obviously not universally) definition is a major SSW. 

I'm not aware that SSWs promote mid-level blocking; generally they disrupt the PV. That means you can rule out BBQ weather, unless you like cooking outside whilst the snow is fallling....

Yes the favoured pattern seems to be a major lobe of the PV settling into the Baltic region, with high pressure to the west of that. How far west it ends up will decide whether we can tap into the cold or see it all confined to mainland Europe.

depends on the extent of the block I suppose

GFSP going for a decent cold snap again. 

Around mid month in line with met Office forecast - nice block building over Greenland too forcing lOW pressure systems to sink southwards. Hopefully we will see some consistency soon?

And the positioning of the block which is crucial. 

The GFSP rescues the 06Z output - but as you rightly point out, we need to see some consistency in the model output for this to materialise, not just between the different models, but between different runs on the same model. Until we see cross model consistency at 10 days plus from now, it is all strictly FI.

Given that there is no consistency between or within models, I guess the NWP must be struggling with the fallout from the SSW.

The consistency in the GFS/P output for the timescale (mid January) has been remarkably high IMO. The 06z output should not come as a surprise.

As Marcus would say, just for fun.... almost the entire country snow covered

great to see the potential there, hopefully a sign of things to come

http://modeles.meteociel.fr/modeles/gfsp/archives/2019010200/gfs-0-360.png?0 

http://modeles.meteociel.fr/modeles/gfsp/archives/2019010112/gfs-0-336.png?12 

http://modeles.meteociel.fr/modeles/gfsp/archives/2019010106/gfs-0-372.png?6 

http://modeles.meteociel.fr/modeles/gfsp/archives/2019010100/gfs-0-348.png?0 

http://modeles.meteociel.fr/modeles/gfsp/archives/2018123118/gfs-0-360.png?18 

GFSP goes into full-on reverse zonality. Looks like a locked in cold spell would follow from that chart!

GEFS are still all over the place. Run 4 is a beaut, as is run 2 and a few others.

I notice Exeter have pushed back the cold spell into late January and put more emphasis on the uncertainty. That's pretty much in line with what I suggested would happen this morning. Interestingly they seem to be plugging in wet and windy conditions for the middle of the month (possibly chilly at times with a snow risk in the north) which is the period we are largely focusing on with GFS/GEFS. 

http://www.emc.ncep.noaa.gov/gmb/STATS/html/new_acz6.html

Thank you for proving my point. That chart shows the customary dip in accuracy from the Christmas lack of data quite nicely, but as you are no doubt aware you won't find any stats for a 6 day forecast one day after it was made!

The variance appears to be almost the same as the usual peaks & troughs through the rest of the year.

Indeed. What that does show is the difficulties with the pattern at Xmas; obviously not restricted to our corner of the world.

Aside from the fact that the variability isn't 20% within each model. ECM varies by less than 10% and UKMO by not much more.