Indeed, a weak ridge and clear skies would lead to some exceptionally cold temperatures. We last saw a -20 in 2010 and as you say there'd be a good chance of the same with that ECM run.

I suspect when we see the T2M ensembles later it'll be one of the coldest, if not the coldest member!

Also a substantial correction west@216 with the atlantic V the milder 12z 

216 brings more in the way of snow to the UK -

I think the effects of the SSW may be starting to be calibrated a little more effectively by the ECM model as time progresses, with the ECM going for some cold scenarios this morning involving both frost for most and snow (for some). Its been fascinating viewing for days and I don't see that changing.  Nothing is particularly nailed on, but the potential for some stellar charts as the days progress, popping up in FI is there.

Today's offerings from the EC, however, may appear understated, but they are also fairly cold!

EC 240 must raise your eyebrow lol 😁

TBH I'm stunned by that ECM run - that'd give a cold spell for the record books.

Waiting for EPS - Although looking at the high res Aperge thats to 96 - it builds a similar blocking ridge 

Should be lots of support today !

ECM is on the verge of bonkersness in a good way I don't think I've seen a run quite like it before . The major models are still quite different to each other but most roads still lead to cold and snow.

A very decent set of GEFS this morning

I was a bit worried when I saw the OP. Thankfully the ENS from the GFS are the best set yet!

The tropospherical response from the SSW appears to finally be manifesting across the NWP this morning in the form of an initial cold col set up. Signals for a significant cold spell have definitely edged up today.

I instantly thought of Steve Murr when I saw the ECM 240 this morning 

The ever westward retrogressing NW/SE jet alignment continues ensuring we eventually end up on the cold side of the jet. We then await a significant height rise in the Greenland / Iceland area circa 23- 26th ish.

With atlantic influences cut off it will be interesting to see how cold it gets and how quickly.

London moves towards the freezer at the end..its not often we can say that !

I think we all remember 2013 when the GFS batted on with the wrong solution to something like T84, in a similar situation. Hopefully we will see corrections on forthcoming OP runs.

Fingers crossed. If we take the ECM as truth we're 4 days away now from the start of the cold spell! 

Good point, it really didn’t want to go with the slider scenario did it! Let’s hope it’s a more straightforward route this time......I doubt it.

www.meteociel.fr/modeles/gefs_table.php?ext=1&x=&lat=51.5085&lon=-0.12574&ville=Londres

This is always a good indicator of what GEFS is up to

The main thing that caught my eye this morning is the significant westward ‘correction’ of the jet profile meaning we are not influenced by the dreaded Azores HP & cut off from further northerly or continental airflow. 

Best set of EC ENS to date from Wed onwards. Still some spread Fri/Sat re: the troublesome low but EC still not buying in to with the GFS on its own. Not much snow in the forecast but longer term some potent E or NE'lies in the clusters as Greenland blocking develops. Interesting

— Matt Hugo (@MattHugo81) January 13, 2019

Decent GFS 6z very different to the ECM but still snowy .  Odd 264h+  with mild 850s easterly.