I bring fairly straightforward advice this evening;
Ignore the GFS beyond around day 7, as it has the wrong manifestation of tropical forcing which causes the amplification and southward dig of troughs driven by the stratosphere to end up in the mid-Atlantic when it should in fact be across Europe.
It seems based on its performance from mid-Dec to so far in Jan that the model is over-reacting to the anomalous warmth in the Indian Ocean. The result is a sort of 'False La Nina' signal in the model. What the model is showing us represents something like what would happen if there was no strong El Nino event currently in place.
I know this sounds overly bold, but I have been in a lot of discussions with expert minds in the field and these deductions are strongly supported. There won't be enough eggs to go around if these deductions prove incorrect!
As far as can be told, ECM seems not to be having such issues. The ensembles display a good appetite for gradually lowering heights across Europe from around day 7 with the jet stream easing south, and the 12z op run shows this nicely. In reality there will probably be a more complex evolution in which we have to put up with some residual low heights across Canada causing a hiccup or two before they get sorted out by the tropical forcing from the El Nino and allow a decent ridge to build toward Greenland/Iceland and take up residence to our NW. At that point the dive south in the jet happens quite suddenly and the game is very much on. This may be achieved as early as in the first week of Feb, but equally could take until the middle of the month.
This outlook is based quite heavily on research carried out by various mets over the past year or two and, as such, has to be considered experimental, with the usual caveats. Yet it has to be said that the research teams will be left scratching their heads and totally unsure where to go next if the reality doesn't turn out to be in keeping with the general theme that I have outlined. So fingers crossed it verifies well, not just for the cold and snow it brings great potential for, but for the sake of progress in meteorological research! 
Oh and I forgot to mention where the potential SSW comes in... it's actually a complication of sorts.
You see, the warming that goes on in the model output out to around day 12 is what's expected to work alongside the tropical forcing to help get the Euro trough in place with ridges/blocking to the W/NW of the UK. The reason is simple enough; the strat. vortex is displaced to a location that very nearly aligns with the expected surface pattern:
I've posted the NW version as it shows the heights as well, something that wouldn't go amiss in the TWO charts perhaps Brian? Pretty please 
The full-on SSW that GFS keeps showing is more about taking the vortex down once and for all. If it does turn out to be as strong as the 12z shows for example (see below), and propagates down to the surface (which is looking quite likely), the already troubled vortex won't be able to put up much of a fight.
Potentially we are looking at a large displacement that produces favourable results thanks to the tropical forcing setup (along with help from factors such as Arctic sea ice distribution as well, for that matter) followed by very extensive blocking later in Feb in response to a SSW - but that is the best case scenario, in which the SSW not only occurs but manages to split the vortex rather than just displace it again. Due to the location, another big displacement would probably do more harm than good as the vortex would end up back nearer Canada. There's😊😊 your worst case scenario!
Phew. That was a big edit!
Originally Posted by: Stormchaser