WeatherOnline suggest different:

12pm

3pm

Wouldn,t be unhappy with this a couple of days before Xmas!

Just for fun? Regen? Means rain over Moors?

FWIW the JMA has the low on Sunday centred over southern Scotland 

GEM disappears the low altogether into a flabby trough

Why not? HP could retrogress and join Greenland HP block!??? - Then we could pull in some proper cold air to the east!?

Gefr. Regen means frozen rain/sleet! now I know.

T+240 is too far away to worry about Beast. All that concerns me at the moment is the prospect for snow on Sunday. Was looking good for the Midlands previously and now not so good - the positioning of the slider low is critical and that won't be nailed down for another couple of runs at least.

Yes, a warm easterly would certainly be the most pyrrhic of victories. As you say, cold in the wrong place for us currently, from the north, from the east. Today's northerly won't be that cold either, at the surface – 6c maxes are not what I call cold. 

It's feasible that none of the models have Sunday right yet. Certainly too early to say which ones may be closer to the mark.

This isn't just favoured spots in the north getting snow though so what Downpour was looking at, I don't know?

Sufficient cold air in the mix during the middle part of next week for marginal snow events to pop up here and there, even in the south as the GFS 06z shows (though this model tends to be a little optimistic with the thresholds required).

A little further ahead and a comparison of the 00z and 06z GFS runs for 8 days range reveals that the significant changes have taken place by this point to set up a colder, more wintry 9-16 day progression; the trough by the UK does not extend so far NW so there is less squeeze on the ridging across Greenland from that side, while the Canadian trough is displaying more of a splitting tendency against the SW tip of Greenland, which sees a secondary low undercutting the ridge that settles down over that island. That's only for a day or so but this is enough to seriously mess with the attempts of the Canadian vortex to advance into the N. Atlantic. This is all aided by better placement of the Arctic High.

These are all changes that depend heavily on the track of various LP systems beforehand and so are subject to much uncertainty, but these improvements from the 06z do at least increase the probability that the 00z runs were getting carried away with a small modification to the U.S. pattern taking place in the nearer-term. Fingers crossed! 

Couldn't resist - ARPEGE 06z a tad south of the 00z so the spread of possibilities continues to include more southern areas as well.

The timing of interaction with the trough to our NE is critical. The strength and position of that feature is also playing a role here so we've two major sources of uncertainty rather than one! 

If the polar vortex sets up over Greenland you can forget about a Scandi block. The northern arm of the jet sinks or forces it back virtually every time.

Not much sign of that happening though, for a while at least. We have almost all the ingredients for a cold spell in place... except the cold 

A great read, great analysis James - many thanks for sharing your thoughts. Hopefully you've got this nailed (and the weather gods are in a benign mood this side of Xmas!)

Isn't the extent of that wedge of upper heights near Greenland/Iceland also playing an initial role?

The OP 6Z run is probably in my top 5 deterministic GFS runs of all time.

GFS 06z  850 hPa ensemble for London is better than 00z especially in the longer term, below average in run up towards Christmas.

I've added fronts to the 6Z GFS output. This is for midday on Sunday

Snow will fall on the northern edge of the main occluded front with a mixture of rain,sleet and snow coming in behind. A secondary feature is also flirting with the SW coast although I suspect that will be all rain.

Close-up view at T72:

Compared to the 78hr 00Z chart:

Meanwhile the ICON high-res chart is very different:

The leading edge low gets absorbed back into the main circulation and crosses southern Scotland.

May the Azores high remain strong with ridging NW and repeatedly swinging and pointing towards Greenland to provide essentials for building the Greenland High.

And the high-res NOAA model has it much further north on the 06Z!

And May the Azores High swing in this direction of the UK once we get to next summer.

Talking of the Azores High, the NAO is forecast to head negative although there's a lot of uncertainty.