IDO

GFS 18z follows the script of the 12z at D6-7:

Yes, as we see at D9: Ec -v- GFS. Also, it is better undercut by EC to keep the cold south of the UK:

ECM looks close to the GFS:

...for "potential" longevity, that option of a deeper digging trough is the way to go. That is definitely the call if the trend had been that way.

The GFS control is bordering on outlier material at D8 and I would not worry at the moment: 2m temps & 850s:

The worry is that the other solution is also paved with twists and turns. See the GEM; it has the surface high that the GFS 0z had, which delays the flow of the cold: After that, the run falls to pieces, and GEM has been the poster boy for the GH potential on previous runs! There are many forks, and the GFS seems the best for a snow event, with the usual caveats. Remember the GH has a relatively short half-life, so getting an event in early would mean less worry.

If we are being honest, something close to the GFS is what we want, even if the south has rain to snow. Getting heavy snow on the ground before the colder air digs in would be ideal, assuming the rest of the run goes to plan! D9 snow:

By D12 we have 5 days of embedded cold, which is what I deemed as a noteworthy cold spell, so anything more is a bonus. D8-12 850s:

The knock-on effect of not having that delay in the cold is highlighted at D10.5: 06z: 0z: No worries for the South on this run, plenty of wiggle room.

The Arctic high is working for us on this run versus the 0z. Sustaining the Asian/Scandi trough, whereas the 0z cut that off for a while: 06z: 0z: We need the positives to eek out longevity and a sharper drop of that trough.

The 06z loses the surface high to our SW showing on the 0z which delays the colder air: 0z: 06z: So much better to this point.

The ens for London suggests good support for the ECM OP pattern, and as other models have also moved that way, we have above-average confidence that the mean/OP have the basic pattern. The OP is on the colder side of the ens D5-7, but within the spread; London: ~ECM mean, D0-10: No complaints from me.

The models are beginning to converge on a solution at D8: In order; ECM, GEM & GFS All around that time, as the Iberian wave relaxes, a surface high springs up from a shortwave slipping down the west side of the Scandi trough. This stalls the cold for around a day, and we may get frontal action here or a warm sector. Thankfully, at the moment, all models are getting the cold air in before the Azores low arrives, but the margin of error is a worry (GFS the worst). If we can avoid that fork, we should get a cold spell from both the ECM and GFS post-D9-10. Looking at the GEFS, there is a considerable scattering of patterns, and we are far from resolving deep FI. The usual, more runs are needed, but it looks like we are in the game now post-D8, and that, for me, was always the doubt!

There is much more WAA pumping to Greenland on this run: Energy from the ESB and the Iberian wave.Cold by D8:

No one wants to rely on the Arctic high being modelled correctly; as you say, it is tough to pindown post-D8. But on this run, from D8, it was a big help, with a good axis (N/S) forcing the trough south towards Scandi and sending that cold air to block the westerly push: When the axis changes to E/W later in the run (see above), we see that it becomes a hindrance, forcing the tPV towards Canada. A signal from the mean lately. Lots of puzzle pieces, from shortwaves to Pacific ridging to Arctic highs, to split-energy, to favourable wedges!

GFS D8-13: The difference between the ECM and GFS, with both showing the Azores low heading east/SE, is that the cold is already established on the GFS, allowing a textbook undercut, dragging the Scandi trough south. The delay in colder uppers on the EC means the angle of the Azores Low(s) is less conducive to an undercut. I am optimistic that the ECM can move towards the GFS later, and hopefully, this alternative to the GEM/UKMO solution means a win-win whatever way events lead.

At D10, it is a lot better than the 0z with deep cold and the main arm of the jet digging south: This is the case where we say, "Get the cold in..." and we will see how post-D10 develops with the pockets of wedges.

That is why I am not concerned about Iberian Heights. The WAA at D8 is still being pushed to Greenland to pump that GH. It also keeps the Azores low west.

GFS 06z very consistent with the 0z, only subtle differences by D6-7: So, again no shortwave drama. See if FI changes post-D7.

They are good runs in getting a cleaner split-flow of energy from the UK High, though there is no complete UK high retrogression. So, the charts we saw 3-4 days ago with a stonking GH are unlikely, as they showed a full retrogression of the high. I would guess that less energy means that without further help, the clock is ticking on GH's longevity. Another fork in the road is the migration of the Azores Low. GEM keeps it furthest away by D8, then GFS, but EC pushes it into the UK, delaying the colder air by two days. With that GH clock ticking down, that is likely two days lost to the cold: ECM is the third chart. Note: There are still fewer yellows on the ECM chart on the tip of Greenland, so the pattern collapses earlier. It's all out in FI, so we must wait and see where this fork takes us whilst models hopefully firm up on a cleaner height flow to Greenland pre-D8. Wedges after the GH dissipates may prolong the cold for 2-3 more days. The GEFS mean suggests a return to a familiar pattern in FI with no re-amplification on the Pacific side, instead a wedge sending the tPV towards our NW:

D7 on ECM and that pesky shortwave stalls the WAA to Greenland long enough to flatten the pattern: That can change and probably will, so we move on and hope for better tomorrow...

It looked messy at 168 and at 192 it gets worse:

It is very messy 168, but we have the Pacific/Arctic high incoming to reinforce the GH:

ECM is a lot better at D6:

The 12z GFS in FI shows the first-time potential for wedges rather than a collapsing GH: That would be the best case for the UK once the GH dissipates.