The critical model divergence remains day 4-5 with GFS, GFSP and JMA faster with the Canadian trough and so flatter with the pattern while ECM, UKMO and GEM are more amplified.
The fact that it's still exactly the same models on each side means we have a disagreement about how the physics of the atmosphere will govern the evolution of the pattern Wed-Fri next week.
So to begin with the more amplified runs, in descending order of magnitude, we have - somewhat uncharacteristically for the model - the UKMO 12z op run (left-side), more or less matched by GEM (not shown), after which comes ECM (right-side):
Transient cold is the theme with these runs. Away from the north where snow showers could be widespread for a time, it only briefly looks cold enough for overnight lowland snow, and by the relevant time, ECM and GEM have built high pressure in enough to wipe out most of any showers. UKMO is inconclusive on that matter.
Really though, I'm not expecting anything more than perhaps a few snowflakes in the wind - usually best to keep expectations low with these things, especially when model support is only half what it could be.
If we get the GFS/GFSP/JMA version of events, a few frosty nights will most likely be the extent of any white deposits from the atmosphere.
What happens beyond this period seems to be of equal or even greater trouble for the models to resolve. All of them look fairly similar for day 7 apart from GEM which is more amplified across Europe as the trough there lingers on.
The same could be said for day 8, with slight differences in the timing of a shortwave trough over the U.S. but nothing much in the grand scheme of things. This includes GEM remaining the exception, as the impacts of it bombing out the troughs again sends it off on it's own journey into a parallel reality.
This is what the consensus among operational runs therefore looks like:
From this point is where ECM, GFS and GFSP all diverge from one another.
What ECM does is transfer the trough over Canada down into the Atlantic on the western flank of the trough that is located between Iceland and Svalbard in the above chart. This then drops down into the North Sea for an interesting finish to the run, the upstream arrangement being sufficient for sustaining the mid-Atlantic ridge to Greenland for an extra day or so. The evolution looks to rely on a strong ridge in the western North Atlantic days 8-9 that is supported by a cut-off low just off the East U.S. Coast.
GFS could have followed the ECM route from day 7, but instead a low develops some way offshore from the East U.S. Coast and races NE (it's the same system as ECM has but treated very differently), merging with the would-be-sliding-low and creating a violent system that then pushes due east. Meanwhile another strong trough develops over Canada and the pattern flattens out again until later in FI.
GFSP is just... boring. The Canadian LP drifts into Greenland and makes itself at home there. This story is repeated over and over through FI and makes for pretty much the most tedious (but probably not unpleasant at the surface) run I've seen so far this season as a Euro High maintains a strong hold over the UK throughout.
So it's a confusing picture really.
ECM represents a very fast version of what a number of experienced members on other forums seem to be anticipating as a trend through the first two weeks of December, while GFS gets there at what they may consider a more likely pace, as a tendency for the U.S. pattern to amplify feeds back to a very strong mid-Atlantic ridge for 14th December, which couples with very low heights dropping down toward the Med. to make for an interesting day 16 setup.
GFSP does show that U.S. amplification right at the end of it's run but it still has too much in the way of residual low heights around Greenland for comfort.
I'm crossing my fingers for more amplification in the flow as I enjoy the greater variation in temperatures that can bring. As for a significant cold/snowy outbreak, any time between 21st December and 1st January would be very satisfactory indeed, even though I intend to be travelling a little way south on the 25th. That's weather fanatics for you!
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T-Max: 30.2°C 9th Sep (...!) | T-Min: -7.1°C 22nd & 23rd Jan | Wettest Day: 25.9mm 2nd Nov | Ice Days: 1 (2nd Dec -1.3°C in freezing fog)
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