OK studied the situation a bit more and with benefit of 06Z GFS run (which
may be an outlier of course but does seem to be a reasonable compromise).

Powerful westerly jet does indeed seem very likely now in western Atlantic
(aided by 564DAM air), this is pointing straight at the block and is hence
a powerful mechanism for breaking it down. But it does not seem to want to
propagate east very quickly, indeed earlier UKMO and ECMWF did not really
want to either so some consistency there. Lows are spawned and run to east
of Greenland but the upper flow starts to disrupt at circa 15-20W and push
low PV air north which will have the effect of transferring the block east
into the Baltic rather than the Balkans, given the very cold air eblished to
our east by then this seems very reasonable. So for the UK I fancy that next
weekend (possibly a little earlier) may see a transition to a strengthening
cold southerly/SSEly given the cold over France as very mild Atlantic air
pushes into Ireland.
Obvious problems then of rain turning to snow moving east. 06Z GFS pushes
everything through beyond T+180 but I feel it will struggle a bit more than
that as the high over the Baltic seems to collapse too quickly in the 12
hours from T+180 to T+192 due to model resolution change.

Probability of block breaking down later this week into a cold
southerly/SSE'ly with fronts coming in from west - 70%
Probability of block not breaking down but staying over UK - 25%
Probability of a bitterly cold spell with block re-establishing to
north/west UK - 5%

Oh what fun!

Will
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