Facepalm time!
At least the reason for the difference is clear to see on this occasion. So what causes it?
Simple actually - there's a shortwave between the Azores and U.S. troughs at +96 hours which does different things in each case;
UKMO merges it with the Azores low which amplifies that feature and enhances the ridge ahead as the poleward flow is enhanced.
GFS on the other hand merges it with the U.S. trough and amplifies that feature, which due to the presence of the Azores low ends up firing a strong jet E then NE.
GFS was closer to the UKMO 12z solution with its 00z run, in which it merged the 'middle shortwave' with both the U.S. and Azores features simultaneously. Then the 06z favoured the U.S. trough and the 12z has taken that idea further still. So the question is, whether UKMO is lagging behind or GFS is being misleading?
Notice that purple blob just off the N coast of Greenland - that's the remnants of the Svalbard low that I talked about this morning,a nd again it looks to be headed for the Pacific trough instead of the Canadian/U.S./Atlantic one.
That latter trough is a fair bit west of the 00z, though. A nice trend if it could be maintained!
So where's ECM going to go... it actually had the 'middle shortwave' as I call it merging with the Azores low on the 00z, which gave the impression that it might persist the cold, blocked theme beyond day 6... but then it had the Svalbard low merging with the Atlantic trough and the blocking was knocked down with fury.
With two key features that need to behave favourably to produce an ECM run that prolongs the cold, you could say we have a 1 in 4 chance tonight. Factor in the chaotic nature of the atmosphere though, and who knows what the true odds are for something like the UKMO 12z...!
Originally Posted by: Stormchaser