It's worth noting that the GFS evolution is rather similar in many ways to that which preceded the 2010 cold spell - which was signalled a long way out (think 10 days+) and eventually came off.
In that instance, the models showed repeated warm advection bursts heading due north, acting in effect as giant waves to mess up the jet stream. Each one made it wobble just a bit more, then bam - an amplified Rossby wave set up and brought most of the UK a remarkable cold spell.
This time around, there's also repeated northwards advection.
The first such attempt has already happened and is in the process of toppling:
There's some rippling of the jet, followed by a northwards burst on day 6: (Note: not due north this time, but enough to draw the Azores High northwards)
That also fails, but the next attempt is an even greater northwards push, on day 10:
This also topples, but in doing so draws a break-away chunk of that upper low SW'wards, which is then trapped. Like a soapy bubble blown across bathwater, the high can't go anywhere else but east - and gives us a textbook Scandi High and easterly setup.
Over on NW, they'd doubtless be going on about wave-breaking and angular momentum pepping up, but you don't need any of that jargon to see what's going on. In effect, it's just like a seesaw with a heavy rock on one end: you push once, nothing happens; push a bit harder and it wobbles, push even harder and the thing flips entirely. And once it flips, it's stuck there for a while!
The key to this is seeing what other models do with that push of heights around 240. GEM doesn't do much (it instead propels it ENE'wards, rather than NNE), whereas ECM this morning had a similar evolution to GFS.
Bear in mind this "push to the north" thing has already started - for once, this isn't entirely pie in the sky stuff, more a case of seeing if the pattern can repeat with increasing amplitude as per ECM/GFS, or whether in the end it all gets shoved away eastwards before it can make the jet kink enough...
Originally Posted by: Retron