Cheshire Freeze

Note Nick's (Bluearmy) post above. It's not one run. If there was a feasible Achilles heel to a decent and long lived cold spell, this was it. I, for one, have been warning of the possibility for a couple of days, along with Nick. Others may have pointed out the possibility too.

Well if we do end up back here- A lot of people are going to scream You can actually trace back the issue to 72 hrs across most of the models...I'll use ECM 72 hr as an example. We see here a pretty much perfect shaped HP for funnelling cold- however it is just too far E. Ordinarily this wouldn't be an issue as it should naturally back west with Atlantic trough disruption. BUT a combination of Azores HP interference (stopping a clean SE ejection of energy) and a pretty intense area of low heights at the eastern end serve to disrupt the balance of the HP. The Azores interference probably the main player here as it completely inhibits enough energy ejection on the SW flank of the high in any semblance of a clean way.

Actually quite hideous at day 9...it just all falls apart

ECM takes winds S'ly and lifts the cold pool away (rather like a more progressive GFS 0z). However, it follows the trend of holding the block further N, so longer term we remain in the game rather than a quick waft from the E before a sink of the HP

It was a genuine question...not a dig at you but if you want to reply in that tone then fair enough. I know full well what you were saying but I was questioning the premise. In my eyes a notable decrease in forecasting skill at day 6 won't manifest in the output until days 7&8 and increase thereafter due to the snowballing of the butterfly effect? It's all a moot point anyway given that we often see fairly significant changes in the 72-96 hrs range.

Control run follows the op to an extent in FI, keeps us in the game and holds blocking at a higher latitude than the 18z did Opportunities galore from there

Longer term, CFS continues with its (I'm guessing) SSW driven March. Brilliant continuity from when I was last checking it 2/3 weeks ago. Fits in brilliantly with longer term GFS 0z. Not seen this level of consistency from CFS since March 2013, another SSW driven month. Could be some fun and games to be had next 6 weeks or so...

This is more like it...shame it's 300 hours plus... Leaching our cold air from Scandi rather than running the risk of importing it from the Balkans! Heights generally lower too so more precipitation likelihood for more people

Pointless analysing it as so different to last run...may not even get that far if UKMO is on the money. We'll just have to wait and see. But yes, at 264 hrs there is a chance to link the N Atlantic and Svalbaard heights EDIT: and on cue...

Wow if only we could pull all that westwards!

It's a common theme starting to creep in to the modelling, earlier and earlier (the HP losing it's shape). The UKMO is actually quite awful at day 6 for this with the 'sagging high' cutting off the cold feed fairly quickly At this rate we might need another attempt at positioning the high favourably (to get a proper snowy easterly) but it would have to be timed meticulously to avoid the Atlantic overpowering the block and sinking it.

There are differences this run....Scandi heights slightly further S and that straight E'ly seen on the 18z days 7&8 may turn more SE'ly as the high is orientated differently. Still all variations on a theme though. We're actually in a SSE'ly by day 7....though not a warm one at this point!

Aye but where do you draw the line on that one? I say this because what happens at day 7, for example, (outside of the sharpest forecast skill) can be determined by what happens at day 4 (within the sharpest forecast skill range). It's not as simple as you make out and when we're dealing with dynamic and complex physics an arbitrary cut off point usually doesn't mean a lot. Personally I will look at day 4 with scepticism when an easterly is progged...

Or a drought. I'm leaning towards a dry summer at the moment. Big trouble potentially if that's the case. Personally, would love a 2007 revisited!

As reflected in the 850s mean....a solid improvement from recent suites. Remember though, it can just easily slip the other way again!

Did you knock up that chart yourself Steve?

Yep, ideally would like to see that addressed. Stop that high from sagging and the mind boggles at the depth of cold we could see. A big hurdle to clear though!

Still all to play for though feb... I'd wager we're nowhere near the evolution we'll probably be seeing in 2-3 days time at that range.

Later on in FI, after a couple of milder days, we begin to try and shear off another Scandi high You can see the little bubble of warmer 850's heading up through Iceland NE The upshot should be a Scandi high which tries to regain its shape and recurve cold uppers back towards the UK

I look at this chart and think....if only we could get LP around the Balkans...it would set up a nice conveyor belt of cold uppers and serve to keep the high propped

Pretty decent easterly flow this... If I was being picky I'd like to see more of an undercut going forward, not sure how far inland any showers would penetrate from the east without disturbances pushing west. Definitely better than the SE'ly that had been modelled though.

At day 7 the ECM really is poor with a poorly aligned block, especially in comparison to what has gone before in previous runs and to other models. I would suggest that the strength of the Azores high at day 5-6 is the main reason for the poorly aligned block and subsequent lessening of the cold pool ejection...you can see here how it diverts too much jet north Which is exactly why I pointed it out as a potential spoiler earlier this morning!

They'll shift one way or another over the next 48 hours. Toss of a coin as to whether we get the deep cold in looking at those. Very few 'flatliners' as Feb would put it which gives some degree of confidence that it may not be a prolonged affair anyway.

Control run shows how the cold is kept in better shape when travelling across a more N'ly latitude As cold as you'll see Scandi that, pretty much

One thing to note, at 144 the GEFS mean is ever so slightly less cold with the Azores high pushed slightly further NE in towards the UK. I'm uncomfortable with its presence TBH as it will just encourage more jet energy to head N in the long run, increasing the likelihood of a sinking block. The GFS 6z low res is completely feasible unfortunately and may not be just 'model bias'