Because you concentrate on the wrong things, the Azores high is only an issue if the rest of the Atlantic is working with it, i.e you have a dipole like this:

When you see red and purple on the map at the same time, that is the point at which you start worrying. An Azors high coupled with a greenland high and Scandinavian low is not an undesirable pattern at all, because its so easy for the azors high to relocate itself to Greenland or dissipate when low heights move in from the north. Likewise a negative AO is worse than useless if the Atlantic is not cooperating. 

To illustrate how things have progressed here is a tile of some of the height charts I have posted over the last few days. Its all 192 so inperfect I know, but still makes a good point I feel.

And here is Today's

Note how run after run there was a strong trend to build heights to the south of Greenland and extend the influence of the Scandi trough. My only issue was that the ECM was originally not on board; now it is and there continue to be upgrades in the 500mb pattern. Like I say these arn't the same time periods obviously, but it still gives an idea of how the pattern has improved at the back end of high res. I reckon this is more important than the temperatures - you guys taught me that when I failed epically to predict, I think it was the 2012 cold spell, based on 'warm easterlies'. 

Look at the tiles I posted in the comment above, its been visible for at least 3 days, and the metoffice were going for a colder period in their forecast, though understandably they were being cautious about it particularly since the ECM was not on board, but now the ECM is on board we can be far more certain. The point I was making is that an Azores high in isolation is not a problem if everything else in the pattern is helpful, just like a -ve AO in isolation is useless. One further thing, although the azors high was impressive at the surface, the 500mb heights were less so, I'd be more worried if there were red colours under it than yellows and oranges, particularly if those colours start to get close to greenland!

There's a clear trend there with the band of high pressure becoming more connected/coherent as days pass.  Hopefully the trend continues and we get a solid block in place to extend and deepen any cold 'snap' we may get after Xmas. 

Anyone????

Obviously very tenuous at this range, but could boxing day be the one to watch out for? The models have xmas day as a transition (except for the north where the cold air is already in place), but by boxing day we have disturbances and the cold air right across the country. This parallel run is rahter agressive so snow for the east and rain for the west, but most runs less agressive.

I know I go on about never taking stock of outputs beyond the five days range, but I'm not really too keen on this one.

This shows the stratospheric vortex trying to push towards North America as we go into the opening days of the New Year. That had instilled a sense of deja vu on me as that would have the effect of firing up mid-Atlantic cyclogenesis once it start interacting with the jet stream - just like last year.



However, it is a million miles away in forecasting terms and it may just be me being pessimistic - and of course there is the positioning of the jet stream to take into account.  

I think both are useful but the Para is maybe becoming more useful as it becomes the main GFS run soon. mid January I think according to the Brian?

GFSP verifies slightly better but I wouldn't really treat them differently. Much more important is consistency between the GFS/GFSP/GEFS/GEM/ECM. All else is chicken feed in the 10 day market.

The thing is though with heights over greenland it always had the potential, and will have the potential to be more than a 2 day cold snap. Of course that doesn't guarantee anything, but it is a better bet than seeing heights over scandanavia or just a midatlantic high, which ofc it still might end up as.

I would again suggest people step through the individual GEFS perturbations.

http://www.theweatheroutlook.com/twodata/datmdlout.aspx

Unless one of the few colder solutions is the outcome the south is unlikely to see snow this side of the New Year. It's a different kettle of fish in the north as it has been for much of December.

The Mean on the GFS Ensemble graph from the 06z for Aberdeen says it all for me personally:

http://www.theweatheroutlook.com/charts/gefs/gefsens850Aberdeen0.png

What I like to do as I'm not that brillant at all this is to simply stick a ruler along the -5 line and see how often/long/far the mean drops below it. On this run it gets to about -6 twice and for about 24hrs each time which to me is hardly awe inspiring and this is for Aberdeen, never mind locations further south.

I know it's not very scientific but it helps me get some sort of steer at this time of year

However there is encouragement in that there are a lot of members dropping below -5 much further and much longer but much more support in general terms (e.g. through the mean) is needed for me to start getting overly excited.

OK at 240hrs 6Z GEFS 500mb heights

Control: Greenland high, 552 isopleth south of greenland 

P1: Mid-atlantic high, Northerly. 

P2: LP over Greenland, transient cold. 

P3: Bartlett. 

P4: Mid-atlantic, Greenland hybrid, 552 isopleth over Greenland. 

P5: Strong greenland high, slower transition with lower heights. 

P6: Mid-Atlantic, Greenland hybrid, 552 Isopleth over Greenland.

P7: Weak greenland, mid-atlantic hybrid, stalemate.

P8: Northerly toppler type conditions.

P9: Strong greenland high, but unhelpful shortwave, likely slow transition to good situ.

P10: Cold sector, but not helpful.

P11: Overly strong Azores.

P12: Strong greenland high, but Azors also strong. 

P13: Cold northerly, Uncertain outlook. 

P14: UK amplification. 

P15: Strong Azores. 

P16: Strong Azores. 

P17: Ditto. 

P18: Some mid-atlantic amplification. 

P19: Euro. 

P20: Mid-atlantic amplification. 

OP: Ditto. 

Where  is ideal,  is promising but leaves much to be desired,  is unfavourable.

We have

4x

9x 

9x 

So the majority going for either promising or ideal. The ECMOP and GFSPOP go into the top categories too. 

you stole my idea! :P

Loving the use of smileys, Q!!

What about the middle chunk?

You've been very generous I'd say. 

P5 brings colder air in as high pressure collapses over the UK.
P6 keeps the -5C line away from the south over central for most of the time before high pressure collapses

I'll not even worry about the  , although as I said these are more interesting on the whole for the north.

I just feel the need to say, I'm not contrary on purpose; I know it might seem like that for the past couple of months; apologies if it has turned out that way. I'm very much with Brian's view about the stratosphere, but any trend towards a Greenland high in my view should be taken extremely seriously, far more seriously than any other type of cold set-up. At the very least, perhaps we can all agree that xmas day is likely to be cold (if not a white xmas for the north!).

I reckon you've got a decent chance of seeing falling snow and possibly some slushy and fairly short lived accumulations. You've probably got a better chance of snow than I have.

To be fair some of the s are even colder than some of the ups  albeit briefly; I was more concerned about the overall pattern than the airmass temp. I think ECM advises that beyond day 6 the 500mb should be the first port of call rather than anything lower, also 850s are often upgraded if the pattern is right; if it was all this fuss over a northerly toppler I would agree with you.

Yes I agree with the point about 500mb charts being the first port of call, although I think you'll find it's considered less important now than it was. Numerical models I believe (when CPU cycles were more expensive) modelled fewer levels with the 500mb being key. Since then there have been big improvements with modelling lower levels and even surface features.

Now less scientifically, experience tells me that more often than not we need the cold air in situ for several days. Why? Because often x, y or z appears and scuppers the potential for snow, so we need several possible disturbances for one of them to 'click' and deliver the widespread snow people are looking for.

Yes that is true, and the discrepancy used to be huge, that said it still exists today, to the extent that a 500mb GFS chart can be as or more accurate than a surface ECM chart of the same time period. Also another advantage is that its relatively easy to see trends in the 500mb level, but quite difficult to do the same in the SLP beyond 168hrs or so. 

Also one thing to note, and this ties in with what you said to another poster, I do live in the north, so what I am saying is naturally going to be more geared to me. By default I do look at London ensembles to keep the comparison the same, but generally I am looking for cold air in my neck of the woods which does look far more likely. Not that I am in the north-north, getting snow to my latitude would require a non-trivial cold spell unlike Wick or Aberdeen!

Yeh I agree with that, but shouldn't it be the case that upgrades are just as likely as downgrades? I think we could see a few downgrades on the OP and still come out okay (perhaps with a 4 day cold spell).

@144 hrs on the 12z. 

On the Cusp of FI - seems ok so far.

Cold plunge is already further south on this run.  will be interesting to see how this chart ends up,

Doesnt look like we can sustain heights over Greenland so a short lived cold spell is the most likely outcome