thanks stormchaser another great post you never disappoint, [ sorry for the 1 liner, ]

That will come, trust me. There is however a decent atmosphere in here this season with some great input from people who make an impressive effort to post with detail and justification.

Good evening Mario. Good to hear from you.

We all salute him, it is part of the Ts & Cs of membership 

No major changes on the 18z GFS op run output compared with earlier runs (up to Christmas Day)

This OP run drops the deep low for a sausage dog

Good news for people who do not welcome very strong winds but beware - the way things have been it could easily reappear in subsequent runs.

The GFSP gives us a blizzard in a week. Can't remember a time when the models were more all over the shop. ECM is better long term but this blizzard would be nice. 

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=168&mode=2&runpara=1

On the 18z the post Christmas northerly succumbs rapidly to the grim reaper toppling high. Pub runs ain't what they used to be 

18z topples even faster and back to default high to the south at the start of low res.

18zp run still going with  deep low to the east (one to watch if it don't drop the idea soon)

http://modeles.meteociel.fr/modeles/gfsp/runs/2014122018/gfsnh-0-168.png?18

There seems to be a choice between where the low comes from - the midatlantic or the NW. If its the former then it means colder conditions during the incursion with snow in the north, but a collapsing block, in the other case it means a brief mild period but a stronger block in the end.

My bet is the later scenario, only because lows love to come from that spot, and the former is just so wierd.

looking at the temps for this chart think it would actually be a lot of cold rain. Not sure why it shows snow.

The 18Z has the block collapsing but some snow still to the SE for a time.

Its marginal, my precip type parameter seems to think it could go either way.

Don't think we are much further down the line to knowing how long this cold spell will be or how cold or if we will see any snow. All we can say is that there will be a cold spell I think?

Sure but I still thought it would be interesting to run it through anyway, I suspect the lapse rate is pretty high so the uppers don't need to be as cold, but yes I agree with you, the track of this trough will be very important in determining who gets snow and its far too early to say.

Maybe evaporative cooling would help? Good charts by the way Q.

Indeed although one thing this doesn't take into account is the wet bulb (related to evap cooling) , I've been meaning to include it but the calculation to actually do that is tiresome to say the least, I really admire the fact that Brian has a wet bulb map on TWO charts; it can't have been easy to make. 

Anyway I have about 5 more winter type charts in the works including snow accumulation and ensemble weighted snow probability. Can't wait to try them out for the short term. I'm like a kid with a new toy! 

I'm not sure we can even say that. Computer modelling is still really quite useless isn't it? :)

Makes a total mockery of people doing long range forecasts when we can't even say whether we'll have a Greenland high or a massive Euro slug in a week's time!

I don't know about that. People on TWO started talking about a cold spell on the 14th, that's when something was brewing. Since then there has been a lot of mad flipping with regard to the position of troughs e.c.t. but the idea of raising heights over greenland and a calming of the atlantic has remained consistent throughout.

One further point: The CFS has trended towards showing high pressure and a weaker atlantic since early December, it did, to be fair, go for blocking in the Azores rather than Greenland (although it was hardly wrong about that!) but it was still successful in being able to show that at all (although because of the displaced nature of the block people using it in LRF would probably have incorrectly gone for a mild end to the month).

Just a slight shift toward ECM within the 6 day range by GFSP and that's arguably the most significant development in the whole run, as it leads to a slower toppling of the high and almost allows LP to become entrenched close enough to the east for interesting results.

The trouble for longer term prospects is that the amount of energy heading south from the split jet in the Eastern U.S. is not even close to what ECM and JMA produce, and this lack of a jet flow underneath the mid-Atlantic high means it has nothing to 'stand on' - hence it sinks and then topples.

This preference for having the northern arm of the jet far stronger than the southern arm is something the 'old' GFS is well known for... and has clearly been even more present in that model's last few runs, with the U.S. low still having the SW-NE alignment, setting the output apart from all of the other models.

The question is, are we seeing another classic case here, or are the likes of ECM leading us up the garden path? As I commented on earlier, the professional view over the other side of the Atlantic appears to support idea of classic GFS bias being at play, which is cause for some hope to be had.

It's certainly the simplest explanation for what is a really quite massive difference between the GEFS mean and ECM ensemble mean, the former really pushing low heights across to our N. while the latter shows a prominent mid-Atlantic ridge keeping the low heights in the U.S./Canada sector, even with some suggestion of the low pressure underneath that can work to sustain it for the foreseeable.

So it seems like we should expect GFS and GEFS to back down dramatically in the near future... but as usual, no guarantees I'm afraid