When you look back at 2010 look how strong that block that grenny was amazing

Apologies James off track

Clearing the low heights from Greenland this side of New Year would be a real bonus, and will come down to in part how far high pressure is able to invade the pole from the East U.S. and where exactly it exerts its influence.

The current consensus is for some kind of mid-Atlantic ridge, with a growing sign that it could be a fair bit more resilient than what we've seen at late thanks to the Pacific/U.S. amplification that now features across the majority of the model output, but little at this point to suggest that a full-blown block across Greenland will be achieved this month.

I'd take a 3-5 day cold spell for the final week of December, as the real fun and games remains on the cards for next month. The GFS 18z only offers two days down here, this cut short by a rapid deepening of low pressure close to Hudson Bay just like seen on the GFSP 12z op run.

I can see where those two runs get the idea of such strong low pressure development from - deep cold air plunges down across the Central U.S. and then gets shunted east, creating a strong thermal gradient between there and the Atlantic of the kind seen in December 2013.

Sounds alarming, but the thing is, GFS has always had a habit of shunting amplified ridge/trough combinations east too rapidly, which means there's a fair chance that the cold/mild boundary will take longer to reach the Atlantic, before which it will be less intense as both sides will be continental in nature.

...and how about that, the GFSP 18z turns out to be slower than the GFS 18z, so here's the evening entertainment:

A considerable NW'rly kicks things off, very interesting for the NW with that shortwave in on the act. However, a troublesome low lies to the SW - it's too progressive and extensive to allow the cold air to win out over England and Wales. A different story for Scotland though.

The blocking to the NW is looking decent tough, and that comes to our aid a little later:

As the low bombs out and shoots NE (looks painful for the south!), the high to our NW feeds in a good shot of cold air behind with a temporary blizzard for the northeast, just some wintry showers elsewhere before it settles down from the SW ahead of the Atlantic jet riding over the top of the ridge.

This is in fact a very unlucky run as far as the south is concerned. I've posted it not only as a result of the slower push east of the U.S. pattern, but because it is a great example of how often we live life on the edge when it comes to snow events in England and Wales! Nearly every winter we have to be strong so that we can handle it. Newcomers reading this, you have been warned 

Continuing the trend with the 18z, there is evidence that the mid-Atlantic high is becoming a more prominent feature. Certainty the Atlantic looks much calmer than of late; note the low anomalies in the southern part of the Atlantic. I know people worry about the azors, but a strong enough Scandinavian low can completely annihilate it, its quite a common route to a cold spell to first begin with an azors high. Alone an azors high will find it difficult to stand if it doesn't have the support from the north.  

Its not true, just before the 2010 cold spell there was an azors.

 That LP in the midatlantic caused the cold air advection which brought the cold air in the later part of the month. 

1963, right before the cold began:

Strong Azors high. 

Not that an azors high is in anyway desired, but its far from the be all and end all, everything else is going in the right way, and when you have very low heights to the east it matters far less.

Anyway GFSP and GFS both going for similar patterns on the 18z:

850s are far from impressive admittedly and both unfortunately do have strong Azors/Euro highs. On a positive note the Atlantic does look very weak with 'non-classical' lows. There does seem to be a good chance of retrogression to greenland as pressure lowers in scandanavia, that would bring us a cold northerly anyway, alternatively the azors high might try and move west which is another possibility some of the models have been playing with. I recon though that the xmas period will be dry if not cold. 

Not to be horrifically pedantic or anything (and I'm enjoying your posts!) but every time you type Azors instead of Azores I have a little involuntary twitch 

My apologies, I will try harder. If it helps I feel the same when anyone says PV (including the necessary self deprecation that I also used to do that).

It's worth mentioning that yesterday's 12z ECM control run was one of the coldest yet this winter for the UK: high pressure builds strongly to the west after day 10, leading to a northerly flow that persists for several days, bringing sub -5C 850 air across the majority of the UK (occasionally warmer in the SW). There are shallow lows caught up in the flow too.

240: Deep low SW Norway, strong cold front moving south over the UK. Thicknesses range from 548dam in far south to 528dam over NW Scotland.

288: Elongated high mid-Atlantic, angled NNW/SSE. NW'lies and NNW'lies over UK, thicknesses from <510dam over northern Scotland to 524dam over Northern Ireland.

336: High mid-Atlantic, centred 54N. Elongated N/S axis. NNW'lies over UK, thicknesses from 518dam northern Scotland to 522dam Northern Ireland.

360: High sinks southwards, still N/S axis, centred at 47N. NNW'lies for UK, thicknesses from 519dam Kent to 533dam Northern Ireland.

By 360 there's a ridge from the Siberian High over Scandinavia and a deep (988) low over Italy.

The ECM ensembles now show a pretty decent colder cluster appearing too, with something like 45% of the runs showing a cold spell from Boxing Day onwards. It certainly ties in with the signs from this time last week - now to see whether the momentum can be maintained!

nice analysis Retron, thanks. Here's hoping for a trend setter.

It's a Sigmund Freud thang, AZors  "RAzors" blades at the ready if the models refuse to budge!

Most of the operational output continues - as it has done for a while now - to show a westerly regime into early next week, with some mild days and cooler, showery ones mixed in.
The 'Christmas Truce' from around the 24th is becoming more prominent, with a break in the train of lows across and to our North resulting in a slack N flow with a transient Atlantic/Greenland link. It looks like a fairly weak flow on the scale of it though and hostilities look like resuming shortly thereafter with the high re-asserting itself over the Azores. The 00Z GFSP illustrates the set-up should the Northerly last a few days, and it's possible that with a bit of luck some areas may see a snowflake on the big day.

It is also possible the majority of us plus the NWP have got it wrong and the Christmas truce develops in to something more prolonged - it is +240 out after all. I wouldn't bank on it though, and the clever money would be on a long, drawn-out war between the prevailing Atlantic forces and the winter brigades into the early part of January at least.

Whilst the GFS 00z operational run has dropped the idea of a mid-Atlantic ridge in FI, it's interesting to note that the 850 ensemble set is the coldest yet for Aberdeen from Christmas Day. 

Both the the operational and control runs are mostly sub -5 uppers from the 25th for the remainder of the run. 

It it will take a while for the models to lock in a solution post Christmas I think. Lots of uncertainty.

Even if the ridge doesnt take hold withers from the Atlantic onslaught, if the lows take a more southerly track and we see trough disruption over us, there will be some snow for those in the right places

http://www.theweatheroutlook.com/charts/gefs/gefsens850london0.png?cb=628

Huge uncertainty after 120 hrs

HERE IS MY LATEST ANALYSIS FROM THE OUTPUTS OF THE WEATHER COMPUTER PREDICTION MODELS FOR TWO WEEKS FROM 8AM TODAY TUESDAY DECEMBER 16TH 2014.

NEXT UPDATE WEDNESDAY DECEMBER 17TH 08:00

THE CURRENT GENERAL SITUATION A ridge of High pressure across the UK will move away East with an active warm front following bringing very mild WSW winds across the UK tomorrow.

MODELS-2 WEEK HEADLINE Remaining changeable with rain at times especially across the North and West with snow on hills there at times. Temperatures alternating close to or a little above or below average day to day but overall no severe weather expected over the period.

THE JET STREAM ENSEMBLE FORECAST The Jet Stream Ensemble forecast continues to show the flow moving West to East across the UK in an undulating form as the polar front moves South then North again within the west to east flow.

GFS OPERATIONAL The GFS operational this morning shows a pattern which changes little over the period. Yes there are variations in day to day conditions with mild winds and rain alternating with colder and more showery interludes but away from high ground most areas away from Northern high ground will see nothing wintry falling from the sky through the period up to the New year with frosts at night patchy and occasional.

THE GFS PARALLEL RUN The GFS Parallel run does show a window of opportunity around Christmas of a period of Northerly winds and some snow showers mostly iver the North while the South sees some rain or sleet for a time but other than that the pattern of High down to the South and SW and Low to the North persists or indeed returns after Christmas on the run up to the New Year.

THE GFS CONTROL The GFS control run shows conditions which are conducive to a Westerly flow of weather being maintained with some days when mild and wet conditions prevail and then a couple of colder polar maritime air type days when sunshine and wintry showers dvelop along with a pathsy frost. Late in the run the weather is shown to become more deeply unsettled for all with rain and windy weather for all but still with colder periods with wintry showers, always most likely over the North.

THE GEFS ENSEMBLES The GEFS Ensembles members in very basic terms though not without exception look like the best chance remains with a continuation of the Low pressure vortex lying to the North of the UK and High to the South all the way up to and including the New year period. In weather terms this means more rain and showers across the UK with average temperatures or a little above at times alternating with brief colder interludes with wintry showers in the North.

UKMO UKMO today shows the UK under a strong Westerly flow to start next week lying between High pressure to the South and Low to the North. A cold front is shown to be approaching the UK from the NW early next week which will likely replace mild weather with some rain with colder and more showery weather as we move towards Christmas though with any wintry showers confined to the high ground of the North.

THE FAX CHARTS The Fax Charts today show a complex series of troughs crossing East over the UK from later today until Friday in moist and mild air. On Friday cold fronts move SE down over the UK clearing the UK by the weekend bringing a temporary colder and brighter NW flow with scattered wintry showers over the North.

GEM GEM also maintains a mild Westerly flow predominating across the UK with just short colder injections of air with wintry showers in the North for a time this weekend and again over Christmas when more prolific amounts of frost may develop over the South for a time as High pressure moves in from the West, slowly slipping away SE post Christmas.

NAVGEM NAVGEM maintains the strong Westerly flow often mild and cloudy with rain mixed with some short 24-48hr periods of brighter and colder weather when wintry showers could occur across the hills of the North.

ECM ECM this morning shows a mobile and strong Westerly type pattern through the period with spells of rain and mild weather alternating with colder and brighter weather with wintry showers in the North. There are two separate shots of colder polar maritime air shown, one this weekend and another coinciding with Christmas but away from northern hills the chances of snow on lowland UK remain low over the period.

ECM ENSEMBLES The ECM Ensembles on face value do not look over inspiring though for the first time there is evidence of change afoot the other side of the Atlantic as Low pressure over Canada looks like beginning to support a rise of pressure over Greenland which although at this stage will mean little longer term a rise of pressure there could raise tha chances of a pattern change for the UK.

NOTABLE TREND CHANGES FROM PREVIOUS RUNS Trends away from the current pattern of weather across the UK remain weak and ill defined with the overwhelming majority in favour of only small variations in the overall pattern the most likely this side of the New Year.

MY THOUGHTS The models continue to yo-yo between mild and strong Westerly winds with rain at times alternating with colder polar maritime air with wintry showers across the North. There are two separate attacks of the colder air from the NW, one this weekend although away from windward coasts and the usual hotspots in the North snow showers are unlikely on low ground but more widespread frost is likely over Friday and Saturday night. Then another perhaps more substantial attack over Christmas itself when if a lot of todays charts verify would mean we would wake to a sparkling and cold, frosty Christmas morning in a NW flow which I'm sure many of us would take over the mild and damp and sometimes wet conditions which prevail before and inbetween these two incursions. Having said all that though the pattern remains stubbornly resilient to meaningful change with the polar vortex remaining in place to the North of the UK and after a temporary weakening of High pressure to the South mid period the end of the run shows the status quo resumed with more periods of rain and wind on a Westerly flow around a strong Azores High and temperatures close to average or alternating just above or below as each front passes. While long term projections from the experts remain positive for a change of pattern later in the Winter it looks unlikely that any major shift will occur until early 2015 at the earliest which means we have to be satisfied with the same old brief incursions of cold from the NW over the coming weeks which will probably disappoint all but those living on the highest ground of the North or the ski resorts of Scotland if it's snow your looking for.

Looking towards the Xmas period and I still maintain anything cold will be a 3-5 day affair at best. As for the big day itself those with altitude and in the far North may see some of the white stuff but it's Boxing day onwards I feel that we'll see a decent nationwide cold snap, thereafter the Azores high looks to flatten things out again with a more mobile Westerly regime.

Longer term signals from the EC are now suggesting +Height anomalies over the pole and a -NAO from JAN-MARCH, I would say the the first week or so of January will start of similar to what we are seeing now with a switch to a colder regime from mid January onwards, hopefully. 

1962/63 started on Boxing day I think - if only weather history could repeat itself. 

4 week forecast to mid Jan see how that pans out

Thank you for such a detailed description. Some support from the de Bilt ensemble, but obviously plenty of scatter too at that range.

http://www.weerplaza.nl/15daagseverwachting/?r=midden&type=eps_pluim

De Bilt is only useful when talking about an easterly.

All of the Charts currently are going for a north westerly into a northerly.  Meaning the deBilt ensemble is about as useful as Londons ensemble.

Which is obviously good for people living in that part of the world or for others looking for trends

 some off those gm members are amazing

Super-Compact Overview of Day 5-6 NWP Trends:

LP progresses east more rapidly on the 00z op run compared to yesterday's 12z op run. GFS, GFSP and UKMO show little development of the LP arriving from the SW of the UK until it has tracked into the North Sea via a route passing just north of Scotland. ECM shows much stronger development before reaching the UK and progress into the North sea is slightly faster than the other models go for and also faster than yesterday's 12z effort by the model.

Looking Further Ahead:

ECM and GFS then show strong LP development in the Eastern Canada/Hudson Bay area which extends a fair way south in the far-western Atlantic. GFS goes on to relentlessly drive the jet NE across the Atlantic with only a brief fight by HP to the NW of the UK.

Would ECM do the same? It's far from clear, but the angle of the jet appears better for at least a slower toppling of the ridge when using the 500 hPa heights as a guide (the ridge in the pale green). The charts below (GFS left, ECM right) show the difference:

A slower toppler could allow the LP over Scandinavia time to dig further towards the Med. which would open up the door to the Atlantic ridge being supported to our NE for a time. This is how yesterday's 12z ECM control evolved going by Retron's report, good signs indeed.

I do think anyone giving the pace of GFS's toppler much credit is not playing their cards wisely. GFSP looks a bit more reasonable at first glance but the development of LP over the U.S. is again on the extreme side, and the way it just takes off for Greenland, while triggered by a plausible Canadian shortwave, still seems very progressive.

Yesterday I proposed that the development of an intense thermal gradient between the U.S. and the western North Atlantic was the catalyst for strong LP development on the GFS and GFSP runs, and this still seems reasonable as of this morning, but I can also see another mechanism that may go further to explaining this desire to dramatically lower heights in the vicinity of Greenland soon after day 10; the models show the strat. vortex at 10hPa stretching in that direction as it comes under pressure from warming events:

So what we're seeing appears to be a rapid trop. response to strat. vortex changes at 30hPa.

This suggests that low heights are likely to re-establish to our NW around the turn of the year, although developments higher up in the strat. indicate that the Greenland lobe should come under serious fire very soon afterwards (10hPa vortex heading for Scandinavia as of day 16).

This does not mean that the ECM control run can't be along the right lines though - that can happen if we see less vigorous LP development to our NW than GFS/GFSP currently show coupled with some low heights to get down into Europe prior to the toppling of the mid-Atlantic ridge.

The 10hPa and 30hPa strat. developments seem to fit in quite nicely with the latest EC-32 and EC-seasonal updates which it has to be said are outstanding if you're seeking significant cold spells in the first few months of 2015!