In Depth Model Discussion, Analysis and Summaries

[SMU]

I thought id tag this one onto the model output this eve-

Since the advent of the internet & the 'archives' on the forum every year since around 2000- even back in the snowatch days on the BBC forum ( some will remember this) we have all referred to Winters of old, the 63's, 47's, 79's ,85's etc etc- We place these with fond memories close to our hearts & are we are all very quick to post up images of what used to be-

Since the advent of this -

http://www.wetterzentrale.de/archive/2009/brack/bracka20091216.gif

some 10 days ago where the first pressure rise over greenland developed -this winter has had a whiff of a classic about it-

Weary seasoned posters would have been sceptical about the pattern redeveloping in such a short space of time- especially with such familiarity as the last evolution, however as we edged to Christmas day & now almost beyond we see a potential 'classic' pattern developing- which in my opinion has a BETTER chance of sustaining cold & the UK seeing widespread snow- ( we havent done half bad this time round though)more of that in a moment & the reasons why-

Firstly the evoltion thats being progged for the early part of next week is marginal at best, dont get me wrong we have a chance of something widespread, but as it stands the angle of attack, surface cold ahead of the front & upper air conditions do NOT favour a widespread snow event-

If we take the ECM 120 which seems the middle ground-

We see that the system moving along the channel bears the resemblence & hallmarks of that classic snow event in the making- but we are missing the vital ingredient- HP to our NE...

We have seen a lot of Southerly tracking lows already- both this year & last year- the shallow shortwaves that skip along the coast- however the South & south east have seen rain events from ALL these- even central areas havent favoured much better-

Take this chart for instance from the 8th of Feb this year-

http://www.wetterzentrale.de/archive/ra/2009/Rrea00120090208.gif

The UK in a cold run of Northerlies & very cold surface air prevailing over the UK- that shortwave undercut some of the UK but the cold got lifted out to quickly for most- it was a quick snow to rain event for nearly all- snow lasting less than 20 mins!!

This has been the plight of the UK for some time-

now look at a couple of classics-

http://www.wetterzentrale.de/archive/ra/1982/Rrea00119820108.gif JAN 1982- the Irish & welsh should have fond memories here-

http://www.wetterzentrale.de/archive/ra/1996/Rrea00119960206.gif Widespread snow over the central belt of the UK-

http://www.wetterzentrale.de/archive/ra/1978/Rrea00119781230.gif THis lead to one of the most significant blizzards over the UK for many many years-

So what have we learnt-

Undercutting troughs- even if they are elongated require HP DIRECTLY to the NE to force the dry air undercut at the surface- with the only exception being if a shortwave is traversing EAST under a East based iceland block ( again more about that later)

Back to the ECM 120-

The trough over Norway is cutting the supply of cold off from the Scandi region- where the coldest airmass originates from-

Sure we have a cold feed from the East but just not cold enough-

taking the 18z at face value its a total rain event for the UK, taking the ECM at face value its a rain event for most in the South - with snow over the North & west & elevation a great assist here-

Is there still hope for the mid part of next week- well yes- but we will need to be VERY VERY lucky with a quick flip back from the models to a track along the brest peninsula through & out East- even then thats not the end of the story - we will need minimal north sea track as well from an upper air perspective-

Excluding what I think is dodgy outputs this eve I think there is a zone from yorkshire through to wales that does see snow ( mainly away from the East coast), although PERHAPS not as significant as initially touted ( I hope im wrong with that)

This wasnt the reason of this post though really- its the background signal though & for the people that want to decipher things into more detail the finite differences between a Greenland block & Iceland block- Also how the amplification of the jet determines a Northerly over an Easterly- even though the blocking seems to be in the same place- ( & why a circular iceland block favours a Channel low as opposed to a Greenland block which favours a polar low)

The background NH signal is a negative AO which if by now you dont know I will breifly summarise-

High pressure at the pole & low heights forced into the mid lattitudes- by definition this then means that theres LOTS more air coming south out of the pole: in this situation as you will usually have 3 or 4 standing waves along with the higher polar pressure zone- - The eastern sides of the standing waves have huge amounts of COLD air advection moving South & the western sides have lots of warm air advection moving North-

Note:- this is why in negative AO months the UK can be warm as we can still be on the wrong side of any potential blocking-

What we are seeing now at day 5-10 is a rapid change in the modality of the NAO ( after a brief rise ) -

http://www.esrl.noaa.gov/psd/forecasts/teleconn/images/nao.png it will appear here in the next few days-

The modality change is showing that Greenland has a rapid rise in Pressure & areas towards the azores will drop pressure- the significance is that the AO is dropping to 3/4 Standard deviations below zero & the NAO is heading that way-

this is the PERFECT menu of cold for NW europe-

Teleconnections however do not give us 'local detail'-- local being the United Kingdom- this boils down to what type of blocking actually exists- as there are Many types-

East based NAO, West Based NAO, mid atlantic block, iceland block-....

What we saw develop last week was Clearly a Greenland block- Very central to Greenland---- this & west based Greenland Highs favour the NW UK & Scotland & NI- as they generally end up with NW winds over the UK-

Fax for the 20th- 2009

http://www.wetterzentrale.de/archive/2009/brack/bracka20091220.gif

Reanalysis for 9th dec 1981-

http://www.wetterzentrale.de/archive/ra/1981/Rrea00119811209.gif

Note the central Greenland blocking - almost west based NAO-

note the flow over the UK is generally NW- with the trough also west over scandi-

Going back to last week the reason the pattern loaded to a Northerly & even N.westerly was because of the initial jet profile & the fact that it was VERY amplified North / South-

We got a flow that went directly north up western greenland then returned sharp south southwards over iceland- hence the Northerly flow & how the vortex got shunted to the NE part of the UK-

Also importantly here - note how close the shortwaves got to the southern part of the UK in the southern arm of the jet--- they didnt....

because the Northerly flow kept them a long way South-

http://www.wetterzentrale.de/archive/2009/brack/bracka20091221.gif

look towards Portugal......

We now though see a different pattern setting up which as I mentioned earlier seems to be the same- but with a couple of key differences-

*) NAO block (high pressure) looks to be positivly tilted & circular as opposed to the squashed sharp North /South oval shape it had last week-

*) at a SLIGHTLY more favourable locale V last week ( we are talking more EAST based)

*) Pressure core looks to be centred towards the SE coast of Greenland

*) THe jet flow off the states is less amplified over the top & doesnt look to cut a sharp gradient back directly south through iceland like it did before-

What does this all mean-

Look at these 3 charts-

ECM 168-

GFS 168-http://91.121.94.83/modeles/gfs/run/gfsnh-0-168.png?18

GFS ENSEMBLE MEAN at 144-

http://www.wetterzentrale.de/pics/Rz500m6.gif

They all agree on a curved East ISH based NAO block that is in closer proximity to Iceland than we have seen in many a year...

What does this all mean-

It means the background signal for cold for the UK is developing at around 96-144, once completed it will force the pattern to where we want it & because of THIS angled flow of set up- ( note the waeker amplified flow off the states)

http://91.121.94.83/modeles/gfs/run/gfsnh-0-96.png?18 IE more positivly tilted -

the net result will be a more Easterly component in the air as its curving westwards on the southern side of the high as opposed to southwards before-

this will eventually deliver for everyone, with a HIGH chance of a snow event coming up from the South west into Southern UK- this is why both models- ECM & GFS are modelling this-

GFS 192-

http://91.121.94.83/modeles/gfs/run/gfsnh-0-216.png?18 notice the block centring more over Iceland- ICELAND BLOCK= Perfect for the UK-

ECM-

Almost an Iceland block- but the key is the curved flow over the atlantic- the shortwave will roll & Spin eastwards because its pathway is being vented & circulated from inflow on the NE quadrant > NE >SW & inflow from the SW quadrant >SW> NE-

In Summary then-

I think we are on the eve of a very special period in UK weather- MAYBE a classic in the making, I think the classic charts come though in the latter half of next week towards the weekend with an attack from the NE & the south west-

If we get a wdespread snow event from the inital low pressure tracking along the channel then treat it as a bonus for the UK.............

fingers crossed this could be the one-

If we get the iceland block I would rate a higher chance of seeing a temporary link up with a Siberian /arctic type high pressure that has been lurking all winter-

S

[danm]

I thought id tag this one onto the model output this eve-

Since the advent of the internet & the 'archives' on the forum every year since around 2000- even back in the snowatch days on the BBC forum ( some will remember this) we have all referred to Winters of old, the 63's, 47's, 79's ,85's etc etc- We place these with fond memories close to our hearts & are we are all very quick to post up images of what used to be-

Since the advent of this -

http://www.wetterzentrale.de/archive/2009/brack/bracka20091216.gif

some 10 days ago where the first pressure rise over greenland developed -this winter has had a whiff of a classic about it-

Weary seasoned posters would have been sceptical about the pattern redeveloping in such a short space of time- especially with such familiarity as the last evolution, however as we edged to Christmas day & now almost beyond we see a potential 'classic' pattern developing- which in my opinion has a BETTER chance of sustaining cold & the UK seeing widespread snow- ( we havent done half bad this time round though)more of that in a moment & the reasons why-

Firstly the evoltion thats being progged for the early part of next week is marginal at best, dont get me wrong we have a chance of something widespread, but as it stands the angle of attack, surface cold ahead of the front & upper air conditions do NOT favour a widespread snow event-

If we take the ECM 120 which seems the middle ground-

We see that the system moving along the channel bears the resemblence & hallmarks of that classic snow event in the making- but we are missing the vital ingredient- HP to our NE...

We have seen a lot of Southerly tracking lows already- both this year & last year- the shallow shortwaves that skip along the coast- however the South & south east have seen rain events from ALL these- even central areas havent favoured much better-

Take this chart for instance from the 8th of Feb this year-

http://www.wetterzentrale.de/archive/ra/2009/Rrea00120090208.gif

The UK in a cold run of Northerlies & very cold surface air prevailing over the UK- that shortwave undercut some of the UK but the cold got lifted out to quickly for most- it was a quick snow to rain event for nearly all- snow lasting less than 20 mins!!

This has been the plight of the UK for some time-

now look at a couple of classics-

http://www.wetterzentrale.de/archive/ra/1982/Rrea00119820108.gif JAN 1982- the Irish & welsh should have fond memories here-

http://www.wetterzentrale.de/archive/ra/1996/Rrea00119960206.gif Widespread snow over the central belt of the UK-

http://www.wetterzentrale.de/archive/ra/1978/Rrea00119781230.gif THis lead to one of the most significant blizzards over the UK for many many years-

So what have we learnt-

Undercutting troughs- even if they are elongated require HP DIRECTLY to the NE to force the dry air undercut at the surface- with the only exception being if a shortwave is traversing EAST under a East based iceland block ( again more about that later)

Back to the ECM 120-

The trough over Norway is cutting the supply of cold off from the Scandi region- where the coldest airmass originates from-

Sure we have a cold feed from the East but just not cold enough-

taking the 18z at face value its a total rain event for the UK, taking the ECM at face value its a rain event for most in the South - with snow over the North & west & elevation a great assist here-

Is there still hope for the mid part of next week- well yes- but we will need to be VERY VERY lucky with a quick flip back from the models to a track along the brest peninsula through & out East- even then thats not the end of the story - we will need minimal north sea track as well from an upper air perspective-

Excluding what I think is dodgy outputs this eve I think there is a zone from yorkshire through to wales that does see snow ( mainly away from the East coast), although PERHAPS not as significant as initially touted ( I hope im wrong with that)

This wasnt the reason of this post though really- its the background signal though & for the people that want to decipher things into more detail the finite differences between a Greenland block & Iceland block- Also how the amplification of the jet determines a Northerly over an Easterly- even though the blocking seems to be in the same place- ( & why a circular iceland block favours a Channel low as opposed to a Greenland block which favours a polar low)

The background NH signal is a negative AO which if by now you dont know I will breifly summarise-

High pressure at the pole & low heights forced into the mid lattitudes- by definition this then means that theres LOTS more air coming south out of the pole: in this situation as you will usually have 3 or 4 standing waves along with the higher polar pressure zone- - The eastern sides of the standing waves have huge amounts of COLD air advection moving South & the western sides have lots of warm air advection moving North-

Note:- this is why in negative AO months the UK can be warm as we can still be on the wrong side of any potential blocking-

What we are seeing now at day 5-10 is a rapid change in the modality of the NAO ( after a brief rise ) -

http://www.esrl.noaa.gov/psd/forecasts/teleconn/images/nao.png it will appear here in the next few days-

The modality change is showing that Greenland has a rapid rise in Pressure & areas towards the azores will drop pressure- the significance is that the AO is dropping to 3/4 Standard deviations below zero & the NAO is heading that way-

this is the PERFECT menu of cold for NW europe-

Teleconnections however do not give us 'local detail'-- local being the United Kingdom- this boils down to what type of blocking actually exists- as there are Many types-

East based NAO, West Based NAO, mid atlantic block, iceland block-....

What we saw develop last week was Clearly a Greenland block- Very central to Greenland---- this & west based Greenland Highs favour the NW UK & Scotland & NI- as they generally end up with NW winds over the UK-

Fax for the 20th- 2009

http://www.wetterzentrale.de/archive/2009/brack/bracka20091220.gif

Reanalysis for 9th dec 1981-

http://www.wetterzentrale.de/archive/ra/1981/Rrea00119811209.gif

Note the central Greenland blocking - almost west based NAO-

note the flow over the UK is generally NW- with the trough also west over scandi-

Going back to last week the reason the pattern loaded to a Northerly & even N.westerly was because of the initial jet profile & the fact that it was VERY amplified North / South-

We got a flow that went directly north up western greenland then returned sharp south southwards over iceland- hence the Northerly flow & how the vortex got shunted to the NE part of the UK-

Also importantly here - note how close the shortwaves got to the southern part of the UK in the southern arm of the jet--- they didnt....

because the Northerly flow kept them a long way South-

http://www.wetterzentrale.de/archive/2009/brack/bracka20091221.gif

look towards Portugal......

We now though see a different pattern setting up which as I mentioned earlier seems to be the same- but with a couple of key differences-

*) NAO block (high pressure) looks to be positivly tilted & circular as opposed to the squashed sharp North /South oval shape it had last week-

*) at a SLIGHTLY more favourable locale V last week ( we are talking more EAST based)

*) Pressure core looks to be centred towards the SE coast of Greenland

*) THe jet flow off the states is less amplified over the top & doesnt look to cut a sharp gradient back directly south through iceland like it did before-

What does this all mean-

Look at these 3 charts-

ECM 168-

GFS 168-http://91.121.94.83/modeles/gfs/run/gfsnh-0-168.png?18

GFS ENSEMBLE MEAN at 144-

http://www.wetterzentrale.de/pics/Rz500m6.gif

They all agree on a curved East ISH based NAO block that is in closer proximity to Iceland than we have seen in many a year...

What does this all mean-

It means the background signal for cold for the UK is developing at around 96-144, once completed it will force the pattern to where we want it & because of THIS angled flow of set up- ( note the waeker amplified flow off the states)

http://91.121.94.83/modeles/gfs/run/gfsnh-0-96.png?18 IE more positivly tilted -

the net result will be a more Easterly component in the air as its curving westwards on the southern side of the high as opposed to southwards before-

this will eventually deliver for everyone, with a HIGH chance of a snow event coming up from the South west into Southern UK- this is why both models- ECM & GFS are modelling this-

GFS 192-

http://91.121.94.83/modeles/gfs/run/gfsnh-0-216.png?18 notice the block centring more over Iceland- ICELAND BLOCK= Perfect for the UK-

ECM-

Almost an Iceland block- but the key is the curved flow over the atlantic- the shortwave will roll & Spin eastwards because its pathway is being vented & circulated from inflow on the NE quadrant > NE >SW & inflow from the SW quadrant >SW> NE-

In Summary then-

I think we are on the eve of a very special period in UK weather- MAYBE a classic in the making, I think the classic charts come though in the latter half of next week towards the weekend with an attack from the NE & the south west-

If we get a wdespread snow event from the inital low pressure tracking along the channel then treat it as a bonus for the UK.............

fingers crossed this could be the one-

If we get the iceland block I would rate a higher chance of seeing a temporary link up with a Siberian /arctic type high pressure that has been lurking all winter-

S

Brilliantly informative post, especially for someone like me who is just learning what exactly to look out for beyond the blatantly obvious. Let's hope it all pans out in the way you are describing. We are long overdue a classic winter. Me being one month shy of 24, the only real severe cold spell i can remember is Feb '91. Feb '09 was the closest we've got since then down here in the SE, but not close enough.

Thanks Steve!

[Optimus Prime]

To compare this years current arctic blocking with previous years you can clearly make out there are some exceptional height rises over the arctic;

Compare this to the unsettled westerly winter of 1998/1999

December 2000 is the only date I can come up with since 1998 that had decent blocking action in the arctic;

The main reason why winter 2000/2001 wasn't as cold as what it could have been was because of a deep feature which had developed and ran up from the west

And heights lowered over the Arctic region.

Unfortunately the GFS N.Hemisphere charts aren't working at the moment so I can't see what's likely to happen over that region over the next 10 days but looking at what we got currently it's hard to see why this winter won't be cold.

[SMU]

Evening ALL-

going on from yesterdays output/post- of note in the evolution now is the fact that the NAO is more a west based greenland block as opposed to last nights iceland block-

Its difficult to know where to begin tonight, so many posts, so much model evaluation-

I suppose seeing as nearly all the models have a grip on the 'type' of track the low is going to take then there isnt much point of disucssing it- other than to say that I still think the trough will elongate along the channel perhaps giving a better profile of colder air to the central & possibly southern portions of the UK-

Its also pointless to post up maps for snow as the track/ elongation of the trough has a direct impact on the PPN distribution- however the marker laid out of Central wales has been pretty rigid from the start & again I cite that this area could be the hardest hit- especially at some minor elevation-

The subsequent evolution then becomes almost tantalising for everyone because if the 'proper'cold is unleashed from the East then that will certainly keep everyone happy-

We do have a problem though - as this surge from the East that I highighted the potential for last night has a rather annoying posive tilted shortwave sitting in the flow over southern Norway-

here it is firmly in the way on the GFS at 150-

http://91.121.94.83/modeles/gfs/run/gfsnh-0-150.png?18

now, no amount of willing from the residents of NW are going to force it South east- it needs to get pushed that way-

Also VERY VERY important here is the fact that when the Siberian/ artic high starts ridging west we do NOT have an infinite timeline to delivery the Easterlies-

We have a window of about 24/36 Hours tops- After this the block that origially started moving wsw piles up on the Eastern flank of the shortwave - the westerly vector then just loses its momentum & things start to head south ( yep you guessed it- towards central Southern Europe)

WHo remembers this- the huge failed easterly-

http://www.wetterzentrale.de/archive/ra/2007/Rrea00120070220.gif

See the positive shortwave over Norway it will not just turn over & face the right way- the southern arm of the jet is moving NE from iberia-

Now look at this example-

http://www.wetterzentrale.de/archive/ra/1987/Rrea00119870110.gif

The jet flow is moving SE through the UK over france - which on turn rotates the shortwave the way we want it- with the northern flank moving back to face ne> Sw NOT AS WE SAW IN 2007 SE>NW.

Its going to be VERY hard to get an Epic easterly from this ( after the initial set up)

the shortwave moving into the SW approaches at around 162 is coming in at EXACTLY the angle we dont want it- because its NE quadrant is acting against the force of the siberian high moving SW-

Ian browns favourite this one- although another epic fail-

http://www.wetterzentrale.de/archive/ra/2001/Rrea00120010203.gif

looks great but the southerly jet moving NE kills the flow stone dead ( even though shetland did well out of it)

http://www.wetterzentrale.de/archive/ra/2001/Rrea00120010204.gif

TO Deliver an easterly we need a flow around t160- that goes either miles & miles further south so a shortwave undercuts the trough over Scandi & supports the cold moving west or the Iberian ridge is much further west & actually that shortwave heads up still NE but it goes towards iceland- as the jet will loop up then back SE like we want it-

In all reality whilst it looks great the killer may well be the southern arm moving North over the atlantic which would again be typical of the UK's luck- with the motto if anything can go wrong it will- even with a cut off greenland high thats now modelled as being slightly more west based & not an iceland block.....

in Summary- plenty available to keep up on our toes especially between 42-96, thereafter a slight lull- but good consensus in modelling of a NE flow, then the probability of the jet sweeping it away to leave us at best with SE winds-

Best chance of sustained cold from the East is if the upper trough gets undercut as it will force it west ( GFS ens PTB 13)

cheers

Steve

[BLAST FROM THE PAST]

Great analysis Steve..as usual.

Now I think the models have it wrong with the sustained SW over Norway....I expect movement of this to allow bitterly cold NE'lies.....AFTER NEW YEAR! Expect a different picture come Tues and for Big Steve's favoured outlook. The real cold in my method isn't due until 3rd onwards.

BFTP

[Sno' problem]

Stunning posts there, really appreciated!

[Glacier Point]

One of the eye-taking developments at the moment is some extreme cold pooling over Scandinavia within the 7-10 day timeframe.

Both GFS and GEM ensemble mean height anomalies programme an exceptionally strong positive height anomaly over Greenland and wedge of cold air drawn out of northern Siberia into the longwave trough t168:

This contains some exceptionally cold air, of the order -20C to -28C 850 hPa values. Ensemble means show this as 850s between -12 and -9C anomaly.

The synoptic pattern thereafter will decide whether or how far west that cold air gets.

As per postings of late, the surge in angular momentum during the middle part of December will leave mountain and frictional torques as the likely determinants for the movement of the Global Wind Oscillation through phases 0-5-6-7. Phases 7 and 0 trend the blocking ridge further west. Phases 5 and 6 trend the blocking ridge further east. Within a 10 - 15 day timeframe, the GWO is likely to shift from orbit in phase 0/7 towards phases 5 and 6 as angular momentum increases once more. Model guidance and assessment of teh GWO suggests a phase 0-7 solution for the first third of January, possibly migrating towards phases 5-6 middle part of the month.

The further east the block, the greater the probability of undercutting lows triggering westward advection of the cold upper pool. Therefore, reasonable expectation that we will see at least some westward motion of that deep cold air, potentially resulting in a very severe spell of wintry weather middle to later third January, in addition to the potential within the more immediate timeframes. However, this is unlikely to be a rapid advection as heights over Scandinavia will be hindered by the polar vortex trending for mean trough solution. The key though is the likely duration of this pattern as it has every chance of being sustained over the large part of January.

[Guest North Sea Snow Convection]

I've tried to mention this deep cold pool of air of late and the fact that the east may serve us much sooner than anticipated.

[johnholmes]

thanks Stewart-I must agree with most of your conclusions although I'm still not certain that we won't get a rather more Atlantic flow for a time, but mainly rPm air other than the far south where temporary Tm inputs would be likely. The majority of teleconnections to me seem to suggest that, even if we did not have this quite intense cold pool you draw attention to, then the overall pattern would suggest blocked in some area north of the UK for longish periods.

[Glacier Point]

The ocean-atmosphere coupling continues to advertise a prolonged below average spell of winter weather, coinciding with our coldest period of the winter.

Latest TAO Triton mean zonal wind flow data plot is very interesting and might suggest that we may see a rapid MJO wave-related progression of the ocean-atmosphere coupling which preserves a convective signal east of the Dateline. Notice on the plot below that the westerly wind burst centred east of 180 degrees is continuing (solid -NAO signal) whilst easterly winds (tropical supression) are developing across much of the tropical Indian Ocean.

This ties in with the rapid migration of the MJO, now centred in phase 3 with traceable convection associated with this tropical wave in the eastern Indian Ocean. Notice on this plot the convective activity centred east of the Dateline:

http://cawcr.gov.au/...m/OLR/m.3d.html

http://cawcr.gov.au/...Last40days.html

What this tends to suggest is that any Nina-ish type flow is going to be very restricted and that the atmospheric system is being more responsive to forcing from El Nino.

Total and relative angular momentum has fallen in line with a return of convective signal to the Indian Ocean with much of this removal of westerly winds resulting from negative mountain and frictional torques:

http://www.esrl.noaa...m.sig.90day.gif

http://www.esrl.noaa...m.sig.90day.gif

http://www.esrl.noaa...ltaum.90day.gif

http://www.esrl.noaa...ltauf.90day.gif

Mean zonal westerly wind anomalies persist across much of 20N-35N in the Pacific however indicative of El Nino conditions. Taking the state of torques and persistent wind flows, it is likely that any falling off of angular momentum has likely bottomed out with the next move upwards.

Frictional torques are likely to remain close to or below average c/o the west wind anomaly in the tropics and inverted pressure patterns which drive the negative torque. As frictional torques largely govern the magnitude and sign of mountain torques, it is unlikely that mountain torques will spike very positive over the next 10 days or so.

This should ensure that the Global Wind Oscillation orbits in low amplitude phase 0-7-6-5, potentially becoming snagged around phase 5-6 towards mid month. At present, the GWO is in phase 0 which teleconnects to the projected pressure anomaly pattern across days 6-10.

http://raleighwx.eas...htAnomalyNH.gif

Composite reanalysis for phases 5-6 indicate that the GWO will trend the mean blocking ridge to work its way back east towards days 11-15, towards Iceland with lows being forced underneath the block. The GFS ensemble mean height anomaly for this period (500hPa) tends to support this notion:

http://raleighwx.eas...htAnomalyNH.gif

Temperatures across this period are likely to remain depressed, particularly so during days 6-10 and not returning to above average during this time. Obviously, at this time of year, beow average conditions are likely to present potential for snowfall, especially given a pronounced trough just to the south west of the UK allowing surface depressions to attempt to work up into the cold reservoir of air lodged over NW Europe. In so far as our locale is concerned, I think mean troughs around NW Europe and depressed jetstream are the more probable, with a trough dropping into Scandinavia from the polar region.

Phase 6 composite for the GWO likely to represent the best fit for days 11-15 (allowing for greater negative height anomalies across much of northern Europe):

Edited December 28, 2009 by Glacier Point

[Glacier Point]

Just to tag this on...

Tonight's CPC 8-14 day outlook is wholly consistent with the analysis above and I would agree with this pattern.

[bluearmy]

Just to tag this on...

Tonight's CPC 8-14 day outlook is wholly consistent with the analysis above and I would agree with this pattern.

i note that the shape of the block is rather more rounded on an E-W axis, ridging east rather than south. does this mean that even if it retrogresses towards canada (as it did last time) a west base -NAO will be more difficult to set up (better for us)?

Edited December 28, 2009 by bluearmy

[Glacier Point]

Yes Nick, it should as we are in essence seeing the impacts of the changing wavelengths and climatological effects which should ensure a much larger and deeper cold pool across the NW parts iof Europe and the NE Atlantic.

For what it's worth, the GEM ensemble mean t300 onwards shows the block to become more orientated towards Iceland in line with the analysis above. It's also worth noting that this set up does lend itself to a potential channel-low type event sometime days 6-12.

[kold weather]

My only fear is if the upper high does set-up too far west the attack that I think will be attmepted around 168-240hrs could end up turning our winds to a more cyclonic S/SW...thats why its very key we keep the NAO either centered in the central zone or even an eastern NAO.

Looking at the GWO I agree with GP in that the models are likely as per normal too progressive into taking it towards phase 0...so whilst the models may be latching onto a good outcome they are probably a good deal too progressive.

Also something that I've been considering lately is the chances of a big east based -ve NAO setting up say around the 15-20th...I think the models are already hinting at such a development with mean heights rising to our east however right now they haven't quite got the resolution to nail onto it.

I expect a SE/ESE airflow to dominate quite a bit of the month, quite a bit of the cold could come with surface cold but of course we could get a big easterly airflow at some point as well. Remember much of the Jan 63 severe cold was because we had a monster cold pool established over the UK, the synoptics themselves weren't amazing compared to some winters of the past but it was more the prolonged nature that allowed it to keep building. For example Feb 1947 was a better synoptic pattern for the UK, hence why it had a good deal more snowfall.

IF the upper high sets up too far west then any attack from the SW becomes a real danger, we saw what can happen over the last few days and whilst its still below averager for most, it obviously makes it all the more uncertain.

Finally, expect the models too slowly prolong the Greenland high set-up, I'd be very surprised if the models are handling the true sustainablity of the high correctly, its not a global set-up we've seen recently where everything is so favourable in Jan on the back of a cold December...we had a shot across the bow so to speak in Jan 2009 and we saw how close we came to sub 3C month, this time I think odds are better for the full blown works.

[Cloud 10]

Interesting to note that they have had to increase the scale on the arctic oscillation graphs to accomodate the deeply negative values observed recently!

A very different AO than what we have been used to.

[❄️❄️❄️❄️❄️❄️❄️❄️❄️]

Interesting to note that they have had to increase the scale on the arctic oscillation graphs to accomodate the deeply negative values observed recently!

A very different AO than what we have been used to.

That is impressive! It has been below -5 and this time it could go to -6!

[geoffw]

have you got any thoughts on feb gp?

[BrickFielder]

One thing I am not so sure about is that frictional torque will not peak. My thinking is that friction torque will rise over the next week or so from its current low value.

Weickmann et al have argued that pressure patterns to the east of an MJO event cause frictional torque which prceeds an increase in mountain torque as a slowly moving easterly pattern causes low pressure systems to cross mountain barriers in the extra tropical regions. How far torque increases this time round is questionable but my feeling is it should reduce global angular momentum for us to get a cycle through phases 1 - 4 of the GWO possibly taking up to 7 days. This gives us an increased possibility of a blocked pattern slowly moving eastwards over the next week or so.

I will take a step back now to look at the MJO which is currently heading towards phase 3.

For those who are not in the know about what the MJO represents I should explain that the MJO is a cycle of thunderstorm activity which moves eastward across the pacific normally over about a 3 week to a month basis setting up a fixed pattern of weather (think eastward moving blocking at the equator) ahead of it. For once the MJO does appear to have some sort of strength leaving a bit of a mystery as to why el nino began to develop. Normally a strong MJO (eastward moving area of thunderstorms) will kill of the trade winds across the pacific equator (westward moving winds). It is the lack of trade winds which stop the lifting of cool water from below which normally lead to el nino. Pressure anomalies in the southern pacific give us a clue to the mystery.

That blocked pattern in the southern pacific for a few months seems to have been key to the formation of el nino and also shows up in the SOI (Southern Oscillation index) which went strongly negative. We should note that the pressure anomaly has dissipated and the SOI has now risen to neutral levels. This for me indicates a short strengthening of el nino as the current MJO event goes through, but it may weaken afterwards.

Right back to the GWO which is currently in phase 0 and should head toward phase 5 giving us a period of lower angular momentum (more blocking possible) over the next week.

No sign yet of another disturbance in the stratospheric vortex, in fact it is looking remarkably robust at upper levels at the moment.I am also not keen that it has parked itself to our north rather than towards the pole or siberia. This is not such a surprise considering that ozone build up in our area has been slower than towards siberia (looks to be changing now). Recent weather patterns and the MJO should provide a trigger for some stratospheric disturbance later, but this is not showing up on the model radars yet.

With the cold flow of air to our north it will be interesting to see how ice builds to our north in the artic as this will be important as we get into late January (northerlies will have less warm sea to cover).

To be honest I expect an MJO driven pattern for the next few weeks which will most likely bring a mixed bag of synoptic situations any of which could bring cold conditions due to the southerly position of the jet stream.

[Glacier Point]

A challenging long range forecast situation right now.

The temperature of the polar stratosphere has continued to frop with 10 hPa and 30 hPa values nose-diving. This, combined with a relaxation of the upwelling wave late January (slow moving planetary wave) will have weather ramifications into February and I can see some brutally warm temperatures ushering in a premature end to this winter, but that's a long wave off and there's plenty of weather between now and then.

The Pacific flow pattern has become very amplified as the westerly wind burst across 30N fades as a trade wind surge develops in response to an upper level tropical wave moving across the Indian Ocean (MJO related). Water vapour satellite imagery shows this amplification nicely:

We are therefore likely to see a more meridional type pattern with more pronounced ridges and troughs across the Northern Hemipshere as the wavelengths are lengthened. Expect the PNA to drive very positive:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna.sprd2.gif

Relative angular momentum has fallen as El Nino wanes slightly and trade winds introduce an easterly component to the momentum budget. What has suprised me somewhat is the degree of the fall in angular momentum although a more Nino-ish type flow persists in the Pacific:

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/glaam.sig.90day.gif

This has driven the GWO deep into phase 8 and composites for this phase capture well the general circulation pattern.

http://www.esrl.noaa.gov/psd/map/images/gcm/gwo_40d.gif

Significantly, with relative angular momentum falling to below average values, this signals the likely emerge of some polar jet flow and height rises in the mid Atlantic / UK locale.

The key questions are here:

1) when (or if) will the tropical convective signal move eastward to re-invigorate a more El Nino type pattern;

2) how will the GWO respond and where will the phase space be positioned mid to late January ?

Both questions may be answered by predictive statements on the MJO as the tropical and extratropical flows are now much more coupled.

Currently, the MJO is in phase

http://cawcr.gov.au/bmrc/clfor/cfstaff/matw/maproom/RMM/phase.Last40days.html

GFS ensemble MJO forecast predict a push towards phase 4 / 5 by mid month:

http://www.cpc.noaa.gov/products/precip/CWlink/MJO/foregfs.shtml

There is not a strong easterly fetch ahead of the MJO wave, so wind shear should not be a significant inhibitor to eastward propagation so the GEFS forecast looks plausible.

As soon as the MJO breaks phase 5, relative angular momentum should begin to increase once more sending the GWO into a phase 4-5-6-7 type orbit mid to late January. Composites as posted before suggest a rebuilding of the blocking to the north for these phases.

Putting this together, the engagement of the northern arm of the jet may put some pressure on the blocking structure to our north. A movement towards Iceland and possibly slightly south looks very likely. There is an outside chance of enough of an eddy in the jet stream to initiate a Scandinavian ridge although this is a marginal outside bet given the position of the polar vortex which still favours a mean trough solution for Scandinavia:

http://strat-www.met.fu-berlin.de/cgi-bin/ecmwf?alert=1&level=100&forecast=f192

Also favouring mean trough, our upwelling wave continues and likely to reach peak intensity mid to late January. This should continue to keep high latitude blocking on the edge of the Arctic - typically southern Greenland and Iceland.

I really like the GFS ensemble mean solution for the next few weeks - large positive height anomaly just east of Greenland programmed t168 to become shifted slightly east and southwards towards Iceland days 11-15. 850 hPa values predicted are up to 9C below normal for days 6-10 and 2C below for days 11-15. Add in surface cold, and that equates to a sustained below average spell, although probably on the dry very cold side but wouldn't rule out perhaps one attempt of a low from the south-west.

Days 11-15 H5 mean height anomaly pattern:

Thereafter, I can see the blocking becoming reinvigorated, probably centred around Iceland to Greenland once more.

[geoffw]

so gp does this mean mid - late may not be the severe cold as originally predicted?

[johnholmes]

geoff?

I suggest you read your post and perhaps alter the month?

it is a good idea to read what you are going to post before you post it.

apologies if it is May that you mean although it seems pretty unlikely to me!

[geoffw]

sorry john my stupidity! of course what i meant is mid - late janruary still on for severe cold as originally predicted?

Edited December 31, 2009 by geoffw

[Yozzer]

sorry john my stupidity! of course what i meant is mid - late janruary still on for severe cold as originally predicted?

I don't want to be presumptious and speak for GP (and I applaud his technical knowledge which I can only aspire to in my dreams!!), but I don't read that into his post. I think he is priming us for a warm February, which given the snowpack that exists up here just now may lead to some pretty serious flooding if combined with rainfall.

Mid-late Jan looks good for continued blocking in the Iceland/S Greenland area with a trough over Scandinavia, which would mean a continued feed of air of Polar/Arctic origin.

[Guest North Sea Snow Convection]

I am surprised to see the stratosphere cooling as much as it is. It has seemed as though the colder tropical stratosphere, solar min, east QBO, and favourable ozone distribution would mitigate/prevent this happening.

I think because the cold has arrived so early this winter (and with such a high impact!) it is easy to forget that we are only at the end of December - and often at this stage we are waiting for the polar vortex to play ball and for the stratosphere to ignite an MMW to slow down the polar westerlies! So in that sense, it it still perfectly feasible for a warming event to occur in January. But the -AO values we have been seeing, and currently are seeing are nothing short of astonishing.

However, the recent/present strong cooling will have to make an impact at some stage - and whilst a three month proper winter would be the icing on the cake, in view of the fact that this winter has already and is/ and will be delivering so much in the shorter and indeed medium term that is way way better than the dross that we have seen so often in recent years in the heart of winter - then maybe we should make allowances for a warmer Feb, if that is what eventually happens?

It does look rather likely, but atm still a long way off with a lot to look forward to it seems

[johnholmes]

I hope I can be forgiven for posting this in here from the model thread-it will make it easier for me to find it in here!

Posted Today, 10:15

nick sussex, on 31 December 2009 - 09:24 , said:

Just one thing to add here from yesterday evening are this mornings NOAA discussions:

A CLOSED HIGH /POSITIVE ANOMALY/ OVER GREENLAND IS EXPECTED TO

PROGRESS THIS PERIOD AS AN INCREASINGLY AMPLIFIED WESTERN RIDGE

DEVELOPS WHICH FORCES TROUGHING DOWNSTREAM IN CENTRAL NORTH

AMERICA WHICH KICKS THE VORTEX NEAR THE NORTHEAST/ATLANTIC CANADA

NORTH AND NORTHEASTWARD.

If the high can stay far enough north as it moves eastwards then a chance of a Scandi high or high pressure centred to the north around the Iceland area with some reinforcing colder upper air backing westwards especially for more se areas.

the quote from NOAA is, as I'm sure you know Nick, one they have posted in one part or other of their short and longer term comments regarding the +ve anomaly over southern Greenland for much of the past week. Their 500mb anomaly charts and also those of GFS and ECMWF are all pretty similar.

The risk of any major LONG TERM breakdwon in the cold spell, note the capitals, as opposed to short term encroaches being possible into parts of southern UK from lows yet to form in the Atlantic, is, in my view, and also noting what GP has just posted, fairly slight for 10-14 days and probably out to 21 days. Sorry its a long sentence. beyond that then as I suggested last evening best to go for a reading from, is it, Mystic Meg, for the rest of the winter!

As to where the major blocking in our area of the northern hemisphere is more likely then I suspect there is no favoured zone. IF and its still relatively early days, this does develop into a winter of some standing, maybe the most notable from a 'cold' viewpoint for over 20 years, then all the classic winters showed blocks varying from the NW (Greenland/Iceland)-north and north east into Scandinavia with occasionally extensions of the climatic Siberian/Asian winter high into this area.

Like I said its very early days yet but there is so much cold pooling going on over much of the northern half of the northern hemisphere that its difficult to see how a major change can take place much before the end of January IF then.

No doubt I'll be asked to eat humble pie in a week or so as the Tropical Maritime air rushes in!