Glacier Point

Agree. 06z GEFS, following on from 00z are a dog's breakfast synoptically, showing wide scatter after day 8. We still get a half decent mean anomaly out of it which secures low heights between the Azores and Iberia (notable conflict with 18z and 6z GFS op) and strong +ve heights in the North Atlantic towards Svalbard (both areas are where our easterlies go wrong usually so an encouraging sign there). Fair to say that the GFS doesn't have a clue beyond day 7, maybe earlier given synoptic developments. In such cases, stick with your means. PV still being modelled to shift its position and decamp from Canada. The cut off lobe over Siberia seems to be causing a lot of instability on global NWP right now. Note also, a consistent trend for retrogression towards Greenland of our mean anomalous ridge - hard to believe if you're looking at operational GFS - but there and consistently there.

12z GEFS suite is very strong on blocking to our N and NE, with an equally strong (if not stronger) signature for block to retrogress towards Greenland in the extended frames as the pv is transferred towards Siberia (which has been a consistent signal now for a few days). Get set. It's coming.

Purely speculative at this stage. However, the inference being that the teleconnectors (AO and NAO) are being modelled at strongly negative values in the next 10-16 days. The timing of the next tropical wave (suggested in the winter forecast at mid Jan, maybe a week either side of that), should excite these already negative tendencies and drive us into a hemispheric deeply negative -AO/-NAO pattern [v. cold].

I saw the ECM seasonal prediction for the AO Nick. It was gunning very positive AO throughout.

Basically across the NH, we would need to see a more Nina type signal arising. That would be a net removal of westerly winds across 20-30N in the first instance (currently we are adding large westerly wind totals here), and a general signal for the polar vortex to stabilise and organise itself around the North Pole (currently in tatters and nowhere near the Pole). We would also need to see the MJO and general tropical activity work its way to phase 3-4 in the far western Pacific and Indian Ocean. Currently anything but, tendinging to be more scattered across the equator which is often associated with a -NAO pattern.

06z GEFS H5 mean anomaly for day 10 showing us a realy strong -NAO signature. The really striking thing for me is the lowering of heights across both the Pacific and the Atlantic in the middle latitudes, as well as low heights across much of Asia. That is a classic sign of an El Nino type atmospheric base state and should maintain us in a predominately GWO phase 5-6-7-8 type pattern, which focusses the main core of +ve height anomalies over the Northern Atlantic and southern Greenland. The next challenge for NWP is what happens to the severed part of the PV over NE Siberia / far NW Pacific (could well help to sustain low heights in the Pacific) and how the pv migrates from Canada back to Siberia in the extended range. That tees us up perfectly for late December, switching the focus from Greenland / Iceland towards Scandinavia for blocking as we really see a strong Siberian High develop in the coming weeks. It's getting really hard now to make the case for anything approaching average in the longer range and we need to see massive structural changes now for us to avoid what is coming in January. The ECM seasonal already looks bust, and forecasts predicated on a January thaw type scenario should be aware of the more El Nino type tendency in the atmosphere right now, these being more often than not being associated with colder than average Januarys (remember 2009/10).

I disagree. December 09 surface values 1-15 December 09 surface values 1-30 Projected GEFS 850s (which might undercook surface values given synoptic) for days 6-10 and 11-15 December 2012 Controversially, the first half of this month will rival or beat December 2010 if continued: -3.5C the goal to aim at.

Yes, makes it all the more impressive doesn't it ? I would go for 3 wave for December, morphing 4 wave when we get the Greeny. If I get time, might be able to post up the 30 hPa zonal wind comparison with 1968. I reckon we are running about a 3 days behind the pattern of that year. And of corse, AO value for that winter ???!

Vortex all over the shop. t204 part breaks off over Siberia t240 elongated and displaced towards Greenland t300 starting to form a dumbell and suggestion of heading back towards Siberia t360 main core over Sibria and another split When you start to see this modelled, you appreciate just what turmoil the vortex is in and what dynamical processes must be going on there. This invariably is a precursor to major cold outbreaks across the northern hemisphere. Expect continued model volatility from here on in.

This looks more in line with the long term trend to shift the core pv towards NW Russia. Still looking like we'll get the 'firing pistol' Pacific low around 20 December (which equates to vortex reacting around 9th Jan). Vortex forecasts doing a good impression of cat on a hot tin roof. Transfer from one location to the other and elongating. Clearly this pv is not a happy bunny.

Lockdown. This is nuanced. It is not a spell binding Greenland High. It is not 2009. It is not 2010. But it is a cold block, and the key aspect is that the below average detail is likely to be maintained throughout. The cumulative impact on the CET may only be apparent to some well into the month. Models have under estimated the -ve zonal wind signal over the far NE Atlantic and the net result is that we see the cold conditions sticking - beware using the operational GFS and keep sticking to those ensemble means and spreads. Looking forward we have something of a battle between the anomalous ridge south of Greenland / N. Atlantic and the developing cold ridge to our NE. This battle goes on I think for the rest of the month with an increasing focus around Iceland. Well below average month. Anomalous ridge to the NW, deep troughing over Europe. Now where have I heard that before ?

GEFS might be quite illuminating tonight.. operational doing all sorts of weird and wonderfull things with the polar field in the extended range. I sense another shuffling of the deck coming on. BTW, Steve - where r you ? http://www.meteociel.fr/modeles/ukmo2.php?nh=1&carte=1007&ech=60&archive=0

Define 'decent', bearing in mind the above ?

06z GEFS mean places the CET at mid month at around 2-3C below average. That's comfortably well below average and noteworthy not for the 2010 type deep cold, but sustained.

It has been severely weakened by the events of the last week. Forecasts suggest a possible further split in the next 10 days in response to wave 2 acitivity before a displacement type influence takes effect shortly after this. In the medium to long term we're looking at the upper vortex reorganising itself towards Canada and then being displaced towards the Atlantic side. Given this influence from the trosphere, the upper vortex is unable to draw the support of the lower part of the atmosphere and I'm thinking as soon as we start to see a North Pacific low modelled reliably, we can start the clock ticking (20 days) on its eventual demise a shattering into probably four residual parts. As part of that process, the upper vortex over Canada should edge towards Greenland and then Svalbard.

Yes, 06z GEFS continues the trend. Structural change over the Arctic with the Kamchatkan ridge transfering into the Pole / NE Siberia shifts a large body of cold air across the polar field into the Canadian interior which pulls the vortex back west and enlarges it at the same time. Reasonable agreement on this at around day 7. Steady build in heights over the North Atlantic which extend NE and merge with developing ridge over Scandinavia / NW Russia. Low heights programmed over southern Europe throughout. So the key all along was more energy into the vortex !

TEITS you smelling that easterly ? Very interesting situation at the moment. ECM and GEFS picking up on the height rises to the far NE Atlantic. As long as the residual vortex is clamped to the west of Greenland we'll see bits of energy cutting their way SE preventing any further building. However, trending around t168 suggests cut off lows developing in the mid and western Atlantic and ridge development towards southern Greenland (strong signal from GEFS around t168 onwards). That shuts down the flow of energy and allows the ridge to the NE to build. GEFS also continuing to do 2 interesting things: 1) shift the residual vortex back towards the Canadian interior which further increases the chances of block development; 2) builds a large high pressure cell over Siberia as the vortex their is relaxed. With the GWO potentially swinging back to a big phase 5 projection (composite below), interesting times ahead. The mean anomalous ridge to our west looks a safe bet, as does the trough solution over Europe. I think a build in pressure between Iceland and Scandinavia week 2 looks very plausible.

I suspect wave 1 activity in mid to late December will deal the killer blow. Already seeing signs that the vortex transfer is raising heights over the Siberian side and drawing up the Kamchatkan ridge into the fold. Next evolution would be for the Aleutian low to develop and bingo, 20 days later you're looking at a vortex literally scattered to the four winds. Meantime, as speculated, episodic wave 2 activity really cutting into this vortex.

Not sure any model has a grip on the situation past day 5 at the moment. The commentary on the upper air modelling for the polar vortex from 06z GFS reads something like this... pulled apart, squeezed, attempted to reform, split, split into three parts. The broad gist from ensemble means holds sway though.. anomalous ridge in the North Atlantic, deep troughing over much of Europe, below average temperatures over a sustained period with the potential for attacks of more colder air coming from the north, north-east and east. The upper air volatility is interesting to watch though - signal that the pv is coming under more pressure from below and registering all the way up to 30 hPa.

Mid Atlantic ridge is the percentage call given the upstream ridge over the Pacific but there are signs of shifting a touch west and that trough over Europe is getting deeper and further south. We're getting close to tipping point where a Scandinavia ridge is viable. What the ECM does suggest is substantive trough disruption signal across the North Atlantic. CPC 8-14 dayer a good capture of current thinking: Whilst GEFS day 10 through 12 has the potential for another cross polar flow event. Also shows a sharp injection of cold air into Scandinavia days 6 through 12.

I must be completely imagining the split vortex and the cold spell that is about to ensue. Just because we are not seeing a glowing orange beacon over Greenland does not imply no blocking. The GEFS mean height and temperature anomalies continue to advertise a well defined trough signature across Europe throughout the 15 day period with temperatures well below average. Whether this is the '2010 factor' at work I don't know but we must consider that this is late November and early December and we are looking down the barrel of a sustained cold spell, unusually so for the time of year. Looking at short to medium term those small scale adjustments in the track of the low driving down from the NW continue almost on every run so by the time we get to 2-3 days out, could wel be interesting, especially with the trough engrained over Europe. Looking further ahead, we have the strong teleconnector in the North Pacific and downstream ridge over the North Atlantic south of Greenland. That essentially keeps us locked in a cold puddle of air for the duration. The AO looks on the negative side too which is a bonus for December as far as I'm concerned given other factors.

Seems like a parallel universe with the modelling I'm seeing and the reasoned assessment of teleconnectors compared to some other interpretations doing the rounds. The interesting period emerging where we establishing just where residual bits of polar vortex are positioned. At the crucial t72-96 range, ensemble means and the balance of operationals appearing to suggest the troughing south of Greenland will disrupt allowing for an advection westwards of the cold air and lowering of pressure across Europe. At t144-t168, we have some reasonable agreement for the second low to break through and head SE towards the European trough. Losts of complications there yet to be resolved but with blocking signature still in place to our north, probably not going to alter things too much - likelihood that the low will become adbsorbed with the main European trough with a temporary rise in temperatures towards average before falling away again, The mean temperature anomaly for the 15 day period looking between 2C and 4C below average (which is not far off ECM mean). The day 6-10 and 11-15 GEFS H5 anomalies and T850 anomalies still very persistent in suggesting a sustained below average start to December. Note the persistence of the upper low over Europe and the development of another ridge in the North Atlantic south of Greenland which re-inforces our cold block towards the end of the period. Note also, the ridge over the North Pacific which has not been well modelled in the last week. That teleconnects very strongly with the mid Atlantic ridge solution. So a cyclonic cold block looks the order of the day for December, pressure higher to the south of Greenland.

The paradox of weather forums much in evidence. When the pattern becomes problematic for NWP, and inter and intra run consistency falls apart, people seem more inclined to buy into operationals rather than less so! Stick with your ensemble means. Pretty much a solid trend still there for a substantively and sustained below average period. The models still trying to resolve mean position and residual energy left in the Canadian sector vortex after the split. An interesting little trend within a trend, shift in the block position around Svalbard and AO becoming more negative in time. I can only assume those commentaries on ensembles are being viewed from a Euro-centric view. If there was an issue, would have expected means to become diluted, but not any sign of this.

GEFS mean showing a pan European trough (and quite a deep one) by day 10: T850 values between 4C and 6C below normal within that trough: You've got to think snow potential there. Moving ahead, GEFS mean also picking up on a slight westward shift in the blocking high allowing an attempted Atlantic attack from the south west. Notice at t300 GEFS mean anomaly showing a second trough over the UK and T850s slightly recovering: That screams increased snow risk week 2 December. Perm low heights and snowcover, could be some very low surface values going on. Thereafter, the logical evolution would be for a northerly attack week before Christmas. Probably not what retailers want to hear.

Analogs for cross polar ridge in Late November / December, rolled forward into January: No end in sight, particularly as our teleconnectors will come again to excite the underlying pattern in 10 day intervals and more especially in mid January. The broad scale evolution remains about how much residual vortex is left over the Canadian sector post split, and where this locates itself. Last few GFS runs have suggested a chunk getting stuck over the Hudsons which is the ideal spot to support a Greenland high.