Glacier Point

EC op is notably more Wave 2 days 8-10 at 10hPa. GFS parallel some days behind on this.

Tropical modelling suggests a coherent MJO wave to enter the Maritime Continent during the coming week. Week 2 forecasts indicate a widespread -ve velocity potential across the central and eastern Pacific, where SSTAs are well above normal and likely to favour enhanced westerlies (weaker Trades). There have been several westerly wind bursts this autumn and early part of the winter and I would expect this to be another one in evolution. So although the MJO signal is relatively weak, this will transpose onto a an El Nino low frequency base state. Total and relative angular momentum will as a result increase. There's no indication of deposition of westerlies via mountain torques over Asia, so a steady laddering up of angular momentum looks most likely. That's a signal for phase 6 evolution in the first week of January. We should then begin to see westerlies scrubbed from the atmosphere over time returning the GWO towards phases 7 and 8 during week 3 (2nd week January, possibly with the MJO in phase 8). That could well coincide with the first lagged impacts of the stratospheric warming. With the net ENSO, GWO and MJO signals, I'd suggest mean trough solution across Scandinavia and into Europe with a Greenland type high is the most likely evolution. What do you say CFS ?

For the first time since early February, NWP forecast tools are suggesting a northern block to develop across the Barents Sea to Greenland. How much credance can we give this ? During March and April, complex dynamical processes (torques and eddies) in the atmosphere have worked to add westerly winds to the overall global circulation indicative of a shift in the synoptical pattern from La Nina to neutral ENSO. The net impact of these processes has seen the Global Wind Oscillation ladder up in phase space 3-4-5 (Nino like). http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif However, since late April / early May the westerly wind input has temporarily declined. As nature abhors a vacuum, easterly winds have begun to progressively replace the westerlies and significant easterly wind tendency has developed in the circulatory pattern. This has largely been driven by frictional torques: http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif Mountain torques are following the lead: http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif and tendency in earth angular momentum is sharply down: http://www.cdc.noaa.gov/map/images/reanaly...teaam.90day.gif The MJO after almost completing a full orbit has beat the retreat in phase 7 - a sure sign of negative (easterly) tendency in the atmopshere: http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html Where the GWO goes in the next 10 days will have a large influence on the global weather patterns. Given that both frictional and mountain torques are negative and likley to remain so for several days (some suggestion of frictional torques heading upwards) and lack of tropical intertia c/o a lack of convective activity, I think we will see the GWO orbit in medium amplitude phase 0 and 1. Composite reanalysis for phase 0 at 500hPa identify a large negative anomaly in the eastern North Atlantic and height rises further north and north-west which is consistent with the NWP outputs. Phase 1 reanalysis suggests the low to become stationary and fill with a height rise over continental Europe - something of a progressive pattern which should result in warmer, drier weather developing from the south and pushing northwards towards day 10 (mid month onwards) - note the trough persisting over Scandinavia. Today's ECM 00z (08/05/09) has a similar evolution t144 onwards and I think this is very plausible. Whilst it looks very likely to become unsettled mid next week onwards, this should only last a few days in the south and indications to me that we should see conditions improve from next weekend onwards provided that we do not see the GWO stall in phase 0. Longer term, even though the atmosphere is becoming more Nina like, I think this is only temporary state and the overall building of warmer SSTAs across large parts of the Indian and Pacific Ocean are likely contribute to increasing westerly (Nino) tendency in the pattern as we head into June, as is the state of the polar atmosphere which remains more conducive to sustained warm and dry periods June through August.

Low angular momentum during the winter makes blocking less likely at high latitude and the overall flow pattern zonal (there are exceptions in terms of specific months and GWO phases). Both this and the west QBO emphasise to me how well we have faired, all things considered. The factors that have been on our side have been the solar minima which has beefed up the polar cell and the hemispheric arrangement of SSTAs.

Before we can qualify what if any impacts the MMW has had, we must I think contextualise this event within a west QBO under low solar minima. There has never been an early breakdown of the polar vortex during such conditions and we can see this I think with the polar vortex remaining intact or attempting to reform. There have I think been several impacts of this warming: 1) angular momentum at the upper levels has dropped since mid January. This can be seen in angular momentum plots: http://www.cdc.noaa.gov/map/images/reanaly...g.qbo.90day.gif 2) the Artic Oscillation has shown 2 large negative phases over February. Nothing special in that unless you consider the west QBO / solar minima. I speculate that had this been an east QBO, the AO would have come up with a value in excess of -4 SD. http://www.cpc.ncep.noaa.gov/products/prec...ex/ao.sprd2.gif note the third drop coming up. 3) the storm track across the NH has been shifted anomalously southwards. The largest positive zonal wind anomalies for February are evident across North Africa. So whilst the spectacular blocking synoptics have yet to materialise, the MMW has resulted in measurable impacts across the NH, perhaps more diffuse but non the less significant. These continue as the anomalously warm lower stratosphere persists. http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif Bar far the most discernible impact from the MMW for me is its reinforcement of a low angular (Nina like) base state. That relates directly to my expectations for the next few weeks. Low angular momentum, phase 2/3 of the GWO = blocking during March when shortening wavelengths allow for the release of colder air, even within the context of the west QBO.

... this brings us nicely to where we find ourselves now and what is likely in the next 10 days. Yet another case of using the GWO framework to give forecasters a distinct edge within long range timescales. Looking further ahead mid March onwards, the GWO story will be dominated by a continued low angular momentum base state. OLR plots show localised levels of anomalous and largely unorganised convective activity around the west Pacific and Indonesia. There are also large anticyclones occupying large parts of Africa, the Indian Ocean and sub-Continent, central Pacific. http://www.bom.gov.au/bmrc/clfor/cfstaff/m...m/OLR/m.3d.html This means that one of the engines driving forward (west) motion in the atmosphere is spluttering. The MJO is a good guide to this and it currently is very weak in the locale of the far east Indian Ocean: http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html Easterly trade winds still persist across large parts of the Pacific and Atlantic and any MJO activity is likely to be suppressed. Since the last posting, the GWO has shifted in high amplitude phase 3/4: http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif This is a result of complex atmospheric processes adding westerly motion to the extratropical atmospheres, principally frictional and some mountain torques shifting westerlies poleward: http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif Note the development of positive frictional torque at 45N however which is indicative of the jetstream being shifted southwards. Overall levels of global angular momentum, in excess of -1.5 Hadleys reflective of the La Nina base state: http://www.cdc.noaa.gov/map/images/reanaly...m.sig.90day.gif The lack of tropical convection and natural processes which oppose westerly wind additions will reverse this most recent spike in tendency in angular momentum: http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif I suspect a GWO orbit along the lines of phases 4-3-2-1 1-2-3 (quasi-stationary 2/3) will occur. Composite reanalysis for medium amplitude phases 2 and 3 look like this: Remember these are H500 field anomalies - not absolute values - but the inference is as follows: 1) disrupted Atlantic longwave pattern with continued tendency for cut off lows to develop in the north- western Atlantic sector; 2) unsettled; 3) winds from a northerly, north westerly, north-easterly and easterly vectors. Therefore the outlook continues to remain below average.

Just time for a very brief update. Key thing, strong upper tropospheric anticyclones have become established across east Africa extending into the western Pacific. http://www.bom.gov.au/bmrc/clfor/cfstaff/m...m/OLR/m.3d.html This is indicative of the tropics becoming strongly supressed. A lack of topical forcing is very significant as this is a major driver in global momentum. Westerly motion in the atmosphere has dropped rapidly. Look at how the MJO has become non existent: http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html Mountain torques are responding to this with negative torques developing once again: http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif This is driving a strong negative tendency in the GWO which, as per last posting, as stalled in phase 2: http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif The synoptic response thus far is as anticipated: mean Atlantic ridge; Scandinavian trough. Moving forwards, March signals a changing wavelength (as January is different to February or December). Composite reanalysis for GWO phase 2 signals a shift in the mean ridge further west edging the Scandinavian trough westwards towards the UK with the jet orientated NW-SE (and south of the UK). As time progresses, the trend should be for height rises over Greenland which also accords with the impacts of the MMW. As Ed Berry notes, the effects of the MMW continue to be manifested in weather patterns - the storm track being shifted anomalously southwards against the climatological trend. I've read some views recently questioning the impacts of the MMW. True, the deep cold expected by this quarter has not materialised, yet. However, study of the anomalous warmth of the upper troposphere and lower stratosphere tells us that the impacts of the stratospheric warming have not gone away. Far from it. They continue to represent a serious inter-seasonal weather issue which will continue to influence our weather patterns for the next 4-6 weeks. Make no mistake, I think there is a serious amount, a shed load, of blocking upcoming in March and early April although this is tempered slightly that this seemed likley for February but has yet to materialise. http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif

.. and all of these factors listed at the PSD Map Room: http://www.cdc.noaa.gov/map/clim/aam.90day.total.shtml Torques are by the largest extratopical influences with tropical convection (MJO component) also included within these monitoring products. The best indicators as to where the GWO will be located are tendency in angular momentum which gives an indication in the rate of change of the GWO (how fast and which way the GWO orbits): http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif and the overall level of angular momentum: http://www.cdc.noaa.gov/map/images/aam/glaam.gif which gives us our magnitude of the GWO. Note that the MJO component (the tropics is becoming very quiet) as convective supression in the west Indian Ocean takes effect and strong easterly trade wind push in the central Pacific:- http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html As can be seen, both suggest a negative tendency high amplitude orbit. Currently in phase 1: http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif

Generally, rising GLAAM is better for promoting a disrupted flow pattern which is supportive of higher latitude blocking. That's why blocking is more prevelant during El Ninos (+GLAAM) rather than La Ninas (-GLAAM) during the winter time and this is related to the location of tropical forcing and momentum budgets. However, this is seasonally dependent. As we move into boreal spring and summer, blocking is more favoured during low angular momentum due to seasonal wavelength changes. That's why the last few summers have been rank. Low angular momentum regime has dominated (and still does). If you are after your traditional winter cold and summer warmth, an El Nino and +GLAAM regime is for you. There is one further (and confusing) anomaly. Falling angular momentum (meaning that the GWO is attracted towards phases 1 and 2) can lock in place the existing pattern during the second half of the winter. The best example of this is February 1963 where the GWO spent most of its time in the low momentum phase 2 at high amplitude. Although we didn't come very close to this happening this year, it did at one stage in late jan / early Feb look plausible. But then we picked up some momentum and moved to phase 3 where we stayed for some time. Torques make up significant parts of the momentum budget. They are large drivers in tendency in angular momentum helping to remove or add winds to the atmosphere. They occur as a result of pressure differentials across mountain ranges or longitudinal air masses. Think of the jet stream as a type of frictional torque by which winds are added or removed. The complex interactions of these torques is best monitored on a daily basis to see how falling and rising torques (they usually go up and down in sequence) can impact on overall tendency of angular momentum and the GWO. Right now we have falling mountain torque which is driving the overall level of GLAAM downwards once more. Easterly winds are being added to the extratropical atmosphere (and some evidence of lack of westerly motion across the tropics).

Morning Steve. Readers of this more technical domain do so under the understanding that the terms used maybe different, and this may from time to time spill over into MO thread. Reanalysis of the H5 field anomaly for the last 90 days shows a weak to moderate strength positive anomaly over Greenland and a stronger one centred in the Atlantic. This is different to you spell binding Greenland High yes, but see the caveat. Moreover, there has been two episodes this winter that have featured significantly greater positive anomalies - over Christmas into the New Year and 1st week February. The former was a good example of retrogression from Scandinavia. I would therefore maintain that references by myself and John have been correct and appropriate. I'm interested in the application of the GWO as a long rang forecasting tool. I will be here during the Spring, Summer and Autumn hopefully adding positive contribution. Steve, read my thoughts on where I think the GWO is heading within the extended range - phase 1-2-3 orbits, probably remaining in a quasi-stationary orbit around phases 2/3 as the atmosphere plunges into deep negative momentum territory for March. The composites spell out what likely pattern (in terms of H5 anomalies) to expect. Updated mountain torques show a sharp decline over Asia in particular. http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif

It's (nbsp) a typo Len - I was timed out on the edit and the recovery function inserted it. Major Mid Winter Warming = MMW

For a normal event John, 21-30 days should have been normal timlag. However, this is clearly not a normal event and it may well be that it will have a slower and much more sustained (inter-seasonal) impact on the circulation of the Northern Hemisphere throughout March and April.

Doh ! let's try again. Clearly some issues with the transference of negative wind anomalies across the upper part of the troposphere which make previous thoughts somewhat waide of the mark in relation to cold. However, the general circulation has progressed as anticipated. The extended range discussion is likely to continue to revolve around a strong reload of La Nina type pattern and when or if the impacts of a significant stratospheric warming will take effect. Latest thermal and zonal wind anomalies clearly show the effects of the warming event to have penetrated between 150 hPa and 200 hPa layer - i.e. across the tropopause. Further downwelling is taking place, although at a painfully slow rate. Crucially, the zonal winds across the polar region remain relatively light which should allow for the atmopshere to respond to tropical forcing although it is noted that polar vortex is attempting to reform towards eastern Siberia and the NW Pacific sectors which might be resisting so far the pressure from above. The impact of downwelling will I think still impact the surface although when remains a big question mark. The chances of a very blocked March and April have very much increased from my perspective (cold and wet spring?) through the timing and magnitude of the stratospheric event. A strong trade wind surge is ongoing in the tropics with anomalous 'Branstrator' mid latitude ridges now evident. Latest tropwave plots depict this surge in the central-western Pacific and also strong anticyclone developing over Equatorial Africa and the western Indian Ocean leading to convective surpression. OLR plots also show this nicely. This is signalling a general lowering in GLAAM. Also signalling this, a strong negative mountain torque has developed over Asia which, in tandem with falling frictional torque, is leading to a sharp decline in momentum over the mid latitudes and generally within the atmospheric framework. This falling tendency in momentum has driven the GWO through phase 3-2-1 (as anticipated) and resulted in a strong mean ridge in the Atlantic featuring within the likely weather for the medium term modelling. Further falls in momentum are likely and we are potentially looking at a high amplitude orbit through phases 1-2-3 within the next few weeks. http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif Composites for thhese phases identify a mean ridge in the Atlantic and trough over Scandinavia as continuing responses. Whether or not the effects of the stratospheric warming detonate into the lower atmosphere during the outlook period is unclear. If they do, this will serve to 'excite' the pattern of ridges and troughs resulting in height rises over Greenland. For my money, we are likely to continue with the pattern of ridges and troughs that we see now, possibly with a hint that the last week of the month will start to to take effect;the impacts of the stratospheric warming with decreased momentum leading to a northward shift in the ridge with split flow. I have more confidence in more progressive blocking taking place during March. Organisers of the Cheltenham Festival (mid March), take note.

Clearly some issues with the transference of negative wind anomalies across the upper part of the troposphere which make previous thoughts somewhat waide of the mark in relation to cold. However, the general circulation has progressed as anticipated. The extended range discussion is likely to continue to revolve around a strong reload of La Nina type pattern and when or if the impacts of a significant stratospheric warming will take effect. Latest thermal and zonal wind anomalies clearly show the effects of the warming event to have penetrated between 150 hPa and 200 hPa layer - i.e. across the tropopause. Further downwelling is taking place, although at a painfully slow rate. Crucially

Reanalysis of the first week of February identifies the following H5 anomaly: Note the marked -ve anomaly over Europe and +ve anomaly over Iceland and southern Greenland. I've depicted this to demonstrate the link between the 500 hPa Geopotential height anomaly and real weather, and to emphasise the link between forecast patterns and verification. This chart delivered the snowfall that many of us have seen and a CET which currently stands at 0.5C. As anticipated, the GWO has exhibited a high amplitude quasi stationary oscillation in phase 2/3. This correlates well with composites for these two stages, allowing for a weaker zonal wind anomaly across the polar field allowing for the ridge to pull northwards. The major stratospheric warming is continuing to propagate down through the tropopause. Note however that this propagation is somewhat discontinuous with sudden increases in zonal winds occuring within the tropopause ahead of downwelling waves. This laddering down of the effects of the warming event is not unusual. It is this switching ahead of the downwelling that is IMO behind the model fragility within the extended range. I had hoped that the ECM as the best upper atmosphere modeller would be able to cope but unfortunately, it would appear vulnerable. Beware, another episode of laddering is likely so perhaps the models to switch like a shoal of fish in a stream and pumping more energy into the polar jet before twigging just what is coming down from the 'headwaters'. I fully expect the huge negative momentum anomaly to have an impact on the Northern Hemisphere's weather patterns for a period over February and at least the first half of March. However, this will intensify and ease off reflecting the nature of downwelling waves. The effects of the stratospheric warming show nicely in terms of a negative momentum anomaly within the upper sigma levels and positive anomaly in the lower layers - the prelude to downwelling. Mountain torques (pressure differential across mountain ranges) have worked to add westerly wind anomalies to the circulatory system (once again, wavebreaking a possible trigger for further stratospheric warming). Note the cycle of positive torques is more or less tied into the cycle of torques over the tropics (purple line) and the addition of westerly winds is now looking like reversing in a natural rubbing out out of these westerlies coinciding with the tropical signal. A fall is likely adding easterly wind anomalies to the circulatory system. These positive mountain torques have added westerly motion to the mid and high latitudes. However, frictional torques (pressure differential between latitudinal air masses) have helped to keep lid on these westerly additions by counteracting and adding easterly winds. Tendency in earth angular momentum is now down: ..and the overall signal here is for tendency in anglar momentum to decrease, especially as decreasing mountain torques take effect. This will drive the GWO through phases 3-2-1 and back again 1-2-3 as easterly wind additions are rubbed out. The composites for GWO phases 1-2-3 once again capture the overall likely longwave pattern - mid Pacific and Atlantic ridges (some suggestion for retrogression) and Scandinavian trough, possibly bridging over as the trough becomes cut off. However, as we have seen already, we need to tweak these anomalies to allow for pressue to rise to our north and north-west in the face of increasing signals for blocking related to downwelling waves (remember the laddering though). Overall, the thoughts discussed at the end of January remain unchanged - below average to continue with more progressive blocking to the second half of the month - winds from a north-east and northerly vectors. Both GEM t240 00z ensemble mean height anomaly, CPC and ECM mean height anomaly depict this well.

So here we go, the real deal is about to begin. The atmosphere is pretty much were it was in mid to late December, albeit with global momentum rammed significantly lower c/o strengthened easterly wind addition consistent with a maturish La Nina base state. Recent negtaive mountain and frictional torques have reinforced this with the GWO in high amplitude phase 3. Lates tropwave imagery identifies a growing easterly wind anomaly through the Tropics and across large parts of the Equator. As a result, anticyclones are now lining up across the Tropics (denoted below by +ve (brown) OLR anomalies) to sustain a strong westerly component across 30N. An increase in sub-tropical momentum and weakened sub-tropical ridges are likely results days 6-10. This shown nicely by frictional torques which are increasing across 30N: Based on where the overall base state is at: http://www.cdc.noaa.gov/map/images/aam/glaam.gif .... a solidly low AAM position, I suspect that we will continue to see these easterly winds across the Tropical and Equatorial Pacific, west Indian Ocean and Atlantic although some convective flare ups towards Indonesia and the Philippines are possible. The crucial point about the comparison with late December is that the addition of westerly winds through frictional torques happend then as now. However, momentum fluxing and frictional torques transfering poleward stoked up the polar jet (allied to stratospheric anomalies) cutting short the potential of that cold event. Crucially, zonal wind anomalies across the polar field are entirely different now compared to late December which should work to deflect any tendency to shift zonal winds poleward over the next 10-15 days and this I think will be entirely attributable to the stratospheric warming. The Major Mid Winter Warming (MMW) event centred on 23rd January has shown signs of propagating efficiently thrpugh the lower stratoshpere to a point where it's first impacts are being felt on the tropopause. Pictured below is the mean zonal wind anomaly over time. Note the development of classic 'propagating node' (discontinuous green) and the hatched line getting as far down as the 100 hPa layer. Based on previous warming events such as this, we are likely to see a further two to three such 'nodes', each with increasing influence bringing about ever more pronounced high latitude blocking. On current form, we are likely to see the first real appreciable wave hit somewhere around 10th February with a peak in blocking 21-28 February. Tendency in Global Angular Momentum is likely to peak within the next 5 days as frictional torques transfer into mountain torques driving the GWO into phase 3, possibly 4. However, a sharp decline is likely to ensue as these additions are rapidly removed from the atmosphere. Considering the very low base state that we are are currently in, I suspect that a very high amplitude phase 1-2 orbit is very possible in the GWO as tendency in momentum drops through the floor. This should time itself with first tangible signs of the MMW. The implications for the Northern Hemisphere are huge with high impact weather events very likely. GWO composites for phase 2/3 provide a rough guide as to where the atmosphere is already at and where it will likely remain over the next 20 days: ... suggesting a north Atlantic ridge and deep Scandinavian trough. The effects of the MMW should be to super-charge or hyper enhance this pattern - therefore more emphasis on a ridge building over Greenland I think. The CPC 8-14 day forecast reasonably captures my thoughts w/r/t/ the longwave pattern... In terms of the 'actual' weather, I'm thinking sustained and quite severe cold (cold intensifying as we progress through Feb) with a polar trough centred just to the east of the UK feeding in cold north or north-easterly winds with snow and very low night time temperatures throughout the outlook period.

The GWO continues to advertise itself as an excellent long range forecasting tool. No updates to some of the angular momentum budgets since Saturday so we working on some assumptions here. Current outgoing longwave radiation anomaly plots depict a reasonably strong convective signal in the Indian Ocean, eastern Pacific, Africa and tropical Atlantic. Anticyclones are lining up above these convectively active areas consistent with the GWO evolving through phase 2/3 and frictional torque is likely be strongly positive at 30N adding westerly flow and positive momentum transport at 40-50N. Net result, fast velocity sub-tropical flow and meridional polar flow. Both the MJO and GWO are in phase 2/3 transition and I strongly suspect that we are seeing one of those phases where the tropical signal (MJO) and extratropical signals (GWO) are harnessed and phasing in sync. to promote a strong wind signal. Composites for GWO phase 3 and MJO phase 3 teleconnect to an upper ridge in the Mid Atlantic and positive height anomalies over Greenland with a deep upper trough over Scandinavia: This looks reasonably conistent with the extended range modelling taking us to day 14 and I support the notion of the model guidance to draw the ridge forming over Scandinavia towards Iceland and then southern Greenland. From previous postings you'll understand my reasons for supporting a continued pulling of the ridge back keeping a deep trough in place over Scandinavia and the trend in the models supports this. t240 ensemble means for the GEM and GFS advertise well this pattern: This is interesting as it opens up an attack from the SW approaches as low pressure attempts to force its way through beneath a block over Greenland towards the Scandinavian trough. Here lies an opportunity for heavy snowfall and the precursor easterly will be crucial here - we need as much cold as possible over the UK and near continent as the lows attack, ideally with as much blocking as possible towards Iceland - which is very possible I think. Beyond the 14 day period, we should start to see the early developments of the MMW on the upper troposphere. Standby for height anomalies resembling this as the AO starts to tank....

The trend continues to be our friend. ECM zonal wind forecasts suggest a rapidly shifting dynamic within the upper layers within t192 (this is yesterday's forecast and therefore we can reasonably assume on today's output that this has continued to t168): http://strat-www.met.fu-berlin.de/cgi-bin/...t=all&var=u Note two key levels - 500 hPa and 200 hPa and the differences between 50N and 80N. There is a shift programmed here with negative zonal wind anomalies (not mean values just yet) propagating downwards over the polar field. NCEP zonal wind anomalies identify mean values of 8m/s and 12m/s at these two layers which best represent the key drivers of overall patterns. These translate to slight negative and nuetral anomalies: http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif On current projections, it would appear that we are seeing a very rapid downwelling of waves through the atmosphere and there are now close to normal zonal wind anomalies prevelent. Crucially, we are now starting to repeat the conditions which allowed what was a cold SSTA-driven pattern in December to manifest itself. This underlying pattern is still at hand and it may well be that the reversal in zonal winds will start to favour a weakened jet and a ridge in the NW Atlantic with cut off low beneath it, in addition to some form of ridge over the NE. So I ask, are we going to get ambushed from both NE and NW vectors in the coming weeks ? 06Z GFS spread for 500hPa values identifies some uncertainty over the pressure over the NW Atlantic indicated by some of the ensembles building pressure off Newfoundland with a cut off low beneath:

Very much looks that way Mark - the upper atmosphere starting to influence the pattern although this is in advance of any potential impacts of the now impending MMW. Both ECM and GFS 70hPa layers (indicative of 100 - 10 hPa layers during this period) show splitting in the vortex across the North Pole with one vortex centred over the Davis Straights, the second into Siberia at t168: Zonal wind means are decreasing rapidly at the 200 hPa, 500 hPa and 1000 hPa layers which will weaken any upper level support for sustained polar westerlies: http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif whilst the ECM (arguably a better model for the stratosphere) programs a rapid weakening of zonal winds at 65N: Some keen eyed will see the QBO running at 10m/s and projected extent of the warming event (negative blue wind anomaly). So some support there for blocking developing across the Pole and towards Svalbard. I stress this is not a direct response to the warming event although it does make the life of any downwelling wave that much easier and we might see a more rapid 21 day propagation. The GWO has evolved as expected so not such a clear case for the flat jet programmed into the future and I'm still thinking that the Atlantic ridge is going to come under some pressure to shift NW and become interrupted. Blocking looks a reasonable prospect on this evidence, it's just the strength of upper level flow across 50N - 55N which could hold thinks up here, but generally the trend is our friend right now.

I think we are looking at three dominoes toppling before we can reasonably expect a cold pattern to re-establish. Domino 1 - the amplification of the Pacific Jet has been achieved. Domino 2 - the breakdown and retrogression of the Pacific West Coast Ridge is now being consistently modelled within the t120-168 timeframe. Domino 3 - may prove to be the most robust, the weakening of the Azores Ridge. Full disc satellite imagery shows the size of the ridge in the Atlantic stretching from the Canaries across to Bermuda and Azores. Wavetrak data shows strong low level easterly winds on the SE flank of the ridge. http://cimss.ssec.wisc.edu/tropic/real-tim...s/wavetrkA.html Until this ridge (and specifically the strong easterly flow underneath) is weakened, it will be difficult to achieve any sustained amplification along the polar front. So what might reduce the Azores Ridge ? Analysis of the longevity of Azores ridges (lat 25-40N) through frictional torques tells us that the ridge is enhanced when frictional torques are decreasing and weakening when torques are increasing. The 'wavelength' of the frictional torques is about about a month or just less. This means that our current ridge enhancement / falling torques should last until the last week of January. The GWO composites for phases 1 and 2 (where we are currently heading for) argue that the ridge will also come under some pressure to shift north and north-west over time: Tendency in angular momentum also indicates some increase in westerly wind motion within the latitude of the sub-tropical ridges and negative tendency above it which should signal the weakening of the ridges although I suspect this is restricted to the Pacific sector at the moment. Given the favourable development of the GWO and natural wavelength, we should continue to see longer range tools advertising a weakening of the strength of the Azores ridge.

I think the atmosphere continues to slowly evolve towards a very cold pattern and NWP tools are picking up on a signal for the longwave pattern to plunge the upper trough SE into Europe with height rises above. Frictional torques (wind additions due to longitudinal pressure differentials) are suggesting a negative tendency to angular momentum. The westerly additions appear to be in the tropics and sub-tropics whilst negative (easterly winds) are being added to the higher and mid latitudes. This negative trend is important as this is usually a precursor to convection returning to the Indian Ocean and shift in the GWO towards phases 0-1-2. Overall tendency in angular mometumn is reflecting this frictional torque and is down: The GWO has responded and is moving towards phase 0 http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif Easterly wind propagation from the tropics should continue to see the GWO shift in a more decisive fashion towards phase 1-2 in the next week. GWO composites with February's with these phases suggest a strong tendency for amplification in the mid Atlantic: NWP tools are starting to hint at this development. It's noticeable that any hints of ridges developing as disturbances off the Gulf Coast are translating to either Scandinavian or Icelandic blocking. The latter ties in particularly well with GWO composites allowing for the general context with a trough located over NW Europe and seasonal trend for the ridge in the Atlantic to get pulled westwards towards Newfoundland, assisted by that warm pool of water. Upper level zonal winds are suprisingly close to normal 200-500 hPa: http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif ... which should assist with any attempts to build blocking structures to our north. So for me, there is an increasing trend for a more amplified upper flow triggering a mid Atlantic ridge and height rises to our north. Current guidance suggest that Scandinavia will be the most appropriate location, and this is most probable given the strength of the flow to the north but I'm not sure, and equally this could evolve over the next few days to being shown more towards Iceland. The most likely timescale for this development to a much more sustained colder spell is the end of Jan into early February. Before that, the jet being aligned NW-SE with disturbances shifting it ocassionally west look about right.

Some very interesting aspects to the angular momentum budgets tonight. Flux convergence reanalysis identifies positive transport across the high latitudes which is supportive of negative (easterly wind) anomalies developing and converse developments over the sub-tropics. This is perhaps a first indicator of easterly propagation shifting poleward - the first stage of a -NAO phase. Frictional torques mirror these fluxes. Over the high latitudes negative torques are adding easterly winds and there has been an end to positive torques over the mid latitudes signalling an end to high velocity jet flow and cue for amplification. Over the sub-tropics we are starting to see +ve torques adding westerlies beneath the sub-tropics and weakening the sub-tropical ridges further north. The Pacific flow will be first impacted and then the Atlantic where we should see the Pacific ridge pulled west and north and subsequently, a series of discontinuous Atlantic ridges develop angling the flow ever more NW then NNW. The GWO has started to shift back to phase 3/4 although I'm suspecting a dramatic fall is just around the corner tied into this being a positive momentum phase which tend to last 15-30 days, and likely ending within the next 10 days with a real Nina 'kick' coming to the atmosphere. This I think will send us into a strong amplitude phase 1/2 orbit late Januay / early February - teleconnecting to a mid or high latitude Atlantic ridge and NW European trough, possibly with a developing ridge over the top.

A little more coherent signal emerging I think which replaces an incorrect call for high pressure solution (failed to pick up a strong +ve momentum transport signal and corresponding increase in the jet). For the last few days, we have been monitoring an increase in easterly winds across the Tropics. These now extend in a broad belt 0-30N for large parts of the Indian, Pacific, Atlantic Oceans as well as now starting to show over Africa. This is finally being registered in a large -ve tendency in angular momentum: http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif This will start to have an impact on the +PNA ridge, serving to retrogress it and shift the mean position further north. This will have an impact on the downstream longwave pattern, with the Azores ridge likely getting shifted westwards away from the SW approaches - so definately no high pressure build over the Continent, in fact quite the reverse. Frictional torques can be seen as a good indicator of the tendency in angular momentum over various parts of the globe. The current increase in polar flow was well shown by +ve frictional torques and corresponding -ve torques to the south. http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif These have started to recede over the mid and sub-tropical latitudes - indicative of the jet flow becoming less strong, especially if the wavelength is changed by a retracting Pacific Jet. This may take some time, and residual flow will remain as sub-tropical ridges will continue. However, the net impact of the increasing easterly trade wind surge over the Tropics will I think lead to the sub-tropical ridges (and jet streams) being flattened somewhat and shifted southwards. The Pacific Jet is already strong, flat and southward displaced. Global Wind Oscillation is continuing its painfully slow orbit at low amplitude towards phase 0/1. The only things preventing a full crash towards a strong Nina type (phases 1,2,3) pattern are the frictional torques (receeding) and +ve mountain torque over Asia (strong Asian jet). There is a corresponding strong -ve mountain torque over North America reflecting the inverted pressure pattern across the mountain ranges and this is adding to net easterly flow. http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif The bottom line here is that we are very close to global momentum crashing into deep negative territory although the Asian mountain torque will likely remain for a time (all eyes on the stratosphere) and prevent a strong Nina type pattern from establishing. Contrary to what you might think, I would argue that at this time, a strong 'attractor' state of the GWO phase 1-2 (falling momentum) would be very good for any prospects of amplified flow. During January this teleconnects to height rises across the north -east Atlantic (clue for southward displaced jet). In February this teleconnects to a similar pattern to that observed in Fbruary with a substantial mid-Atlantic ridge and deep trough over NW Europe which is a logical evolution if the upstream pattern regresses. So whilst it may take some time for these changes to take effect, I can pretty much support the idea of the jet to become more southward displaced and a more amplified flow to take effect towards the end of the month. In the meantime, a strong signal for an unusual phase of unsettled weather (temperatures close to seasonal average) but trending cooler in the longer term once again.

Very interesting wind profile showing up at the moment with a strong alignment of easterly winds across the tropics. Latest wavetrak satellite imagery shows the easterly wind surge across Pacific and Atlantic: This is supporting strong sub-tropical ridge development - note how the Azores / Bermuda ridge is showing up as a large well defined feature in the modelling. Angular momentum flux convergence plot shows how these easterly wind anomalies are being propagated both poleward and equatorward: Notice the tendency for easterly wind anomaly transport 35-45N and also poleward eddies fluxing easterly winds above 60N (some suggestion of weak blocking). The overall hemispheric pattern is being dominated by a Pacific sector amplification with the northward displaced sub-tropical ridges fuelling thermal gradients and strong jet flow (although I'm pretty sure that the Atlantic signal is to slow velocities). What's of interest is which way this jet flow migrates. The extended range modelling has been toying with the idea of shifting south and flooding the North Atlantic with cold air and this is worth keeping an eye on. Ofcourse the alternative is a northward shift with a strong polar vortex. The global wind oscillation should provide a good guide to where the solution lies. Currently in phase 3/4 following a backward movement yesterday, I was surprised at this given the strength of the easterly surge over the tropics. If we see the GWO and tendency in angular momentum fall back (move towards phases 1/2), then southward movement is on the cards. If it hovers around phases 3-4-5, then something akin to the modelling of the next 120 hours is likely. http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif One area I am particularly interested in is Iceland. Several model runs (e.g. today's parallel GFS) have tried to develop heights over this area associated with a split jet and southward displacement as surface depressions steer a path south-east. With strong (but not anomalously so just yet) upper level zonal winds, some shifting of the position of this ridge towards Svalbard / Scanindavia is possible (an unusual way to develop such a block). Several historical analogues make such an evolution plausible. One aspect of the modelling which seems rock solid is the strong Azores ridge and high velocity jet flow coming off the US (but decelerating) given the upstream +PNA ridge - but where is that jet going ?

Tendency in angular momentum is down once more, mainly due to a strengthening easterly trade wind surge and slight easterly mountain torque over the sub-tropics. Latest wavetrak image depicts this over the Pacific and Indian Ocean sectors: http://cimss.ssec.wisc.edu/tropic/real-tim...s/wavetrkP.html We still have some poleward positive momentum transport supporting anomalous westerly flow 50-55N, but also hints of increased sub-tropical flow as a result of the tropical winds picking up. Frictional torques are suggesting a slight northward shift in the jet towards the sub-polar region which, in combination with the sub-tropical flow is supportive of the amplified pattern being programmed right now t144 onwards. http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif Overall, the general direction of travel is positive for cold returning via height rises over the UK and Scandinavia.