Glacier Point

.. and there are differences between -NAO west for December and February. Factor in a potentially massively distorted hemispheric profile, and traditional rules of interpreting this feature may not wholly apply.

Could be Nick, although the warming / split signal won't be in view until another few days. Then we should start to get an idea from GEFS / GFS ops on where the bits of the primary vortex go. I suspect there will be a strongly -AO signal projected across the NH, including Arctic ridge, which is currently lacking.

The 06z and 00z GFS, with ensemble support, look very consistent with 00z EC deterministic at day 10. The GEFS interpretation of this is for a reversal in zonal winds at 10 hPa at 60N, so we are in the realms of a technical stratospheric warming with a 10-12 day time frame. The evolution of this, a substantive displacement followed by a likely wave 2 split of the primary vortex centred over Siberia, looks to take a few more days on top of this (say 14-20). Time to wheel these out.... From: http://www.inscc.utah.edu/~reichler/research/projects/TS/TS.shtml and http://www.atmos.colostate.edu/~davet/ao/ThompsonPapers/ThompsonBaldwinSolomon.pdf The zonal wind profile in the lower and middle stratosphere and mid troposphere is a whole lot more negative compared to almost all weak vortex events, so the lagged times may vary somewhat this time round, probably a lot more progressive and responsive to stratospheric lead. So assuming first lagged influence gives target date of 10th December for first tangible stratospheric impact (some argument could be made for instantaneous type response to fast propagation waves).

The GEFS, CMC and EPS solutions upstream for the longwave pattern over the NE Pacific and Western Seaboard are all different for the 11-15 day range which has ramifications for the placement of a substantive high latitude block. GEFS furthest east, EPS furthest west (although EPS beginning to raise heights / thicknesses over Scandinavia). With the MJO convective wave dampening down in the West Indian Ocean, and the 'noise' from the intra/sub-seasonal forcing further east over the Maritime Continent becoming a bit louder, that's not a definitive signal for the models to latch onto just yet. The exception being that all models show the high latitude block. With a natural evolution in the atmosphere and eastward migration of the tropical wave, we should see the GEFS solution gain the upper hand, although probably a relatively slow process given the speed that trop waves can move at in the Indian Ocean. I think a fair summary for the opening salvo for week 1 winter would be below average temps, with the scope for downward corrections as the margin for error, and increasing downward pressure in time. The pace of eastward migration of tropical waves in the Indian Ocean the key here to timing of models picking up a more coherent cold signal.

Not sure I would agree with you there Nick. It is a crucial part in helping to bring the polar vortex down. Short term pain, long term gain and all that..

GFS op and EPS / GEFS means beginning to offer a little more evidence to a substantive stratospheric disturbance as we start the winter. Keep in mind that the pattern from around 19th Nov for a period of several days builds a strong mid tropospheric ridge over East Europe / Western Russia. That is perfect for upward wave 2 deflection into the stratosphere. The vortex is already very weak and it won't take much to break it down, wave 2 probably timed for late Nov into early December. My early season analogues hinting at a new vortex forming over the Canadian sector, well displaced from the Pole. This would be my punt for early December as to how the pieces of the jigsaw will be arranged in the stratosphere... Door wide open for Arctic ridge and deeply negative AO.

..another GFS op showing a highly distressed upper vortex. The detail will change, but the broad gist isn't dissimilar to 18z, 00z and 06z...(you need to view as contour heights not temperature) This stemming from troposheric pattern early November. There is more to come c/o wave 2 likely to hit end of November into December.

That's a good tee up for this.. From a subseasonal and seasonal perspective, the gift that has kept on giving is the pattern over the Maritime Continent. I've used this a lot (to success) over the mid summer and autumn periods as a key driver influencing the hemispheric pattern. It was an excellent lead for the high pressure signal centred over Europe and troughing in the Atlantic. Through September and October, this pattern has remained an only recently has it diverged. This is really useful in understanding why we are getting divergence in the pattern, and key to understanding if or more appropriately when it will return as the key driver. So what am I discussing here ? Well, let's look at the spatial arrangement of sea temperatures across the Indian Ocean and West Pacific. There's a text book signature for enhanced low level westerly winds through the Indian Ocean and easterly winds through the central Pacific. This is evident in surface wind anomalies for the region. Where winds converge, upward movement of anomalously warm air leads to convective activity and again this footprint is evident in rainfall anomalies. What's crucial here is that this convective pattern is mirrored in the upper and mid troposheric levels. Here we see an enhanced (negative) velocity potential associated with a substantive standing wave. Analogues for this provide a clue as to how September - October should have looked.. Compared to observed.. That's actually not a bad match across the Northern hemisphere, allowing for a more blocky type pattern over the Arctic related to other drivers, and particularly within the Atlantic sector. So we have a useful blueprint as to how November and December should pan out, given the likely persistence of the underlying driver, the spatial arrangement of SSTAs in the Indian Ocean and Maritime Continent. However, in the last week we've seen the modelling develop a deviation from this pattern. Why ? The answer to me lies in destructive interference from an emerging MJO wave. GEFS and EC EPS are consistent on the development of a convectively coupled wave moving through the central Pacific and Western Hemisphere (phases 7-8-1). Composite for these phases, and you'll understand why we're seeing the trough signal over NW Europe. Note the development of higher than normal heights over Easter Europe and Western Russia there. That is the perfect geoeffective position for upward wave reflection into the stratosphere. Is that part of the beautiful choreography of the atmosphere to bring about a stratospheric warming end November / early Dec ? So assuming the MJO is a player, where do we go from here ? A stall in the MJO in phases 7 or 8 is unlikely. It rarely gets stuck there. Where it does sometimes linger is the Indian Ocean, phases 1, 2 and 3. This seems unlikely in terms of what we've seen and underlying sea temperatures in the region. That leaves me with a likely return of the convective wave quickly to the Maritime Continent as per GEFS. That may well enhance or excite the standing wave once more. Analogues for this for November / December show a very coherent signal for +ve height anomalies in the mid Atlantic, to which you would need to adjust for greater amplification on the back of zonal wind anomalies (weaker than normal) and warming of the column around Barents Sea. If the GEFS is correct here, we should see a return to the mid Atlantic ridge or even blocking further north in the medium to long range. I would expect to see a Scandinavian trough also emerge. This will be tied into the evolution of the MJO towards phases 4 and 5. One further line of thought is relevant here. As others have keenly observed angular momentum remains disconnected from the ENSO state. For a weak La Nina, we should be seeing -1SD, not +1SD. This is reflective of an atmosphere which is primed for amplification. If the MJO swiftly propagates eastward and enhances the convective / upper level velocity potential over the Maritime Continent, we should see a corresponding upward spike in tendency in relative angular momentum giving a large projection of the GWO towards phase 4. Analogues for this with a ten day lag applied are interesting.. So for the next 10 days or so, cyclonic and troughy over NW Europe, temps not far off average or above. Thereafter, would expect to see a transition to a more blocky type pattern with height rises in the mid Atlantic and points north over time.

.. Think we may be talking about wave 2 in the coming weeks. Latest run of modelling builds mid tropospheric heights over Eastern Europe / western Russia in the week 2 and 3 period. That's a near perfect amplitude for wave flux into the polar stratosphere. latest op gfs probably jumping the gun a bit, but you can appreciate where it's getting its signal from.

06z GEFS mean looking a lot like EC week 4 at the end of the run. Add to that the continued CFS week 3 / 4 forecasts, the jet profile over the US looking very interesting right now during the second half of the month leading into early December.

Will be very informative Nick where we go once two current factors mature out. I suspect the MJO (projected to enter central Pacific) is destructively interfering with the -AO/-NAO signal. Model forecasts are reasonably consistent on a phase 7-8 projection which mirrors the 11-15 day mean height anomalies. This is the first time in a while where the MJO / tropical convective pattern has deviated away from the Indonesian standing wave. Note the 3-wave pattern there, anomalous ridges over Alaska, mid Atlantic and Siberia. The ongoing disturbance to the stratospheric polar vortex will be another inhibitor to the -AO for the time being, although with a strong signal for negative zonal wind anomalies, surely just a matter of time.

Hmm, new EC week 4 (start of winter) is a zinger. Coherent -AO signature, even allowing for the differences between the 00z and 12z.

It's the difference in 500hPa heights between 00z and 12z runs. FWIW the 12z was 140m lower with its heights to the west of the UK mid next week during that critical phase so support for the op. The EPS also broadly supports the spirit of the op to return to a mid Atlantic ridge scenario in the extended range, and trough returning to Scandinavia. I suspect our sojourn with pv over Greenland will be a brief one.

Either way, those chasing rain are not going to be best pleased. Following a dry first two thirds to autumn on the back of a dry second half to the summer, winter recharge is going to become an issue.

OK, more precisely, NOx pulled down from the layers above which bond with O3 and facilitate additional complexes with chlorine and bromine both of which result in an endothermic reaction resulting in cooling.

The current projected Wave 1 displacement of the stratospheric vortex looks to be following the script nicely. During the last few days and for the next 2-3 we will see a series of unusually deep low pressure systems within a large upper trough over the North Pacific region. This leads to Wave 1 activity lagged by 14-21 days and takes us to the current projection for an unusually weak zonal wind flow throughout much of the mid and high latitudes. Both the GEFS and EC EPS are reasonably consistent on a weak westerly zonal wind at high latitudes. This extends throughout the troposphere, and you get some idea here as to how ingrained this pattern is. Looking forward, another series of deep upper low dominate the North Pacific in the 6-10 and more particularly the 11-15 day timeframe (and also +ve mountain torque during this time as well). Net result, more displacement of an already weak stratospheric vortex almost certain towards the end of November and start of December. That is peak vortex cooling season, so we are going to be totally against recent climatology at that point. The lack of any detectable influence of relatively high geomagnetic activity and westerly QBO is stark. So the key question, what happens after that in the stratosphere ? Clearly at some point, any downwelling of easterly zonal wind anomaly from the upper layers will pull down colder air. The vortex will cool. However, feedback loops may already make that cooling ineffective and ultimately may provide a killer blow for the vortex. Check out those years where cooling was delayed into December - it doesn't make happy reading for strong vortices in January !

I'd echo that. The week 3 500z anomaly pattern remains similar to the week 2 and 4 pattern, so interpretation of a mean 2m temp value should be based on this and not model-based projection. I would not suggest that the week 3 value is average as suggested in the tweet, particularly as we've seen models begin to drop systems underneath the block, increasing a NW-SE tilt to the upper flow. Worth also noting that the EC extended run intensifies the -NAO / -AO at the end of November / start of December.

somewhat mischievously on my part.. comparison with what is currently being modelled and 'selected' years at the same stage for 10m zonal mean winds 60-65N..

I know it sounds counter intuitive tcc, but there are good reasons for a weak vortex and more precisely a weak zonal wind flow during November to substantiate a tropospheric ridge in the mid Atlantic. Other drivers [ENSO state, Maritime Continent forcing] are aligned nicely to deliver this.

Well this is getting interesting and a possible game changer. Why chop down a tree when you can squash a seedling ? Following the lead of the models of late last week, we have begun to tick down on a warming signal and significant weakening of zonal wind speeds in the middle and lower stratosphere. Both GEFS, GFS and EPS remain solid on developing this. The operational GFS getting close to a reversal.. And right down the middle of the GEFS mean, some members dropping into reversal.. The EC EPS is similar and would give a sizeable clustering around +5 to +8 m/s. That is one weak vortex. Already. Even if we were to assume worst case scenario, those two GFS members going up would still translate to a sizeable negative zonal wind anomaly. The mid tropospheric modelling continues to develop a deep upper lower dominating the North Pacific in the week 2 time frame. That is near perfect geoeffective positioning for upward wave (Wave 1) deflection into the vortex leading to a push of the vortex towards the North Atlantic. We are yet to see these being manifested in the strat modelling as they are a little way off, so a weak vortex looks set to become event weaker.

That signal not just confined to the GFS op. GEFS and EC means all showing a similar signature. Not in itself a large warm anomaly, but of interest if you are following the logic that high geomagnetic activity equates to cooling of the upper layers and vortex intensification and swift transfer to a +NAO regime. We are travelling in the opposite direction to that right now. Still quite some time to go and we know how quickly cooling can develop. For what it's worth, EC and GEFS at extended range showing conditions ripe for Wave 1 development, possibly Wave 2 into early / mid November. A distinct lack of westerly zonal wind anomaly in the troposphere right now, all restricted to the Southern hemisphere.

Messy - if nothing else on the basis of the messy and slightly incoherent composites for phase 6-7 of the GWO for January and how we relate those to the back end of January into February where these become more coherent.

GFS starting to pick up the amplification coming out of the Western Pacific 23/24th January - where will the anomalous ridge manifest itself over NW Europe ? 12z going for Scandinavia. Not sure I'd agree with it there, and much will depend on modelling of the decline of the vortex over Greenland.

So, verification for posting in as much I can do so given we are in the midst of model mayhem and projecting forward using NWP is not going to be convincing. The only things I can verify at this stage are: GWO passing through phases 5, 6 and then into 7 with a lagged atmospheric response up to 10 days. Well, Check. Atmospheric response for phases 5 and 6 and observed in this critical period: and the observed mid month so far.. Not bad. Timing was specified out to 10 days from the 9th/10th but still what I would regard as the margin for error, particularly as the posting was dated end December when none of this was sniffed at. Also note the slight west based -NAO observed that was well signalled, long in advance of any model solution. So mid month northerly, followed by lull as the pattern becomes more Atlantic driven as the NAO becomes more west based for a time. This is now, so reanalysis in a few days will be interesting, but I reckon I'm good for that. Turning to the final third January into early Feb, this does present some difficulties as monthly change overs are problematic - do we use composites for January or February ? Notwithstanding this, broad guidance on the assumption of a stalled GWO or even heading back to phase 5 is towards the mid Atlantic / southern Greenland anomalous ridge and northerly / northwesterly./ northeasterly vectors. Granted, current long range modelling is not there (but caution advised in relying on this, moreso at this juncture given synoptic variability), but trending there (06Z GEFS mean has a strong but flat mid Atlantic ridge). Here comes the usefulness of being able to understand why that might be and when that might change. I would suggest the current strengthening of the stratospheric vortex is destructively interfering with the tropospheric pattern by strengthening the vortex west of Greenland. This prevents the Atlantic ridge from extending northward into Greenland. When or more importantly how will this change ? With the GWO heading back to phase 5 around 23rd -28th January, we should expect some more amplification to be levered into the upstream flow. That gives us back end of January or more realistically early Feb, although you'll appreciate the margins for error and the final few days of January would not be beyond reasonably possible. That also coincides with the likely displacement of the stratospheric vortex away from Greenland. So bottom line here is that we continue to see signals for mid Atlantic ridge extension towards Greenland and -NAO conditions during late January into early February. That should be put alongside current model consensus which does not suggest this.

Updated upper level velocity potential update (GFS) is interesting.. Continued migration east of the -ve anomaly towards the Maritime Continent during week 2 but note also split emerging between North and South Hemispheres with the Northern Hemisphere element well into the Western Pacific. That suggests tendency in angular momentum will show an increase during week 2. No dramatic fall off in angular momentum. Also, GEFS and EC EPS consistent on dropping an upper level trough through the Western Pacific (i.e. downslope of the Asian Massif) 23-24th Jan, with upslope ridges and troughs - good precursor pattern for generating westerly inertia into the Asian Jet. Net result, GWO will continue to orbit high amplitude phases 6-7 and, with the renewed injection of westerlies coming, shade phase 5 as well.. Stress again, these composites are for February, and Jan composites not so keen on ridges in these locations but still suggestive of Scandinavian troughing (bet messy otherwise). So the close we get to Feb, the more I would expect the ridge to our west and north-west and trough to the east to bear out. Strat allowing..