Glacier Point

Evidence suggests a strong alignment of strengthened sub-tropical ridges across the Northern Hemisphere which is helping to force westerly addition to the polar flow and easterly addition to sub-tropical flows:- http://www.cdc.noaa.gov/map/images/reanaly...leaam.90day.gif There is a large +ve frictional torque in the mid latitudes and corresponding -ve torque in the sub-tropics:- http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif Overall the balance of frictional torques is negative, which is indicative of falling tendency in global momentum, usually a precursor to negative mountain torques which looks to be ocurring:- http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif Within the tropics, wavetrak images indicate easterly trades setting up:- http://cimss.ssec.wisc.edu/tropic/real-tim...s/wavetrkP.html which is consistent with latest OLR data showing strong -ve anomalies (convection) in the western Pacific and supression in the central Pacific, http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif and also likely forcing -ve tendency in momentum over the Tropics. Coriolis torques are also adding easterly flow to the atmopshere:- http://www.cdc.noaa.gov/map/images/reanaly...ltauc.90day.gif Put all these together, and there are good grounds for driving tendency in angular momentum down (although I'm dubious about the speed - maybe another day or two of twists and turns), but the net result will be to force the GWO into phase 8/1/2:- http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif Zonal wind anomalies remain close to normal over the polar field at 200 hPa to 500 hPa, but some suggestion that these will increase over time as downwelling waves increase speeds from the cooling stratosphere:- http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif I'm not sure what impact this will have as the polar vortex has shown a marked tendency to spit into two - over the Hudsons and Siberia - one to watch although it will not be until after mid month that we see an increase in organised polar westerlies. Composite reanalysis for the centred tropical forcing (convection) suggests a band of high pressure in the Atlantic and retracting ridge over western US into the Pacific:- Composite reanalysis for GWO phase 2 looks a good solution for polar flow and tendency for ridging in the Atlantic:- CPC 8-14 day analogues (2nd panel):- http://www.cpc.ncep.noaa.gov/products/pred...4analog.off.gif Roll these forward 10 days ..... ... another signal for high pressure blocking out the mid latitudes over the E Atlantic, and not also the tendency for the Pacific ridge to retrograde which will send an amplification (trough led) downstream with a ridge likely building behind, possibly working towards southern Scandinavia over time and possibly, possibly retrogressing slighty NW towards Shetland. The composites for GWO phase 2 for me best capture where I think we are heading in the 10-20 day timeframe - mid Atlantic ridge to develop and transfer slightly eastwards and northwards towards Ireland (winds from the NW initially), taking up residence over the UK and southern Scandinavia. Therefore remaining largely dry and temperatures close to or below average.

Interesting.... A strong +ve mountain torque is underway over Asia, mainly 30N (worth keeping an eye on stratospheric temperatures within the next 10 - 12 days for signs of a rise), http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif Frictional torque has receeded somewhat and the mountain torque is the main thing supporting a westerly motion within the overall ocean-atmosphere framework. The global wind oscillation is in stage 4. Within 5-10 days of strong +ve torques, we generally see the atmosphere respond by removing westerlies with easterlies (note the impacts of the last +ve mountain torque). With a strong Nina base state now embedded, we should see GLAAM drop sharply. This will serve to force tendency in global angular momentum downwards and back towards phases 1-2, which correspond to a weak and amplified polar flow over the Atlantic. Within the tropics, we see the latest MJO wave shift eastwards, although this has detatched itself from the main body of convection centred in the west Pacific around the Philippines. Compare here the MJO plot (phase 5/6) with tropical convection centred more in phase 4 area: http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif Composite reanalysis for the centre of this convection:- analogues> expected outcome> (I would adjust these anologues somewhat to take account of increased upper level zonal flows forcing the core ridge eastwards towards the UK) If you perm the expected GWO signal and tropical convection signal, it is overwhelmingly for high pressure solution in the east Atlantic and NW Europe sector. This contradicts somewhat some of the longer range NWP guidance which suggests an unsettled zonal pattern.

I read that as below average probability signal days 12-42 post warming date. So earl/mid Jan through early Feb ?

Just a reminder. Stratospheric time scales are extremely slow. Modelled effects of the warming not likely to show until 7th January at the earliest. That's still a few days yet before it gets into GFS range.

Some conflicting views expressed here on the desirability of high pressure centred UK immediately after a strat warming.. https://twitter.com/WorldClimateSvc/status/1075165825290723333

Seriously ? Have you looked at the mean height anomalies for the next 5-15 days ? Total opposite of what you're suggesting. Wouldn't read too much into these plots as they don't show the whole picture. Strong region of suppression developing across the Indian Ocean and broadscale favourable environment for westerlies (weakened easterlies) across the central and eastern Pacific. Net increase in angular momentum incoming and atmosphere taking on a very El Nino look to it.

EPS lending support, split signature day 11

Decent signal for a strong mid latitude ridge signal across Christmas week in the EPS and GEFS for NW Europe. This looks plausible on tropical and extratropical signals right now. Thereafter, EC locks in a -AO out until February and drives the NAO negative mid January onwards. I would be expect movement of mid latitude ridges north and north-westwards over time as the impacts of the stratospheric warming take effect.

Struggling to find comparable displacement episodes of this magnitude based off GEFS and EPS.. This one possibly:

EPS is top down, but for indicative purposes, looks very similar at 10 hPa to gfs 06z wrt placement of ridge and residual parts of SPV.

Suspect Mike Ventrice will be tweeting the eps mean at t360 soon. Very similar to gfs 18z and 06z. Strong mid winter warming looking very likely.

Would concur with this, although NE vector may be more appropriate early Jan.

So timing looks like this: Stratospheric (displacement) warming 27-29th December. First tropospheric response would on that basis be around 10th January, peaking on first wave response 15th-25th January. Second wave early to mid Feb, potentially very interesting if split vortex occurs. Bear in mind, this doesn't take into account troposheric forcing. MJO cycles suggest mid latitude ridges early January, but GWO lagged impact of westerly inertia being deposited as of 9th December suggests more -NAO variant to this. Either way here, January and early February hold the greatest cold weather potential for this winter (and has been the case for some time).

Ticking along nicely. Big convective anomaly traversing through the Indian Ocean with clear extratropical impacts. Best analogues early December 1965 and early January 2004 for a strong displacement type warming, Day 1: 10 hPa By day 26 indication of split vortex (and therefore a sustained event): Tropospheric impacts start day 12 on these analogues (not allowing for fast propagation waves), running through day 30. Day 16 here for reference: If there were a split vortex, trop impacts would continue well beyond this. All of this occurring when tropospheric drivers signalling low pressure in the Atlantic and pressure rise over Greenland for January. The key issues will be: getting cold air embedded over Europe during the initial stages, and not to much high pressure over North Africa and Iberia and sufficient upper level flow through central Europe. Interestingly, 1984/5 continues to resonate here. MJO evolution that year suggests another opportunity for a strat event during February.

12z GEFS 10 hPa mean is a heartbeat away from a technical strat warming event.....(closer than 00z)

Actually thinking here that there will be two, possibly discrete episodes of ridging to our NE depending on the extent of Atlantic incursion on the back of this. The first being reasonably modelled but probably too progressive in its break down / eastward displacement and a second phase around the 21st - 23rd December.

Tend to disagree with this statement Nick. Mike Ventrice's plot shows increasing depth VP centred east Indian Ocean / Maritime Continent week 2: These tend to under represent anomalies in the week 2 range, so an increase in depth of the -ve VP anomaly is significant (and this increased from yesterday's plot as well). So whilst the RMM values might be weak, the angular momentum tendency signal will be increasing once more. The eastward propagation of this will lead to an increase in total AAM as well weeks 3 -4 through the Pacific. So whilst the +AAM signal through 30N will decay, new inertia off the tropics will hold up the global AAM budget to elevated Nino levels. The real fun and games start when that signal begins to decay, and should coincide with the MJO in phase 8.

Worth noting that the recovery of the upper vortex is not necessarily a bad thing from a downwelling -ve zonal wind anomaly. I've had a lot of success this year with 1984 polar analogue. A lot of similarities since that late warming / early final warming in February. We are currently at the end of downwelling signal for the autumn in the troposphere, AO set to bounce around as per late November to mid December 1984. What followed in 1985: There was an entrenched weak -ENSO signal that year and east QBO, so need to factor this in (although could argue that weak +ENSO signal and solar are more conducive to rapid tropopsheric response during the second half of the winter). Note the +ve zonal wind anomaly / colder upper stratospheric temperatures following the (downward propagating) warming late 1994 / early 1985. Strat warming looking like 26th December IMO.

Whilst the gist of the extended GEFS and EPS looks credible (+ve height anomalies centred Hudson Bay), there's any awful lot of white space across the rest of the Northern Hemisphere. Suspect that modelling remains too progressive w/r/t Scandinavian ridge demise, and, possibility that we will retain some weakish +ve heights to our north in the run up to Christmas. My punt of technical strat warming date would be 26th December.

Yes, this would be bang on with expectation. So a less cold (T850) phase after the northerly then easterly next week

Although relative angular momentum is very high right now, that is partly a reflection of a persistent +AAM signal in both hemispheres. The greatest +AMM signal in the Northern Hemisphere is that westerly inertia signal deposited 30N. That is essentially past peak and likely to decay over time. However, with the tropical VP signal migrating a tropical wave eastward during week 2: We're likely to see jet extension and +AAM signal renewed week 2. That takes GWO signal back to phase 5 (Scandinavian ridge signal). Because of the slow speed of this tropical wave, suspect that the GWO signal will remain more in phase 5 than would normally be expected. Hence ridge to our NE likely to be more durable and models likely to be too progressive with this feature. Looking beyond this, the GWO suggests a phase 6-7 evolution on the cards as the decaying +AAM signal and maturing momentum signal from the tropical wave take effect. Ridge signal to shift further west and NW. Disregard any model output that does not follow these broad lines. The final third of December into early January is stacked with cold potential. Think a decaying +AAM signal and MJO returning back to the Indian Ocean. High chance that the GWO will be heading into high amplitude phases 7 and 8 and overlaid by a collapsing polar vortex and very weak polar westerly flow. This I think is where we are heading into January.

EC 45 dayer shows marked drop in AO forecast second week of January to -2SD (mean), commencing in trend 26th December. Wonder what's driving that then ? Consistency with GLOSEA on stratospheric forecast. NAO forecasts not as negative, but worth pointing out that a Scandinavian block is not well represented within that index. FWIW, thinking would be for the NAO to move in concert with the AO here as pressure will likely be anomalously low in the Atlantic.

So, how are we doing here ? The tropical wave evolution identified has been slower and that shifts any comparative analysis back by 2-3 days. Suspicion that it may continue to be slower than forecasts currently advertise. That places GWO in a likely tilt towards phase 5 around 12th December For 12th December, GEFS vs original vs GWO composite phase 5: So a continued indication of anomalous ridge development to our NE for this time scale. Regular readers will understand the value of this approach, particularly in view of the volatile model output in the last few days and particularly back end of last week. Looking ahead, the tropical wave development has week 2 and possibly into week 3 as most favoured for the GWO to be in phases 5 and 6. For high amplitude phase 6: That suggests we're looking around a week's to five days worth of blocking ridge to our NE mid month, trending towards ridge transferring to our west and NW in the lead up to Christmas, troughing into Scandinavia. That I think gives us a first proper opportunity to pull some genuinely cold air into Europe, although the for Western Europe I suggest that this continues to mark a skirmishing phase of the winter which would be typical of similar ENSO events. The real interest lies in the final week of December into January given the stratospheric vortex will be under intense pressure and may well have collapsed. Synthesising the GWO attracted to a phase 6-7 evolution suggests a proper episode of Greenland blocking (and more identifiably from the composites distinctive -ve height anomaly in the North Atlantic) and impacts of the strat warming being overlaid with that pattern.

Displacement forecast for mid December, but expectation would be for full on warming event to occur towards the end of the month. MJO wave passage through the Indian Ocean will likely delivery the knock out punch.

FWIW, UK Met model showing a full on warming event for end of the year, as is CFS (which has continued to do so for the last few weeks). That's not wholly surprising given long lead thoughts of stratospheric behaviour and this plot which, if verified, would be signalling a stratospheric warming within 9 days: