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Showing content with the highest reputation since 15/11/18 in Posts

  1. 68 points
    Evening Based on the pretty successful thread around the current seasons NH patterns I thought I would lay out the thoughts for Winter 2018. Headlines - Following on from Autumn > Winter 18 Globally is set to smash records- From record warmth to record cold, record rainfall & record drought... This will be driven by weak westerlies & a possible SSW in mid December- Teleconnection notables **could** be * Lowest negative AO on record ( or at least a challenger ) * Lowest & or most positive POL on record ( this metric can be good for us in either positive or negative mode, however its the extremes of pressure that support the -AO ) * Most negative 100 or 10HPA zonal wind on record in Merra2 data at any point during the Winter ( most likely earlier rather than late ) UK wise - The overall theme is a very high probability of a below average winter with the sub 3c breeched twice - ( maybe 3 times ) Winter NH patterns For a winter forecast this last 10 or so years a traditional method by most has been adopted - taking the state of the Teleconnective background signals & matching them to analogue years in the 1900s - & I still believe there is some merit in this especially when we are in very high NINO index years as this can be the overuling global driver over & above any favourable ones - The last 5 years or so have seen attempts to create a winter forecast by morphing analogue years for the stratosphere - again with some limited success- However there is one fundemental issue with using these analogues which is the assumption that should the same metrics repeat themselves then the tropospheric responses would all align & the H500 height anomalies would be at least very similar across the NH. Well now, as we get better understanding of how climate change is impacting the weather (most notably the ice & arctic heat ) then all these analogues potentially become redundant due to the troposphere becoming more & more independent in relation to not allowing the stratosphere to downwell - ( otherwise known as being 'decoupled' ) The main driver of this is the surplus warmth left over in the arctic at the end of the summer & Autumn as well as the record breaking ice loss along the peripheral ice edges - Not only this, as the mid lattitudes heat up we have grown larger more 'intense' areas of high pressure transporting yet more heat upwards into the pole- ( NB even now this week in November we see record High pressure developing towards Scandi pumping more warmth upwards ) The feedback then becomes self perpetuating- I would also like to add to this the 2018 'spin' that not only does this year have a massive heat anomaly,but perhaps ( IMHO ) a dipole of heat / cold across the areas with early ice build up V no early ice What you then allow is even more inconsistency with heat creating vast gradients across the pole allowing for yet more blocking - & because of the dipole the balance of heat then allows potential displacement of the tropospheric vortex - Its no wonder then that we see the amount of wave 1 & wave 2 blocking attacking the early vortex & the inability of the stratosphere to couple downwards as the resistence upwards is balancing it out - What you get as a net of all this is no tropospheric analogues because there isnt a teleconnection that quantifies Arctic Amplification ( AA ) & there are no stratospheric analogues because if the strat cant downwell then its overall strength is much less impactful on the troposphere - - So as mentioned then- Autumn has been one of blocking highs - even when the stratospheric jet has increased along its seasonal pathway the troposphere has refused to couple with it- ive highlighted this is due to the anomalous vertical heat flux & perpetual bombardment of upward waves thats served to offset the downward propergation of the strat leaving the polar cap sitting in equilibrium Seen here ( Source AER ) Also shown is the continual warmth in waves pushing upwards from the troposphere - & now potentially down from the strat- Remember the upper vortex is *Stronger* than average this year so while its vulnerable to attack - early displacement to russia * Isnt * the same as a SSW split - for this we need to see a reversal of the zonal wind @60N/10HPA * Classic winters like 1962 had early warming over the canadian side which took the vortex out for the entire winter then went on to allow for blocking to remain in situ for long periods of time- I believe this winters signal of a tropospheric decoupling could allow for a similar scenario especially if the SSW underpins it - QBO support - The initial concern is that a downwelling WQBO would omit us from the chances of a SSW however we are still in the transition phase where 30MB moves to positive but the 50MB lower down holds negative- The impacts of this are that the QBO will not act as a blocker early on the potential SSW this winter - Ive uploaded the image & I should have shortened the timeline but hey ho! h Possible Strat warming ~ Mid December... The strat as we know has been under attack this winter & the lower vortex especially has recently born the brunt of any deceleration - dropping to around 5M/S where as higher up the 10HPA wind has remained less impacted The day 16 disks which when the next wave of vertical flux peak starts to show a canadian warming & relocation of the vortex - So worst case scenario a dipsplaced vortex towards Russia, best case is a split & even less eastward zonal wind speed !! ** Key date proposed for major displacement / poss warming Dec 15th but remember because of the decoupling this may only serve to reinforce the blocking thats already there - ** DJF UK CET - Viewpoint & Anomaly plot The seasonal winter CET stands from Hadley @ 4.13C broken down into D 4.7 J 3.9 F 3.8 Statistically across the last 100 years sub 0c months are - 8/300 which is about 1 every 40 years ~ 2.5% chance. sub 1c months are 17/300 which is about 1 in every 18 years ~ 6% chance sub 2c months are 29/300 which is about 1 in every 10 years ~ 10% chance. So we can see the numbers are low- in terms of our chances I try to quantify this winters numbers by saying that a sub 2c is as high as 50/50at this stage ! & sub 1c maybe 25% - Sub 0c maybe 10%... Overall projected winter CET SUB 3c ~ 2-2.5c Overall pressure anomaly locations for Winter 18- These may wax & wain however general locations feel about right- Storm track is in green - Cold winter for central states as well as possibly the east depending on whether the blocking is more East based towards a +PNA or more west based -EPO ( the further west you get the -EPO you do encourage a ridge on the SE coast ) UK sits on the NW side of the low anomaly in Europe - so in that respect continental air is sent west in our direction- which will drive snow events- Europe as a whole cold & below ave - especially central & Western, however places like Greece / Turkey could benefit from warmth up from the south - It is my belief that confidence in the projected patterns for this winter are about as high as they can be in terms of potential blocking- especially in the locations we need it - what you will see from the models is perpetual ridging to the pole from every angle across the globe ( -EPO / -NAO / -POL ) If the vortex remains uncoupled then the speed of the 100HPA vortex is more crucial than the 10HPA one higher up- This will continue into December where the idea of a 'front loaded but possibly fully loaded winter comes from' - The nature of reduced westeries also impacts the UK around rainfall- Mean averages especially for the NW will be lower however what you lose over a month in terms of loss of westerly driven wind & rain events may well be offset by slow moving PPN events ( The same as Autumn ) So there you have it - 2018/19 Global extremes of weather & for the UK higher probabilities of exceptional weather which for once is more in favour of exceptional cold instead of warmth.. S
  2. 55 points
    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.
  3. 54 points
    So I mentioned earlier on about very high angular momentum levels, which would normally be associated with a strongish El Nino. GWO in high amplitude phase 6 orbit, expected to move towards phase 7 as the MJO component moves through the West Indian Ocean this week. There is a persistent +ve momentum signal across the tropics and sub-tropics which is holding up this unusually high angular momentum signal. Not much scope for a drastic drop in inertia although frictional torques are as expected strongly negative. When the MJO signal swings through the East Indian and Maritime Continent, angular momentum tendency will rise, driving a phase 5 projection in the GWO around 10th December. Composite: This doesn't fully capture the blocking signal over the Eastern Arctic, so some manual adjustment on this required, but the key messages - Alaskan trough, +ve height anomalies to the NE. GEFS for that time alongside the composite: Again, don't get hung upon on exact placement, the broad gist is that at that range, reasonable confidence that the GEFS mean is about right. The strong presumption from there would be for a phase 5-6-7-6 type evolution (herein the beauty of a conceptual model): There is some manual adjustment necessary to these to inflate +ve height anomalies to the NE and reduce +ve height anomalies to the west of Greenland, but some westward pull of the man trough over the North Pacific is the key thing here. That allows any blocking to our NE to manifest itself, and would consolidate a monthly expectation for +ve height anomalies across Scandinavia. Either way, a challenge to any view of returning zonality at any point during December.
  4. 53 points
    Worth pulling this post out from 11 days ago - because it has proved overoptimistic though not totally incorrect in perceived development. The pattern has in fact "relaxed" a great deal - calculated tendency of AAM has started a sharp fall Why? The simple answer is that overall Glaam hit such a height (+3SD) that there had to be a reaction....and therefore the downward phase of the to and fro nature of the momentum budget has been strong. I was guilty of not, perhaps, placing enough emphasis on that +3SD peak of the global momentum budget, largely because I didn't expect it to get quite so high. But - hold on. This is not a post of resignation (from an openly slightly biased snow lover...) - quite the opposite. The pattern was always going to relax at this point, and we have ended up with a trough that appears to have blown away the blocking pattern and led to gloom on here. But remember that these cycles are predictable - and while the current phase has proved to offer more atlantic south westerlies than were hoped for the medium term prognosis is still very good. Note the cyclical nature of momentum budgets...and the certainty that an upwards trend will return before too long. Key to this will be the MJO. We continue to see forecasts of a strong MJO wave and without doubt the signal for blocking to return under a Nino base state will return: Meanwhile the very favourable positioning of blocks through November is having the expected impact on the vortex as we enter December - this is a strong wave 1 attack and is sustained for many days What do these signals therefore suggest? Frictional and then mountain torques will reengage in the next 5 - 10 days and this will initiate a surge in momentum as the pacific jet extends, and poleward wave energy forces a much more meridional pattern to the north atlantic. With increased westerlies over the tropics, sub tropical ridges will be undermined and the signal for falling pressure over the azores and/or Europe will increase. We will see ridging return. Given the pattern at present I will hold to the timetable advertised previously - mid December for the true start to winter, and if we can get engagement with the pool of very cold air that is concentrating over Russia then some snowy fun for some is certainly a possibility. The extent to which the vortex is impacted will have a bearing on where we then go forward. I don't think we have a serious chance of a full reversal in December - but perhaps we are setting the vortex up for a lethal blow in early to mid January. If the blocking pattern that returns mid month sets a scandy/russian ridge up once again, then this will initiate the second phase of necessary warm air attacks on the vortex, with a lag effect impact into January. So - cold in mid December, and the potential for winter to accelerate into January. There will always be ebb and flow within this broad scale pattern - but right now we are still on course for some great model watching kicking off early next week as NWP picks up on the mid December signal. Optimism still in place.
  5. 52 points
    One further observation. Both EPS and GEFS very keen to dislodge the Siberian lobe of PV into the Pacific sector and nudge the Canadian lobe further west. Net result - the hemispheric wave number becomes much reduced and aggregated. That's a massive signal for high pressure development in the high latitudes across the eastern side of the hemisphere. Not a lot of change is going to happen to that wave pattern so we are lining ourselves up for the winter pattern here. Slowly slowly catchy monkey.
  6. 49 points
    We really couldn't ask to be in a better place at the beginning of winter than where we are now. Incredible Scandi blocking/moving to Greenland with continued warm pulses of WAA into the Arctic making things favourable for more blocking and more WAA = it becomes a self sustained pattern. MJO moving into phase 8 which for December is suggestive of Northern blocking and then when you take into account the increased chance of blocking with low solar activity/low sea ice and combine that with a weak ENSO not overriding any other signals, we've basically hit the jackpot. I see no real Atlantic threat going forward, the atmosphere is primed for blocking to continue probably well into December, we then have the possibility of an SSW which could cause a split/displacement in the Vortex and bam, you've got December/January, 2 months of potential almost continuous below average temps/snow. Blocking doesn't = cold of course so we need to be cautious there, but things are looking fantastic. Cold this week, perhaps slightly less cold in the South by the weekend but with models agreeing on the slider/undercut of the low, don't be surprised to see further upgrades as things trickle from FI into the reliable. This isn't currently about "will it snow here, there or anywhere" for me, it's about the bigger pattern and going forward it looks fantastic!
  7. 46 points
    If we take the ECM at +168 two days ago And compare it with the ECM +120 from tonight You can clearly see that as we’ve got closer to reality, the model has; 1) Underestimated heights around Iceland area 2) Has started to disrupt the trough to our west more, it is more negatively tilted and a little more elongated. 3) heights in Europe lower than previously modelled If that trend continues in the next two days and the same for the subsequent frames after this, perhaps Day 10 will look a lot different come T+0 than it does now. This is the same for GFS too, not just ECM. Uncertain outlook but certainly upgrades in the mid term from the models. UKMO pretty solid. Going to wait a couple of days at least yet.
  8. 46 points
    Aside of the significant and near record kicking the stratospheric vortex will be receiving for this part of the early cold season (c/o wave 2), interesting to note that the tropics are choreographic themselves nicely in the -NAO context. The week 2 forecast depicts a strong suppression signal in the Indian Ocean and, consistent with El Nino events (of similar magnitude), a convective signal across much of the central Pacific basin. That's consistent with top 10 -NAO Decembers: Even more so when +10 Decembers are subtracted: Decembers with similar other drivers for context.. We are at the point where, if models are near correct, feedback loops begin to be established - cooling of the sub-tropical Atlantic and warming at higher latitudes, development of strong Siberian ridge and Aleutian low, stratospheric feedback into January and February.
  9. 44 points
    Through the mist people should be starting to put the jigsaw together- Scandi / Iceland / Scandi oscillations- becoming ever colder... We are set for Winter - Winter forecast out later tonight....
  10. 43 points
    The models have retracted away from the warm NW push of southerly air to be replaced by trough disruption & energy moving ESE A classic case of models moving away bias into reliable modelling--... Its like for once the models have listened to what the forecasters on here have been saying.....
  11. 42 points
    Hi all, I'm not seeing anything wintry as such from the latest models, par for the course really!...still, at least we have brexit to look forward to..or not as the case may be!..not sure I will be posting much in the weeks ahead but thanks to those who messaged me during my long absence..much appreciated.
  12. 42 points
    Nino is here, and has been for some time since October. We are playing real time Nino climatology here, which suggests December may be a touch too soon on cold backing sufficiently far west. That said, +ve height anomalies to our east and NE and low to the west are exactly what we should expect. GEFS mean 12z starting to rebuild a ridge across NW Europe as the MJO quickly cycles through the Indian Ocean. CFS stratospheric modelling continues to look 'interesting' for last week of December, which would have some merit given recent musings.
  13. 40 points
    Amazes me that in a thread titled 'hunt for cold' some are hellbent on writing off colder weather for several weeks because the Atlantic gains control and GFS has been showing mobile / mild southwesterlies deep into FI. We all know a pattern change can occur within a week or so and takes the models a week until they start to cotton on to the changes sometimes. I don't think the usual 'mobile/zonal takes weeks to budge' applies atm given state of the upper patterns to start winter. Plenty of clues to the triggers for this potential change already been posted by myself and others last few days, so lets not write off all of December for starters! Back to the 12z, day 10 onward shaping up nicely on the GFS operational, the ridge building to the N/NE kind of follows what I might expect for the response to the MJO heading into P7-8 albeit expecting this to occur a little later than advertised.
  14. 40 points
    Guys I concur with @bluearmy Lots to look forward to as we enter December - Also the zonal winds dropping throuhh the floor- Im off to New york - So offline till Sat next week Have fun !
  15. 40 points
    Morning all, snow arrived on time, just as predicted by the high spatial resolution snow model from 48 hour out. Amazing accuracy produced, even predicted the start time and snow depth to the hour and cm with my location 1km x 1km. As usual its result was spot on, 10pm start time Sunday evening and snow depth of 5 cm 6am Monday morning. Some of its data is fed from the longer range single powerful model our experts use over here. This model has been rock solid in showing a strong Arctic ridge towards the last week of this month with low pressure over Southern Scandinavia to produce a more meridional flow. Yesterday our team indicated the low would be near Denmark at day 10 and with any Atlantic milder air incursion only making limited progress to remove the cold block developing to the NE ( if any ) So that was a optimistic update for colder forecast. Will get an update from them in about 2 hours time to see how things are looking and will ask for specifics with regards to UK interests. Meanwhile we now have the excellent service of the snow fine mesh model now providing us with hourly information here in resort. Try and get some pictures of the snow this morning to wet your appetite . C
  16. 38 points
    T324 gefs mean across the polar field .... I doubt this place will be so quiet in a weeks time ...
  17. 38 points
    An interesting viewpoint. Here is another. GLAAM remains high - in fact it is very high and the last AAM cycle really did not dip very far - it is back on the up and let's not forget the MJO forecast I posted a day or so ago What is interesting is that the current uptick in AAM once again has happened without input from frictional/mountain torque, as these normally precede such activity. Meanwhile we are in the middle of modest wave 2 action on the vortex, with a ramping up of subsequent wave 1 in the forecast With these background forcings I cant see any return to zonal for any period of time. We might get a trough stuck in a position that causes some grief to cold lovers, but sweeping west to south westerlies in the true "zonal" fashion look long odds at the moment. I'm not sure that NWP will be resolving this data particularly well, even assuming the vortex keeps its shape and coherence. I dont have the skill of GP or Chio on wider implications, and if Tamara is still surfing these pages then she has far greater experience in analysing GSDM patterns than I.....but optimism regarding the broad pattern (if you want snow chances that is) looks pretty well founded to me at the moment. It's still early days.... but confidence in a cold spell around mid December continues to grow....and the current shape of things would suggest that this could be only the beginning of a recurrent pattern.
  18. 38 points
  19. 38 points
  20. 37 points
    I'll do an update with the 500 mb anomaly charts later today, not all gloom and doom for you coldies though!
  21. 37 points
    Same model (with a few tweaks), same data, same run. Two different outcomes: The inherent dangers of using operationals at that range laid bare. What we do glean, and more particularly from the means, is that there is a decent signal for height rises to the north-east. That's broadly in line with thinking. Progression of the MJO signal through the Indian Ocean perhaps a touch slower than EC and GEM forecasts, GFS maybe a bit more on the money. Broadscale hemispheric and stratospheric developments on the back of this as before. One thought to add, angular momentum is sky high (+2.5SD). That's well in advance of the oceanic signal. That limits the scope for large swings in tendency, reducing the scope for amplification signals. That may well mean we head to a destination via a slower, but more steady and surer evolution. That would suggest block to the NE is a long time player which will progressively influence the circulation for our sector. It also means that the atmospheric 'cupboard' is being well stocked to deliver a period of high angular momentum but negative tendency (Phases 7 and 8 of the GWO) for the core of the winter.
  22. 37 points
    Compare days 1-5 GEFS z500 mean height anomaly: With days 11-15: Substantive shift with mean trough in the North Pacific being replaced with a mean ridge. That ties in with this: MJO fairly rapidly moving through the Western Hemisphere and into the Indian Ocean. That's a Nina like cycle within a broadscale Nino base pattern. Usually this type of juxtaposition is good from a -NAO evolution. Next evolution would be for the MJO / convective signal emerging across the Maritime Continent and associated tropospheric impacts to the stratosphere, which continue to look like mid to late December, with perhaps earlier evolution of a -NAO through more conventional (MJO/GWO) feedbacks.
  23. 36 points
    Worth noting that in the day 4 - 10 timeline the sub-tropical jet will get very energised over the Southern US. That ties in nicely with an El Nino tropical signal, enhanced through passage of a tropical wave. Without any strong blocking feature to the north / far NE, and no scrambling of the upper level polar westerlies across the North Atlantic, we will be exposed to a poleward returning 200 hPa flow. That's not expected to last much beyond day 10, so chances are that an eddy will form in our locale setting up a ridge over NW Europe and trough becomes more slow moving to our west. Next phase of ridge development over NW Europe very much on the cards during the 10-21 day period. Stress that doesn't necessarily mean instant cold, and we could still get trapped the wrong side of a ridge for a time.
  24. 36 points
    Perhaps this thread will be better considering you seem hell bent on only ever posting the worst looking charts in terms of UK cold NAVGEM is perhaps the worst performing model currently on the planet so not entirely worried about what its showing. Given the UKMO's solid consistency, the GFS/GEFS moving towards the undercut option and the ECM also showing the undercut, GFS(P) is currently largely out on it's own for the time being. I wish we could see it's ensembles, could be the warmest one out of the bunch.
  25. 35 points
    This one is tight, that one is wrong .....etc etc etc none of them are ever right ....... that’s why we do this ! There can be little doubt that ec op has led the way on any initial attempt to bring deep cold in failing. That’s to do it’s the way it has never been happy to leave the upper low to just dissipate to our advantage and was always progressive in taking it further west and phasing with the trough headed in from the sw. has it been totally correct? No it hasn’t and it still isn’t now. Has it been more correct in respect of what happens to the weather in nw Europe? Yes it has. Anyway, the clusters will reveal later that there is definitely one which brings winter to w Europe days 9/10. How big is unknown but I haven’t noticed it thus far on the model getting this far west. All is not lost ....... still two weeks of autumn left ............ the overall mean pattern establishing for the end of vobember and into winter is one which has been trailed on the seasonals the Atlantic storm track is deflected into w Europe by northern/ n Atlantic blocking in this scenario, the uk will often lie in the broad battleground area ...... that looks like it’s coming for later this month but whether we end with Scotland or Hampshire being closer to the dividing line is the question. Any failure of low heights to our south will affect things and we know how fickle they can be ...... despite the current general malaise, ( which is totally misplaced) you know you will all be back later ............ that’s why your reading this now ................
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