Glacier Point

Don't confuse being careful and precise with trigger mechanisms with downbeat ! Just one storm in the Indian Ocean (tropical storm four). Meantime 06z GFS continues with a highly amplified hemispheric long wave pattern. Oh, and a nice trough developing in the Western Pacific

I think broad scale we are still on track. Some oddities with the Indian Ocean right now, which are precluding the cast iron signal one would want. The phase 1 progression not in itself important, more it means we have to go back through phases 2-3, but noted the upper level signal may be ahead of this in the Maritimes. This conflicts with the Global Synoptic Dynamical Model which suggests that following a very positive frictional torque we should see positive mountain torque event with associated extension and retraction of the Asian Jet. Possible indication of this around the 8th-10th December, which is where our amplification would stem from. Two (very low level) M class flares in the last 24 hrs observed. Again not in itself important, but need to keep an eye on this to make sure it's not a sign of increased activity phase.

Worth noting that we have a tropical storm (FOUR) in the Indian Ocean forecast to track across southern India and westwards. This is confusing the MJO signal (sending it back to phase 1) and also likely behind GFS dropping angular momentum. A likely pointer to model meh at present. The broader analysis continues to suggest upper level velocity potential heading into the Maritimes and eastward, which would assist in reamplifying the pattern. GEFS continue to show around 7-8 members per run going for the expected mid Atlantic amplification. Just hope the sun doesn't get in on the act right now.

Extended eps beginning to show height rises over Greenland and a severely perturbed lower vortex with the main lobe centred Siberian side. Would be some proper cold beginning to establish in western Russia if that were to be near the mark. notably warm stratosphere on the Siberian side also.

This from Cohen's latest blog which I would agree with in respect of how the stratosphere - troposphere will interplay during the coming few weeks.. "If the WAFz does remain relatively quiet I believe that for the first time in a while the troposphere can settle into a more stable large-scale pattern, consistent with the PV disruption, for an extended period lasting several weeks. Key gross features include northerly flow out of the Arctic directed at Northern Europe, troughing over Siberia, ridging in the eastern North Pacific with downstream troughing over North America. It is harder to anticipate the details that will be important in determining the sensible weather in Western and Central Europe and the Eastern US. However, if the pattern is progressive (moving to the east) this will likely favor colder temperatures in the Eastern US relative to Europe while a retrograding (moving to the west) pattern will favor colder temperatures in Europe relative to the Eastern US. Also if the long-wave pattern starts to rotate then the WAFz could become more active and potentially lead to a significant rearranging of the large-scale wave pattern across the NH." https://www.aer.com/science-research/climate-weather/arctic-oscillation Beyond the culmination of the autumn blocking phase (should mature in the mid stratosphere during mid / early December), I suspect we will see a cooling in the upper and mid stratospheric layers as we loose favourable WAFz patterns. Zonal winds should increase at these levels. However, as I Cohen notes, that should pave the way for the downwelling of negative zonal winds currently in the mid and lower stratospheric layers. There is therefore likely to be disconnect between the lower stratosphere and mid and upper layers. Some very good examples of this.. There is a almost a tangible tightening of the troposheric blocking signal as the stratospheric blocking signal matures and fades. So analysis of stratospheric temperatures and zonal winds may not be wholly useful in the coming weeks - it's more about what the troposphere does in response to downwelling and other drivers.

Actually, record of EC weeklies has been broadly trended +ve height anomaly to the west / north-west for first week of Dec and an inferred -ve temperature signal. That looks pretty much what we're going to land.

I have 7s for Birmingham and Norwich (available with Hills). But to bring this back, it's always useful when a model aligns itself with a broad signal you're reading from the atmospheric signals.

EC week 4, let's just say, what price a white christmas ? Genuine -ve temp signal there at that range is stark. Week 3 continues the theme as discussed, cross polar ridging.

....and a big intrusion if the N Pacific ridge into the Arctic towards the mid Atlantic ridge. Strongly negative AO latter stages of the EPS

Models beginning to lock onto a below / well below return for the first week of winter proper... Thereafter, I think a window of opportunity arising for something more substantive in terms of cold for mid month onwards. Both GEFS, GFS op and EPS sniffing a shift in the stratospheric vortex related to the wave 2 precursor observed final third of November and some decent hints of wave 1 picking up briefly in the coming few days. That squeezes an already weak vortex and moves the axis of the stratospheric trough towards Alaska. This exposes the pattern to vortex splitting and Arctic ridge development. At the same time, we have a substantive North Pacific ridge and a North Atlantic profile which is ripe for amplification. Tropical forcing signals are becoming much more supportive of the type of pattern observed in late September and October (ie. amplified) and risk of a spike in tendency on angular momentum are increasing as a result (again genuine amplified signal). Be worth keeping an eye on how much upper cold can be pulled out to the NE.

Some low level westerlies pushing through the West Indian Ocean, but upper level easterlies still dominant, so no coherent wave signature just yet but forecasts depict some eastward propagation and wouldn't be surprised to see convective anomalies begin to strengthen over Indonesia. Convective anomalies further east are Kelvin wave related, with main MJO component centred over East Indian Ocean. GFS forecast for eastward propagation, and takes us to a similar point to mid September. Prospects for a sustained high angular momentum phase looming.

So GFS comes up with a split.. Timing is towards the end of the envelope, but still within the traceable period for tropospheric ridging in mid to late November. Could well be worth keeping a eye on this as we have model agreement on the North Atlantic stratospheric ridge. Any Wave 1 activity on the Pacific side will squeeze and pinch the vortex. In the next two to three days we get a deep upper low passing through the Aleutians (probably the last for some time).... Note also the plot annotated above. That's primarily based on Indo-Pacific tropical forcing, which looks increasingly likely over the coming weeks.

Broad scale, the GEFS is more likely to be the way December pans out. There may be timing issues here though as the ducks are not quite yet lined up. I'm looking for tropical convective wave to develop east of Indonesia and an attendant +ve mountain torque to develop as the necessary triggers to a truly cold pattern to embed. That's still a little way off just yet, so the choreography of the 06z GFS may be a little fast. My position 1st week of December would be for the UK in a mean trough with a broadly north of west flow, temps below average. The more extreme version advertised by the GFS 06z is plausible given the background drivers - extremely weak polar westerlies, massive disconnect between angular momentum and ENSO status, so I would not be keen to discount it, rather have it on my side as a second choice (particularly if you've already hedged this way some time ago).

The GEFS and EPS could not be more different for week Friday for NW Europe. One says deep trough, the other moderate ridge.

worth noting we now have both North American ops singing the same tune here..

Seems to me the reasons not to go cold are being removed one by one... Compare Tuesday's week 2 forecast for upper level velocity potential in the tropics and today's.. You'll get some idea of the pace of change here - not fast, but the steady trend is to shift the convective wave associated with this towards the Maritime continent. Trend in GFS modelling a reflection of this, mid Atlantic ridge and developing North Pacific ridge. Notably EC modelling has been much slower with this wave, and more broadly convective wave passage through the Maritimes shows lower skill. So expect poor model reliability to get worse. GEFS and GEM modelling of the stratosphere is different to EPS, although broad agreement on a Canadian Warming and slight rotation of the stratospheric ridge on an anticlockwise rotation towards Greenland. GFS is a good representation of this, although on the weak side for this run, but the prospects of a squeeze on the upper vortex from both sides of the Pole from Atlantic and Pacific sides looks like a long term trend here. Where that takes us.. should see increasing propensity for tropospheric ridge across the polar region, and increasing meridionality in the Atlantic sector. Latest EPS very unconvincing in its anomaly fields being very weak signature across its mean with the exception of the North Pacific ridge.

Fascinating circumstances and prospects right now. First week of December coming into view, and, with not all the ducks properly aligned (just yet), we are looking at below average temps for much of UK, more so Scotland. The convective anomaly centred over the Indian Ocean is showing eastward propagation. Slow, but steady. As soon as that hits the Maritimes, we should see a more amplified hemispheric pattern develop centred on a poleward displaced North Pacific ridge. We are already beginning to see this in the extended range modelling. The mid Atlantic ridge also likely to gain traction under this regime. Angular momentum will also spike once the convective wave pushes into the West Pacific with a +ve mountain torque, with likely hemispheric reinforcement of Rossby Wave pattern. The stratosphere also holding deep interest. That ridge associated with the initial warming phase is starting to look like it will drift around the Arctic basin, becoming Atlantic based week 2-3. With another warming over Siberia, the Aleutian stratospheric ridge likely to be in play over the top of a tropospheric ridge - could well be a sizeable height anomaly developing in the North Pacific through Alaska during that time. So that means we may well have two ridges peprendicular to the Pole - ripe conditions to support tropospheric cross polar ridges. Keep an eye on the GFS and GEFS as they are just starting to advertise early stages of this.

NAEFS should be quite bullish on the sharpening Euro trough week 2, as the GEM ensembles (as were the 00z) quite a bit keener on persisting +ve heights further east over Greenland. Continued painful slow evolution in the convective wave over the Indian Ocean, but 12z outputs seem to be latching onto it judging by the appearance of a well defined North Pacific ridge. Just also a whiff from the GEFS of twin vortex development either side of the pole with a ridge between, encouraging the Pacific ridge poleward. Effectively mimicking a split vortex.

Yes, it is good. Confirms the view that trop wave will continue to push east towards the maritimes. That suggests the mean ridge to our west will regain strength weeks 3 and 4, and there will be a ramification after this as angular momentum tendency spikes once more. Definitely looking west for the first half of December (more appropriately north of west) for the ridge to sharpen the trough to our east.

Indeed, notable cluster of members on the 850s heading 'south' within a cold trough. EC46 was -AO/-NAO throughout, slight trend to neutral NAO just before Christmas but not sustained.

Ec32 maintains weakish +ve height anomaly to NW week 3, becoming stronger week 4. Scandinavian trough signal throughout weeks 3 and 4. Not so different from that composite I posted earlier on. AO negative throughout and noted downward trend on 12z EPS

MJO heading back to the Western Hemisphere ? Intraseasonal low level forcing also in the Maritime Continent (100E) also making a comeback ? Which would favour mid Atlantic ridge scenario.

Just outside my 'embargo' period I'm afraid.

Another run, another ensemble mean suggesting quite an aggressive +ve height anomaly developing over Hudson Bay in the week 2 period with a switch in the flow to the southern arm of the jet over the US. Western Europe left under a weakish upper trough signal (temps marginally below average). Two things to ponder here. What's to stop the westward movement of the upper ridge over the Canadian Arctic ? There may be some stratospheric-tropospheric coupling going on, but over time we would normally expect such features to shift their position westwards. That allows for the controlling influence of the Canadian ridge to be negated downstream. With the pattern of tropical convective rainfall beginning to creep back towards the Maritime Continent (non-MJO component), this suggest mid Atlantic / possibly Barents Sea / Scandinavia area modest ridge development. These won't be too obvious on week 2 means or even spreads, but small scale mid to high latitude ridges developing within a flow which is well displaced of its norm would be interesting. Within this context, stratospheric warming impacts will be around the week 3 / week 4 periods. The tell tale signature for vortex splits is very cold 850 values pouring into NW Russia and Northern Scandinavia. Helsinki, Moscow and Murmansk the locations to keep an eye on for extended range ensembles. Some twitching of ensembles for these locations staring to occur but nothing stark just yet. Intriguing times.

Now that would be interesting - the mjo showing an affinity with the Western Hemisphere. Would fit the idea of some choreography between a strat warming event and how that transposed itself in the mid and high latitudes. Think the mjo will trend more neutral though, and intraseasonal forcing over the maritime continent more to the fore.