Glacier Point

..and the value of this approach ? Q: how likely is GFS P t300 ? A: based on reasoned assumptions, likely.

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.

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.

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.

Wave amplitudes this cold season are very high. Wave 2 first ... Wave 1 now ... Very similar in evolution to 1986, wave 1 likely to be shorter duration this time round though as the Pacifc ridge becomes more of a trop feature in the coming 2 weeks. GEFS and EPS modelling continues to advertise a steadily weakening stratospheric vortex as we enter the core climatological vortex intensification period.

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.

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.

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.

Here's the importance of the wave 2 attack that is happening right now. That is opening the channel and the low frequency forcing that will manifest itself during the early and middle part of December will keep that (Scandinavian) channel open. So when the next phase of vortex disruption takes place, the response time will be a whole lot faster than the strat warming which was associated with the strong MJO last middle February.

GFS still very keen on developing a strong suppression signal for the tropics weeks 1 -2, and a very strong MJO / GWO phase 8 signal attached. Twin anticyclones developing in the east Indo / Asian region, drifting slowly east and an associated downstream anticyclone impact on mountain torque and upper level flows across the eastern half of Asia. Essentially an ingredient for suppressed levels of wave activity flux, and colder air now being forecast to develop across Asia at 10 hPa. Both GEFS and EPS in agreement on a stronger than normal North Pacific ridge in the Stratosphere to be in place in the meantime, and displacement continuing into middle part of December. This looks plausible.. With the transition of the next MJO forecast for middle of December (passage through east India Ocean key), latter part of December would place us on full alert for stratospheric warming potential. Split vortex type.

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.

In the longer term, the interest will be what sort of feedback we can reasonably assume from a large injection of cold air across western Eurasia. Expectation would be for an anomalous Siberian High to develop, possibly in tandem with a Scandinavian ridge. 06z and 00z GEFS and 00z EPS notably more pronounced than recent GFS ops with the warmth and degree of displacement at 10 hPa over the Pole.

Stratospheric disturbances were expected. Trop pattern looks to be happening a lot earlier than anticipated.

Convectively coupled tropical wave passage through the Indian Ocean ... check. Resultant spike in the Asian Jet and introduction of westerlies on the back equatorward momentum transport...GWO spikes phase 4 ... check. Hemispheric pattern becomes highly amplified as a result... check Stratosphere warming on the back of upward wave activity.. check. All the ingredients here being set out for what we are seeing in GEFS extended.

Trust me, that's not a tropospheric vortex that's about to set up shop for the duration. More the exact opposite, particularly when you look at what's going on in the stratosphere. Think lorries about to jacknife.

Key thing about 1986 for me is that the embryonic vortex took a substantive early season hit via 2 separate wave 2 events both tied into tropospheric patterns. This in effect destabilised the vortex allowing for its eventual demise via displacement event. This gave the tropospheric pattern breathing space during December to manifest what we would associated with a moderate end Nino event with regard to December - January evolution of a -NAO. Note two separate Wave 2 events during the autumn and early winter, the second resulted in a substantive deceleration of the upper level winds registered in both MERRA and NCEP data sets. 10 hPa zonal wind declined from that point onwards, culminating in the warming event later in January. So, as is nearly always the case, a combination of factors at play and no one smoking gun, but the choreography of the upper and lower parts of the atmosphere can be followed through the autumn and subsequent winter months.

Some useful tropospheric developments upcoming which are likely to have stratospheric impacts towards the end of November and more particularly into December. A strong convectively coupled tropical wave is being forecast to develop in the Indian Ocean in the next week and migrate eastwards during week 2 (MJO phase 2-3-4-5 transition). We are likely to see jet extension and subsequent retraction of the Asian and Pacific Jets leading to a increase in tendency in relative angular momentum and the GWO spiking phases 4 and 5. Mountain torque territiory for the Himalayas. At the same time, long wave pattern over eastern Europe maintains a favourable Wave 2 projection. This is likely to be enhanced over time as the amplification resulting from retraction of the Pacific Jet works its way downstream. Both tropical wave and resultant wave 2 look very similar to 1986 evolution. The AO looks to be tanking +ve in the week 2 timeframe, ideal for wave activity flux (again similarities with Nov 1986 there). So bottom line is ingredients for stratospheric vortex being significantly weakened during latter part of December.

Depends on your forecast - in line with hirlam, gfs and arpege. I think most here would agree met office not coming out of this well.

One last hurrah? Looks like this locale (east of Pershore) is about to get hit with the heaviest burst since the showers

Radar seems to have more of an east to west component, so ppm over London might be a player for some surprise additions tonight?

You’re more than welcome.

Thanks - not so much the roads but accumulations in the fields around Kirby Bedon

Quick request if I may: I'm due to survey a site south of Norwich on Monday. can anyone give me an idea of lying snow depths in the local area ? Thanks

This band now beginning to look organised, and interesting most reporting a sudden drop in temps along the northern edge. Forecasts earlier had this front stalling. Could get interesting..

You should be OK with a 4x4, and the intensity will be such that gritters / ploughs will be able to cope on major roads.