Glacier Point

Here you go.. http://centaureaweather.com/weather-news/some-thoughts-on-the-european-winter/ Standard La Nina climatology may not apply here.

Just getting the inclining that the Pacific pattern is edging westwards (mjo would be a proxy here - heading towards phase 7). GEFS members should be well worth a view wrt mid Atlantic and Greenland areas as the North American trough is pulled slightly further west. Hope to get a blog entry on thoughts up asap.

a little premature with those Scandinavian blocks Nick given the likely influence of the mid Atlantic ridge ? Deeper into December and January, hmm, different story.

Eps seems to have refound the mid Atlantic ridge, which I think is right given the background Nina state. Temps showing a negative trend 6th - 8th onwards, but nothing special which again I think is about right given the deep trough over the US. The thing that really takes the eye is the building ridge over the eastern Arctic. A key player long term? December heading for a very negative AO value. Is this standard Nina climo, soon to be reversed in Jan, or symptomatic of a Nina beginning to morph into something else?

The development of a mean anomalous ridge to our west is very consistent with the strong and coherent La Nina signal. With the modelling agreeing on the development of a strong Alaskan ridge in the coming 10 days, the resultant North American trough will put pressure on this mid Atlantic ridge, so unless there's much undercutting, which doesn't look likely, dry and cold with milder phases from the NW looks best call at the moment with a more muted but still discernible ridge to our west, trough to east. Longer term, La Nina climatology would strongly favour a ridge development to our NE during January. That, allied to the development of a strong +ve height anomaly over the Arctic in the next 10-14 days holds some interest for coldies. If the ridge over the Arctic gains sufficient strength, then some clockwise motion in the Arctic circulation would be expected, bringing the ridge closer to our NE (also over anomalously warm surfaces in that locale likely to aid higher surface pressures). So, getting a cold trough bedded in over Scandinavia and Western Russia will be key to anything that develops in January.

Both EPS and GEFS 00z and 06z near identical at T360 w/r/t 10hPa temps and heights, suggesting zonal winds becoming very light and close to technical warming. Warming signal at 50hPa working its way up.

And the importance of the Siberian High - Aleutian Low combo,

GEFS going close to a reversal at 10hPa at the extended, although it has to be stated EC EPS not there yet. This year's vortex refuses to cool in its core, the cooling events always around the periphery..so temperature plots are not going to be a reliable guide. The H5 field days 5 through 10 are ideally configured for a mountain torque event over Asia. MSLP days 10 onwards really beefs up the Siberian High, displacing flow southwards over central / southern Asia. Both argue for increased upwards wave activity flux, and being reflected in the GEFS if not a little premature. Getting some cold air to our east is going to be critical for January's prospects given the now strong likelihood of + ve pressure anomalies to our NE related to a fully coupled atmospheric - oceanic Nina event. 2006 being a superb example of how things might evolve. That boundary between cold and warm is going to be very critical.

Such a crucial next week for so many being the start of the school holidays. If you're looking at the GFS and parallel GFS ops, looking good for the south and SW, probably a slower start with the UKMO. GEM, hmm, that might take a little longer but the GFS banging the drum for a quicker return to summer.

Are we starting to see some stabilising in the models ? I suspect modelling has had one of its 'once every six months' type failures, reacting to false signals (or perhaps more accurately over-reacting). Funny how we tend to get these at the peak of winter and summer seasons. GFS angular momentum plots highlight the recent upsurge in tendency in relative angular momentum, followed by a rapid loss of inertia (phase 4 to phase 8 transition). NWP almost certainly saw the phase 4 evolution and projected more of the same resulting in an inflated Azores ridge, and then overcompensated when this started looking like a phase 8 transition, with too much of a loss of angular momentum being forecast. So perhaps a period of us moving back to the pattern observed through much of June, but with more influence of ridging over Scandinavia and a broadly +AO / +NAO regime.

Not surprised we're getting either N/NW or southerly options from the models. All modelling agrees in a displaced vortex over the Canadian Maritime. The issue here is how longwave troughs passing through this axis interact. If the get pinched off and closed upper lows form off Newfoundland, we get the NW option where the sub-tropical ridge (which is very displaced poleward right now) gets pulled NW. If the troughs remain broadly intact and sweep NE through the Atlantic, the Azores sits very close to the UK or even displaces just to our east. The sign and scale of the AO would be a good proxy here. More positive, the more eastward the sub-tropical ridge. More negative, the further west the sub-tropical ridge and cooler more unsettled for NW Europe. I'm playing the percentages here. Atlantic predisposition to troughing allied to a +AO (polar easterlies are the weakest since 1981) translate to better conditions for the UK. Also worth re-iterating that there remain issues for modelling in the longer term across the Atlantic just to our west. The flow is complicated and ripe for development of lows driving SE through ridges. All of which gives you better chances of ridges getting thrown up over or just east of the UK, particularly the closer we get to August (background signals become more cohesive towards this for August as opposed to July). Longer term modelling towards month's end does retreat the ridge more towards the Atlantic, which I think is not unreasonable given we are likely to wax and wane with this over a 10 day oscillation - not withstanding my comment above about small scale disturbances driving us towards warmer, drier solutions in the short range. So what follows a wane at months end is a waxing again, so Azores ridge + driving low to the SE part 2......

So, how is the Southern Hemisphere responding to the drivers that will in all probability be in place come the end of the Autumn ? Hmm, look away now stuff for cold prospects early doors.

Indeed Rob. With the broadscale drivers all well set, next week's little low cutting SE will be all important in determining how much heat gets into Europe (after all, this isn't without precedent having occurred twice already this season). This will become increasingly difficult to shift over the next 6 to 8 weeks (note how 06z GFS retains heat over a large swath of Continental Europe). The tantalising prospect of low heights over Greenland engaging with Atlantic troughs to sweep up wave after wave of warm air across Europe. A hybrid 1995/2003 pattern ?

Next 10 days looking a little mixed, but not washout territory and I have to question just how unsettled it will be, principally a belief that models aren't handling the trough to the NE Atlantic at all well. Thereafter, it does get interesting. My expectation for August has been for pressure rising (in tandem with that for Europe) over Scandinavia. That places most of Europe on the eastern flank of a trough shifted west in the Atlantic. So, we have EC monthly last two runs going for this and CFS also advertising the pattern. EC EPS this morning showing a notable upward trend in T850 for a large proportion of members from day 10 onwards. Operational GFS for the last few runs also beginning to drop hints with pattern changes and rising pressure south and north-east of us.

06z GFS being an excellent case in point. Little disturbances in the flow dropping to our west driving warmer air poleward. With the +AO signal, a flat Azores ridge is not straightforward and subject to the type of variation we are seeing at T0, T96 and possible again at day 10.

Just worth bearing in mind how the models have been handling events just recently... ECM mean 500 hPa height anomaly day 10 ten days ago: And the initialised version: Broadly OK across the NH but a little off in the NE Pacific, but quite a way off for our corner. Way too flat with the Azores ridge and didn't capture the split flow in the NE Atlantic. A relaxation in angular momentum looks about right given tropical developments, but no where near GFS projection. I'd go closer to seasonal norm, possibly a touch below for AAM state. Expect once again the trough in our locale not to be modelled correctly in the day 10+ timeframe.

Latest 500mb height anomaly shows no low heights over Greenland. It's not until day 7 we get proper widespread -ve height anomalies across this region..

Quite like the output at present. Next week looking very pleasant, particularly the further south and we are starting to get good hints of lowering heights over Greenland and the sub-tropical ridge beginning to exert itself in a poleward displaced position north of the Azores. Next week also advertising the low cutting south-east through Iberia and Southern France, something long range modelling just didn't show. With a strengthened Azores ridge and low heights over Greenland, bear in mind that the models will probably not pick up disturbances in the upper level flow interacting with the subseasonal signal for troughing in the North Atlantic. The GFS also probably going to have too much bias for low angular momentum (compare its MJO forecasting with other dynamical forecasts), and it will try to shift the Azores ridge westwards over time. Net result, shift the ridge signal further east in the 10-15 day timescale. I would favour ridge development across Iberia and sufficient to influence at least Southern UK in this time frame. For this time of year, that equates to fine and dry for much of the southern half of the UK.

Extended range modelling going for lowish heights over the polar field and flatish type mid latitude type pattern as we tip into a more tropically influenced period and away from the polar influence of the spring. This from GEFS day 15 would be a fair summary of the modelling.. I suspect that once this gets closer to higher resolution, the trough to our west and continental ridge will get picked up better. Currently no real model support for a reboot of the heat days 10 - 15, but I would be hedging towards further heat towards day 15.

As summer proper comes into view, I know the model output comes in for greater attention given the trend of the last few summers (starting with 2007) for the weather at the start of June to be a precursor to the remainder of the season. How are we looking at the moment ? My seasonal assessments to clients have been very critical of the chances of an El Nino event evolving, and it looks like that will be the case. Upper ocean heat content just hasn't been supportive of a warm ENSO event developing. So that's a non-factor. What then of what has been much more significant as a sub-seasonal and inter-seasonal driver since last autumn, what we might term Indian Maritime Tropical Forcing (or a persistence of coupled ocean-atmospheric windlfow patterns denoted by thunderstorm activity around Indonesia and New Guinea) ? Of late the sea temperature anomalies leading this have weakened and rainfall patterns shifted in the spring. This was more to do with the lagged effect of a sharp breakdown of the stratospheric polar vortex. Although sea temperatures across the Tropical Indian and Pacific Oceans remain only weakly conducive, this driver looks to get a bit of a boost from the passage of a MJO wave in the next two weeks. Strangely, the MJO was suppressed in this region during the winter, and its renewed strength might indicate that this lo level tropical forcing is no longer as sustained. Either way, MJO phases 4, 5 and 6 and tropical forcing in this region during tend to support ridge development over northern and western Europe in response to troughing in the Atlantic. My eyes are really drawn to two factors which I think will be key players (or more indicative of other forcing and the atmospheric responses). The sea temperature anomaly profile of the North Atlantic is near perfectly aligned for troughing and downstream ridge development over Europe. Anomalously warm pools off the Eastern US coast and to the west of Africa and Iberia have developed over the last few months whilst cold pools all the way through the central part of the North Atlantic have developed and been reinforced by weather patterns in the last month (again related to stratospheric breakdown). That is a warm signal for much of Europe. Repeating cut off lows off the Eastern US, and possible enhancement later in the season by tropical systems should persist that cold pool and lock down the warm-cold-warm spatial arrangement of sea temperatures. The other key development has been in the polar atmosphere. This isn't necessarily a driver per se (although it was in terms of the breakdown of the polar stratospheric vortex in April), but it serves as a useful tool in understanding how the atmosphere is responding to global drivers. The breakdown in the polar vortex in April serves as useful starting point. Sharp downwelling of negative zonal winds (positive height anomalies) were observed, and this basically imprinted the atmosphere with a blocking (-AO) signal, which manifested itself throughout the circulation, even over the Tropics). The anomalous warmth over the Arctic will have served to constructively interfere with this signal, culminating in those destructive night frosts recorded during May. However, since the breakdown of the stratospheric vortex, the upper part of the polar atmosphere has cooled and heights have fallen quite substantially in the upper part of the atmosphere. There are two years which illustrate why the coming few weeks will be keenly monitored. During 1995 and 2007, two completely different summers (dry and warm versus blocked and wet), the evolution leading up to and during the breakdown of the polar vortex was very similar. However, what followed in the stratosphere and upper part of the troposphere were completely different. During 1995, a cooler stratosphere and lower heights extended their influence much lower into the atmosphere and the resultant weak polar easterly flow was characterised by much less blocking. Contrast that with 2007. The extension of low heights was much less, with blocking (positive height anomalies or red shading) much more robust in the lower part of the atmosphere. Looking at the profile to date, the extension of low heights is impressive and suggests something more akin to 1995 is the case (although there is still some time for similarities with 2007 to rear their head). It is the speed of the atmospheric response to the breakdown of the stratospheric polar vortex that is most telling for me. So we have a weak polar easterly (less risk of influential blocking over the Arctic) signal allied to a trough signal in the North Atlantic. Those two suggest a higher pressure tendency for Western Europe and summer where the continental ridge is allowed to develop as the season progresses. So in essence, the June through August signal is more weighted towards above average temperatures and below average rainfall for much of Western Europe.

Sounding a lot like my seasonal outlook there ! Within the shorter and medium range we are beginning to get the influence of an Atlantic trough signal and somewhat split flow over Europe giving a tendency to mid latitude high pressure. As a real positive, a colder than normal upper stratosphere over the Arctic and low heights developing through the upper and middle stratosphere is a recipe for fine spells increasingly developing into June.

Not sure the upstream teleconnections are as favourable as being advertised by RMM MJO plots. 200hPa velocity potential is much further east than would be considered for phase 8, and threatening an Indian Ocean evolution: That's more phase 1 territory which is no where near as favourable for high latitude blocking. Factor in the displacement of the upper vortex towards Siberia and you get a signal for Euro ridging as per NWP extended means. Very similar to the mid / late November episode. That's not to dismiss the text book spike in tendency in angular momentum that has occurred on the back of the evolution of the tropical wave through the Pacific, but my take on that would be for more of a hit to the stratospheric vortex at the end of the month and more robust blocking signal to the NE / N at this time, and into early March, although for the lowland snow chasers that's too late I appreciate.

Nice addendum there Nick. The key from this will be how fast we move the MJO as a proxy for angular momentum back across the Indian Ocean. The faster the better if you're looking for a swifter return of any easterly type pattern. The potential northerly advertised by week 2 GS and EC ops, just a case of wait and see on that one as its not fitting any GWO / tropical influence and must entirely by largely atmosphere driven- which is notoriously difficult to get a handle on.

Quite like the GFS op 06z as a road map for how the next 3 weeks might evolve. We have a strong push of equatorial easterly winds extending across the West Pacific and Indian Oceans, with developing convection in the West Indian Ocean. This is consistent with a developing U200 wind picked up by Mike Ventrices' product: MJO forecasts also consistent with a phase 1 - 2 projection. I think that the eastward propagation is also a reasonable assumption (having my seasonal reasons for it to do so which I can't go too much into). So a stage 1-2-3-4 evolution likely. In terms of how this transposes to the Global Wind Oscillation, we need to consider that relative angular momentum budgets remain close to climatology, so high amplitude phase 2 is not likely (this would be high amplitude if angular momentum were well below normal (i.e, strong Nina). A drop in angular momentum resulting from -ve torques very consistent with a big phase 8-1 projection in the GWO. Analogues broadly consistent with longwave pattern week 1, particularly weakness in heights across sub-tropical North Atlantic - but need to adjust for ongoing low heights to our North. So support for this idea that the trough to our south will lift poleward and westward over time and merge with the trough dropping from the north. Thereafter, low amplitude phase 2 to 3 transition likely as tendency in momentum begins to rise. Composite analogue consistent with the idea of Euro trough evolving to some kind of ridge over NW Europe in response to a dominant upper trough in the Atlantic. Quite how that plays out with any dropping of the main lobe of the pv into western Siberian Coast open to doubt, but fwiw, gfs seems logical. After that, rising angular momentum with the MJO transitioning through the Indian Ocean into the Maritime Continent signals another bout of hemispheric amplification as the GWO enters phase 4. With a low frequency signal for Atlantic troughing and NW European ridge, I wouldn't be surprised to see us back we we start now. Ridge developing over Scandinavia / UK and pushing westwards. With the polar stratosphere exhibiting some unusual behaviour, these composites may need to treated with a degree of caution on exact placement of ridges and troughs, but the broad scale ideas should be there abouts. One aspect I'm really not sold on at all is the idea of +ve heights remaining for very long over the East Canadian sector for very long. So weatherwise that should translate to a blocky type pattern with shallow ridges and troughs and essentially feeling raw for week 1 into week 2 with temps below average across much of Europe. Risk of above average temps returning to western Europe week 2 into 3 but that will be dictated by any push of colder air southwards during week 2 and a rather messy mix could arise with surface cold persisting. Concern would be if the MJO stalled in phase 3 resulting in a more meridional pattern with the UK on the warm side of any Atlantic trough. Into week 3, more amplification developing should see us regain an easterly feed. So back around to where we began...

There is a view, one which I would endorse, that the RMM Indices are not capturing fully MJO activity. For example, right now, upper and mid tropospheric winds are flowing all the way across the West and Central Pacific all the way to the East African Coast. Likely significantly enhanced convergence and convective activity around West Indian Ocean (MJO Phase 1).