Glacier Point

..and the GEFS Ian !

Day 10 GEFS H5 and ECM ensemble mean anomalies are reasonably in agreement. That might superficially suggest a mean westerly flow. However, the logical evolution for both would likely be for a secondary build in heights in the Atlantic angling the track of any shortwaves NW-SE. Therefore, we could reasonably assume any cold pooling to remain in situ and begin to back west again (if it's not already there). The interest from my point of view is that the longer range ensemble means are suggesting a bouncing back and forward in angular momentum, which would typically favour this type of setup. The diltution of T2 and 850 values will be greatest within the suite due to the range of outcomes. A better guide will be the H5 anomalies (if there is a clear pattern there) and for you to project temperature anomalies on that basis. For example, GEFS H5 anomaly at t348 is discernible and with a mean trough solution for the UK would be colder than normal (say 2-4C below average), yet this is not registering at this stage on the ensemble means at T850.

I would go all amplitudes (control) and high amplitude (>1SD) to see if there is a meaningful difference between the two. We have an excellent case in point right now. Really strong phase 8 projection which is in part driven by a stratospheric coupling through forcing of tropical convection. H5 anomaly in the next few days: Is this more amplified because of a more amplified GWO signal ?

an all amps H5 anomaly would be useful as a 'control', perhaps to smooth out inherent variability and to compare against ENSO state.

So we're looking for 6 plots per phase per month 6 x 12 x 8. That is a lot of work. If you feel you have to cut back, I would go for amplitude >1 H5 anomalies first, then 850 hPa anomalies for amp >1. Best wishes for a stirling effort.

For what it's worth, I don't think the GEFS ensemble mean is that far off what the op ECM has been consistently advertising of late. Day 10 GEFS mean H5 anomaly takes the upper level weakness across southern tip of Greenland and drives it (with some inevitable disruption) south-east into the western flank of the upper trough dominating Europe over the next 10 days. Operational GFS very clearly intent on driving this feature eastwards. Note the development of anomalous +ve heights Svalbard as the trough digs SE and forces height rises to the NE. The 850 hPa temperature anomalies to go with that suggest in the range 2-4C below average. Days 11-15 GEFS H5 anoms are interesting in developing another ridge in the Atlantic (more upstream amplification). I suggest that this is consistent with the overall state of the GWO right now, favouring this type of anomalous ridge. That makes the presence of the trough over Europe even more long lived as shortwaves are deflected NW-SE track, helping to advect the cold puddle westwards. With 60 days of the season elapsed, we can also use persistence as a valid tool. H5 anomaly so far, might surprise a few.... For me this centres around the Euro trough and associated cold pool not really going anywhere fast. Just how much of an influence on the UK this has, open to some debate, but the broad pattern and general trending of ensemble means looks reasonable to give some confidence.

Recretos I have long wanted to do this for the GWO, but sadly not had the time. I would strongly advocate that if you want to do this, look at the GWO rather than the MJO. The GWO is a better, more refined measure of both the tropics and extratropics' influence on the general circulation. The data set goes back to 1958 and the MJO has been done to a degree by Allan Huffman. The best way of structuring it (from an operational perspective) would be: - Phase space (all magnitudes) by month, 500 hPa GPH anomaly and mean, T 850 hPa anomaly. - Phase space (for magnitudes > 1SD) by month, 500 hPa GPH anomaly and mean, T 850 hPa anomaly. - Sifted by ENSO signal <1 >1 SD (perhaps take MEI as the measure data set?) If you could do this, and Paul could host it, I'm sure we would have a tool that would be of great interest to a global met community.

Modelling seems to be following a very similar path to early December: a deepening Euro trough, shortwaves cutting NW-SE across the UK. I wonder how soon before models begin to develop heights to the NE, and what if any will the fate of any attempted easterly ? Will it share the same fate of the mid December one ? The sun has been spokily quiet of late.

Indeed CC. Taking Joe Bs theory from 2009/10, increased angular momentum (El Nino type) during a cold PDO phase (low angular momentum) is a recipe for cold outbreaks in the middle latitudes. What we have in front of us is a microcosm of that theory.

... and just to put this into pictorial expression, those two options are for a fall in angular momentum or for continued cycling back and forth in the global wind oscillation at high angular momentum values.

GEFS day 10: ... very amplified and phase 7/8 and liable to deliver T850 anomalies of around 3-4C below normal. The real insight here is this. Longer range modelling failed to pick this amplification. There is more to come as angular momentum is sky high at the moment and we're likely to see the GWO flick through phases 5-6-7-8 and back again. That builds further amplification into the system, so temperatures 3-4C below normal as a starting point.

..but notably the Global Wind Oscillation being held back in a more phase 5 orbit due to the manifestation of the extra-tropical measures in this oscilation compared to the MJO which has a tropical compenent only. As we see the GWO signal 'mature', more of a signal to develop that disrupted flow in the Atlantic.

The really interesting aspect to this warmig event is the distortion to the hemispheric U-fields that is occurring, and helping to heavily accentuate a pattern of tropical waves. At 70 hPa, the tropical stratosphere is at record low levels and almost certainly bringing about strong cooperation with the tropopause to shift and develop deep tropical convection. Upper sigma relative angular momentum anomalies depict a -ve zonal wind anomaly in the mid to high latitudes in the NH and quite strong -ve wind anomaly just south of the equator (corresponds to lifted tropopause and development of taller thunder clouds). This is likely to stress the U-field setting up a poleward shift in eddy flux. Overall relative angular momentum in the middle and lower layers is registering a massive increase in westerlies, in excess of +3 SD. That would be impressive in an El Nino winter, let alone an ENSO neutral winter. This increase being principally over the tropics, for the time being. Meantime, are we seeing a nodal propagation of temperature anomalies through the polar stratosphere? Likely tropospheric response continues to be anomalous mid and high latitude ridges in response to eddies developing off the sub-tropical ridges.

Angular momentum is the net product of interaction between sea surface temperature anomalies over the tropical Pacific, the impacts of the tropical monsoon season, the multidecadal oscillations in the oceans and the inter-seasonal influences of the polar stratosphere. Oh, and the sun !

CH - yesterday's value was +3.9 SD, so we could take that as peak no. 1 with a possible even larger peak in the next few days (note that there will be at least 2 amplification waves there). Why do the bods at Exeter not share this ? Well, can't say for sure (and note the low confidence rider at the start of extended range preamble), but generally in my experience mets are very model centric. Trust me, the idea that you can arrive at a forecast without using a model tool totally freaks practising forecasters out (but not in a bad way, just challenging received wisdom). That's not to say you don't use models, they are after all the product of a lot of investment and good minds, but use both to your advantage (see the sig!). If you have time, Ed Berry's back catalogue of blogs (former National Weather Service guru in the States, now private sector) gives you a feel for where I'm coming from. Although dated 2009, they are as current now as they were then. http://weatherclimatelink.blogspot.co.uk/

.. There will be key differences due to seasonal wavelengths being different, and because we are well into the winter when the pv has matured. However, the point of the exercise was to perhaps establish a timescale for amplification given a very strong +AAM tendency spike within the modelling - how long it took for the previous spike in angular momentum to register with the GFS. Answer, about 11-12 days initially.

Angular momentum continuing to tank, and good evidence of poleward fluxing of that energy as the Global Wind Oscillation gains amplitude in phases 5-6-7. It could well go even higher in the coming days as mountain and frictional torques get to work (might check record GWO phase 5 value). NWP beginning to wake up at t168 mark to amplification signal coming out of the Pacific. I would advise anyone to look at archived GFS modelling dated 18th November 2012 as your reference point here, and then maybe 12z control GFS (you'll need to substitute Kamchtakan ridge for an Alaskan ridge, but the general gist for our locale about the same). Flat and zonal ?

.. much more substantive +ve angular momentum signal this time round. Note the tropics are just alive with westerly tendency at the moment. The same posting in November was however a good heads up to the ridge / trough solution that characterised the first half December. That's not to say all of February will be cold, the further we go the more problematic in determining longevity, and I suspect the key variable for us to deal with will be how far west the +ve height anomaly goes.

Thar she blows... A big surge in +AAM tendency being registered, with a probable second spike to come in the next 5-8 days. Total angular momentum is really quit impressive at the moment with a large +ve anomaly coming out of the tropics driving us into El Nino type atmospheric response. That spike in relative angular momentum will build amplification into the system with a likely second wave following on behind. We should shortly see anomalous tropical convection in the central Pacific and Africa, lending credence to GEFS MJO projection for phase 8.

Reasonable grounds for thinking that the global synoptic dynamical model is moving towards a third cold pattern this winter (the waning of this present one being well advertised) and the last few GEFS ensemble means have leant support to this idea. Tropical convection has been focussed in the western Pacific in the last 10 days. This has introduced anomalous westerly winds to the tropical Pacific which in turn has helped lift tendency in angular momentum towards an El Nino type pattern. Latest TOA Triton data indicates anomalous ocean heat content centred just west of the Dateline and encroaching anomalous westerly winds on the back of the MJO. Trade winds are also decreasing, which is typical of MJO progression towards phase 7/8. The GEFS MJO forecast has come to life in the last day or so, suggesting a strong amplitude wave to develop in phase 8.This seems reasonable on the basis of favourable conditions for convection due to increased heat avalability and cooling upper atmosphere (especially lower stratosphere). These tropical developments are beginning to take effect in the sub-tropics. A strong +ve mountain torque has occured and tendency in angular momentum is already postive and likely to spike very positive in the coming days. Overall relative angular momentum similar to November. The shrewd will recognise that the last uptick in relative angular momentum heralded our first pattern change and cold trough in December, by about 2 weeks lead time as positive (westerly) wind anomalies are fluxed poleward introducing anomalous mind and high latitude ridges and circumglobal troughs (Pacific and Atlantic). Meantime, our first downwelling wave from a very strong polar stratospheric warming (and sustained reversal in zonal winds) has matured. Stratospheric time scales are extremely long, and we can expect further downwelling episodes lower down in the atmosphere for the next few weeks. Based on Baldwin'c composite of weak vortex events, the next episode of increased eddy activity is due around 28th January although the timing of this is not exact. The temperature anomaly section shows a reasonable time scale for 'nodal' type propogation extending into February, perhaps more towards mid February. A typical tropospheric response to this would be very similar to an MJO phase 8 projection, so its interesting the atmosphere choreographed in this way. So teleconnectively we have a reasoned case for anomalous heights to our north end January into February. The timing of this highly problematic as the models were to a degree blind-sided by the first downwelling wave, only latching onto it 2 days before the warming took place. Were this happen again, a sudden turnaround in modelling could well occur. The last five GEFS mean height anomaly for the extended range has 'hit the target' on all five occasions to varying degrees, which is interesting in itself. Todays GFS super ensemble mean height anomaly.... Putting this together, expect further cold pattern, becoming stronger probability the further we go into February but I wouldn't be suprised to see it pop up from 28th January onwards. Winds from the NE and E the favoured scenario.

I think we need to start to plan contingencies for a potentially sustained and very severe spell of wintry weather, of which Friday could just be the start. NWP is presenting a number of potential snow events next week, which could well provide substantive accumulations. The longwave trough migrating eastwards off the eastern coast of North America is consistently being modelled by ensemble means as cutting through a developing mid Atlantic ridge mid to late next week. That provides an eddy in the flow which is likely to enhance the easterly flow off an already frigid land mass to our east and north-east. Thereafter, the ridge could retrogress towards the NE Atlantic and Greenland bring in another surge of Arctic air from the NE. That would give us a sustained period (with snowcover for much of the UK) of below normal temperatures. I've grabbed the 06z GEFS at day 13 which depicts the synoptic pattern with a trough firmly ensconsed over Europe and high pressure to the north.

Good agreement between ECM and GFS ensemble means at day 10 pattern. Remaining cold throughout that period with high risk of snowfall. Thereafter, the modelling remains keen on the draining of the pv lobe over Hudson Bay within a general eastward moving longwave trough exiting the North American seaboard. So the question is posed, where does that energy go ? South-east into Europe where there is an existing trough and block of cold air to the north, or more NE allowing for a pressure rise over Europe ? Given we already have a trough in place and cold block, allied to the 2nd wave of downwelling into the troposphere from this major midwinter warming of the stratosphere, I suspect cold intensifying is the favoured solution. Look NE.

12z GEFS beginning to amplify in the mid Atlantic. Comparison with yesterday at the same time we can see the anomalous ridge extending further north on todays day 9 comapared to yesterday's day 10. That is important because it delays the eastward progression of the longwave trough exiting the eastern US (exceptional cold coming there) and sharpens the trough over Europe, increasing the chance of further undercutting shortwaves and a more substantive eddy developing when the jet relents (which invariably end up strengthening heights over Scandinavia). We can see this is trend today for a renewed surge or perhaps 2 surges from the NE.

12z GEM and another undercut for this Sunday.. At this rate I'm glad we're attempting to bring a front in to have a go, the rest are just wimping out. The inter-run revisions to models at the moment is pretty rare, nearly always going from positively tilted to negative tilted the closer in we get. Notice also trend for the Canadian PV lobe to drain away and increasing influence of th pv to our NE.

If UKM GM and ECM follow this...I suspect so Nick.. Heavy snow into the SW (with a possible new depression ready to cut beneath the UK a day later) .. a nice way to start the afternoon's runs...