snowking

Repeat after me... Boom. SK

Corbyn or Madden? Or some terrifying new hybrid: The medium term model output holds a little hope for coldies, with both the ECMWF EPS and GEFS means giving this: Nothing spectacular but if you can shift enough of the vortex core over to the Siberian sector you set yourself up for generally cooler zonality at the very least. Of course if you can shift the remaining vortex energy from NW Greenland even better, but right now that looks unlikely. We still see some more amplified GEFS solutions, but when you take a look at this: You can see why - the GEFS still far too keen to bring us in to phase 7 versus model consensus. This is a known bias and when we take a look at the bias corrected output: It still gets us there but only barely. And indeed other models that just about scrape in to Phase 7 do so at a very low amplitude: Phase 6, declining Phase 7, leaves us in a bit of a muddle analogue wise. The bigger emphasis is likely to be on Phase 6, which leaves us with slightly more amplification across the Atlantic, but not sustainably so - toppling ridges likely to be the final scenario I would suggest at this juncture. As much as the next week to 10 days may not feel like it we do find ourselves in a very different position hemispherically to last winter (the emphasis should probably be on the first part of this sentence). Unlike last year with a solid, central polar vortex to contend with, seasonal wavelength changes could still make their presence felt a little later in the winter. That is more devils advocate than a solid prediction but I am far from losing hope at the moment with much of the season still to come and the knowledge that it will take far less work to derail zonality this year in comparison to 13/14...just not in the coming days. SK

Unfortunately all the GEFS images on the CPC site are blank today. Last update I saw (Thursday) had the raw output headed in to phase 7, but I did not get a chance to peruse the BC output SK

Hi HC, Both the ECMWF and JMA MJO forecasts go out beyond day 10 (out to day 15 I believe) - the MOGREPS forecast displayed there only goes to 10 days, hence the attached note. It is in that crucial Day 10-15 period that the disagreement takes place in terms of the MJO phasing. As stated, just because the GEFS was poor on predicting it last time does not mean that will necessarily be the case this time, but it would be wise to remain cautious until we see a more unanimous agreement between suites on this signal. Still some question marks as to whether this is MJO induced, but given the close resemblance to MJO phase 7 composites that would be an acceptable assumption for now. SK

It may be wise for now to remain a little cautious about FI GFS (and indeed GEFS) output. Why? Well the GEFS is fairly solidly behind a progression towards MJO phase 7 (unfortunately there are issues with the MJO images today), however it is rather isolated in this assertion, with just some limited support on offer from the BOM: Otherwise we see the other major suites repressing the signal in to Phase 6: (Should be noted the last one there from MOGREPS-G may be a little more difficult to call) We can see where the FI GFS suggestions are coming from if we take a look at the Phase 7 composite: However during the previous MJO wave the GEFS was pretty poor. That is not to say that will be the case this time around, but right now it looks very much like GFS v the rest. Until we see a more coherent signal, caution is advised SK

Just some subtle changes to note on the latest ECMWF stratospheric output We do see a slower downwelling of the temperature wave from the Atlantic sector (may be linked to subtle hints from Mr. Fergusson over on the MOD with regard to later January signals from GloSea5) down as far as 50mb by day 10. The dynamics of the wave activity at this stage are very odd indeed, but we continue to see a too-ing and fro-ing of the upper vortex between Siberia/Scandi and Greenland/Canada. What is interesting about this for me is the shift of the core of the vortex towards days 8-10 sends it over towards Canada/Greenland, something which doesn't sound promising, however at the same time we see the next possible geopotential wave come from an E. Atlantic/W. Europe type sector, at a pretty useful angle to potentially set up some sort of Scandinavian high. So that is the next thing on my watchlist towards the second half of January. So what we effectively end up with is an Atlantic wave 1. Not too sure about any historical case studies to compare this to potentially, but amongst the doom and gloom of the model thread this actually looks quite promising to me. It may take a few more days yet but I would suspect towards day 15 we will start to see a signal appear for some movement NE of the Azores high - assuming of course this signal is sustained as we move towards T+0 and the downwelling continues. Tomorrow would be a really good day to get Berlin back! SK

First of all I think this is a bit of a misconception - take a look on YouTube at some of Mr's Schafenaker, Hammond, McElwee, Taylor etc forecasts around the time of snow. To quote Mr McElwee back in either 09 or 10 (I forget which), "proper winter".However, even if this was true it should not be surprising - the less extreme the conditions the easier a forecasters job becomes. Nobody wants week after week of difficult to predict endless extreme weather if your job is to inform the public of said conditions, keeping them safe and helping them to make potentially life threatening decisions (perhaps extreme in terminology). From my short time as a paid forecaster believe me, you are praying for settled quiet weather - and in my instance it was not just affecting lives, it was potentially affecting commercial revenue too! Sorry for the off topic post but worth pointing out I feel SK

That is precisely what we should be looking for TEITS and nicely summed up Western Greenland vortice to back westwards Siberian/Scandi vortice to strengthen Potential ridging somewhere in between We see that almost instantly on the 12z ECM, though perhaps this is a little too quick with the modelled response right now. Time will tell. But those are the key signatures to watch for in the troposphere should the current ECM det. Stratosphere guidance prove to be correct...and of course that is still a huge assumption at this stage, but not without support. Also worth noting with warnings that sometimes we see an initial response, and then a lagged response too - tonight's ECM control certainly hints at that sort of scenario at least in terms of the initial response with a brief collapsing mid atlantic ridge around days 11-15 Plenty to keep us interested if people know the right places to look in SK

Re: Model Bias I think some are perhaps confusing "bias" with fundamental model physics. Due to the direction of rotation of the earth, angular momentum conservation will always result in a Westerly flow overall - and yes this will form part of the extremely complex physics programmed in to the models. So is there a risk of an over-westerly bias? Potentially yes, and we have seen this in blocking scenarios with the GFS many times over the years, but that bias does not come about due to the models simply programming Westerly tendencies and nothing else, it comes down to failures elsewhere in modelling the dynamics of blocking highs, for example. So do biases exist - yes. The charts often posted by IDO prove this (and also prove that the likes of NOAA etc. are tracking these). But are the biases built in purposefully (outside of the aforementioned angular momentum principle)? No. This is not to say, however, that under low angular momentum conditions the models will not struggle. SK

This tweet from Eric Webb might be relevant in answering this: In short, take note of the image at the bottom right (theorised precursor pattern to an SSW) and the composite above it showing the 500mb anomaly pattern over the past 10 days SK

As per Nick's post above ECM tops out at +228 at -7.7c, and by T+240 (with continued warming around -12c) breaks through the middle of the elongated vortex and obliterates the vortice in the vicinity of Greenland, forcing it well away to the West, and leaving the dominant player the Siberian/Scandi vortice instead...sound familiar? It should do for anyone who has seen the 12z ECMWF tropospheric output: The easiest way I can say to you of visualising this is to look at the purple colours on the charts above and see what happens to their make-up and positioning. This is exactly what is born out at 10mb too. Strong signs of a very quick downwelling phenomenon too down as far as 50mb by day 10 (unfortunately we don't get to see any lower than this as of yet on WxBell) with an almost identical geopotential height transition taking place at this level too - and quite evidently the same in to the troposphere too. Still all out at day 10 right now, but the signs over the last 48 hours are that the warming and potential (2nd) split are not only ramping up, but becoming more and more favourable for the Atlantic profile too. I remain cautious but increasingly optimistic SK EDIT: NAVGEM wants to join the party too, and I am more than happy to let him:

It's a question that is far too vast to answer in great detail. UK cold chances - almost impossible to say at this juncture. I think people need to be able to make a distinction between the kind of talk that goes on in the model thread (far more specific and focussed) and the discussion that takes place here. Where as in the model thread we can look at charts showing the effects directly on the ground, what we are dealing with here is conditions spread across the entirety the Northern Hemisphere, and with a greater depth to forecast too (when we forecast at the surface we tend to look 2 dimensionally much of the time, where as in the stratosphere we are having to think about things in a more 3 dimensional manner). No two stratospheric warming events produce absolutely identical results. However, some of the better (and more instant responses to) stratospheric warming events have occurred when the warming takes place around the Atlantic/Greenland vicinity. Again, just to repeat, Nothing is guaranteed, and it is important for everyone to remember this when viewing this thread, to prevent it spiralling in to the chaos we see over in the model thread at times. But the reason for the desire for a Greenland warming this year is the stratospheric activity up until now has been largely Wave 1 based from the Pacific sector, and this has had the net effect of pushing much of the vortex energy (note: not a solid vortex, but still there nonetheless) over towards the Atlantic and Greenland - so a warming here could help to significantly reduce the influence of the polar vortex in this particular area. In terms of are these charts FI, well whilst conditions are a little easier to predict in the stratosphere (due to the lack of surface effects, such as mountain ranges and oceans etc.) than in the troposphere, no model is infallible. The definition of FI for the stratosphere is a liquid one, and tends to depend upon intra-model (and indeed intra-run) differences. At the moment, with a strong disagreement with the GFS and ECMWF suites, I would recommend viewing current output as FI-type output. If by this time tomorrow the ECMWF (with greater stratospheric modelling overall vs GFS) is showing something similar to today's 12z run then a few in here may start to get excited. Of course then the next stage is to decipher what the effects in the troposphere will be... SK

Further support to a certain extent elsewhere for tonight's ECMWF 10mb explosion, most notably from JMA: And to a slightly lesser extent, the NAVGEM: It should be noted that whilst the GFS det. remains rather muted with this, the parallel run does still have some hints at an Atlantic sector warming: So the signal for some sort of Atlantic warming is very much there, it's just the intensity which is open to question - let's hope for more of the ECMWF trend. Would be interesting if Andrej is around to see what the extended JMA looks like SK

It is indeed sleet.

Should arrive here (M11 corridor) within the next 30 mins or so, DP's seem a little high at the moment but will let you know what falls. SK

Unfortunately we are still looking towards the tail end of runs, but this is much more promising from the last few runs of the GFS(P) - it should be noted the Parallel was pretty adamant on not supporting the brief Greenland warming suggestion of the Det. Try to think of Ed's analogies of the vortex being like a balloon. Up until now our warming waves have come almost exclusively from the Pacific sector in the form of a Wave 1 pattern. Whilst this is placing a considerable strain on the vortex (and a follow up Wave 2 attack causes the split in the mid-term), it is also having the effect of displacing much of the vortex energy over to the Atlantic sector, and we just can't quite manage, in current NWP at least, to see the Northern arm of the jet stream relent for long enough to build any meaningful blocking in the right position for long enough. So suggestions of a temperature wave from the Atlantic sector may provide a little more interest towards the middle of this month. There is no doubt the vortex is staggering around and just waiting for that final jab....coming back to the balloon, perhaps it needs that final jab from the opposite side this time. If somebody is pressing at the balloon from one side, if somebody can come around to the other side and jab at it there is a greater chance of it bursting. It may not necessarily be a Wave 2 attack, but it may just be enough to tip us over the edge. Let's hope so because I saw my first few flakes this morning and none of them settled... SK

That stansted airport TAF is about the least certain I have seen Light rain, becoming heavy rain/rain-snow, probability of snow between midnight and 3am, but also rain-snow between midnight and 9am. Just goes to show to anybody expecting a definitive forecast, it's going to be hard to come by. The joys of the UK climate SK

That's the key one to keep an eye on as the evening goes on is how the dew points drop. Model estimates were for around 6pm this evening, but looking at the radar at the moment I dare say it could be an hour or two earlier than that. It will slow down over time as it starts to pivot, and ideally you want to be to the North of the triple point as it begins to pivot through this evening. SK

It's a little more widespread than expected already, though generally the occluding of the fronts will lead to a more concentrated area of precipitation as the day goes on, especially as the low deepens a little further and we see an increase in shear. SK

Hi John, Potentially so. It may just be that the precipitation was more intense than forecast, but I suspect we are seeing the triple point developing more quickly than expected. The effects could be numerous (both positive and negative), but generally speaking we want the precipitation to be as heavy as possible for as long as possible to help cool the surface. SK

Absolutely - I suspect we will see snow falling but accumulation will be another matter. I'll be up in Ashdon this evening so not far from you at all. Would be nice to come out at midnight to a nice white scene! SK

Latest Euro4 keeps folk in the North of the region happy - perhaps further South less so: This does not preclude snow falling of course, but accumulation wise it's no surprise to see those are the areas underneath the projected tailing occlusion: Model projections would suggest Northamptonshire and Norfolk may be the places to be for this one in our region, though as noted in the model thread the radar returns across Ireland at present are far more intense than suggested by the 6z Euro4. The heavier this precipitation becomes the better the chances for snow. SK

Latest Euro4 takes us towards the aforementioned ECMWF 'middle ground' type scenario: Spot totals of 20cm showing across parts of the E Midlands there SK Edit: It should be noted that current radar returns across Ireland are more intense than forecast by Euro4 for midday - could be a sign of the fronts occluding a little sooner than expected, which would prove better news for more widespread snowfall hopes later.

FYI from model thread: Still so much uncertainty for our region, especially when you look at the updated met office warnings, I can only assume these are slightly UKV based, because even the Euro4 signal is a little more muted, for example, in to North Kent. ECMWF tends to make it an East-Anglia type event (perhaps snow falling but not accumulating South of this), and GFS takes all but Northamptonshire and Norfolk out of it altogether. Most recent Stansted Airport TAF suggest Rain/Sleet until midnight, then 40% chance of light snow from midnight until 9am - so far from certain just yet. SK

Quick update with regards to today's snow potential Still a very tricky cool with temperature profiles proving very marginal indeed. I would say above 200m, and indeed North of the M4, it looks a fairly dead cert. For the remaining 95% of the landmass affected by today's precipitation, a lot may come down to evaporative cooling, and hoping the precipitation stays intense enough all the way through in to this evening. GFS, and hence NMM, remains the furthest North solution: As outlined by Mucka, Euro4 keeps the precipitation much further South: And at present the best time for the widest cross section of England and Wales, at least according to the Euro4, is around 3am in terms of dew points: Though as you can see, still marginal (though at that resolution it is tough to decipher just how much of an area might be above/below 0c). Two things to note here. Firstly, no model is really able to accurately calculate the effects of evaporative cooling (quick FYI for those newer members - as precipitation falls from the clouds, it carries down parcels of air. These parcels of air, having come from much higher up in the atmosphere, are much cooler than the air temperature closer to the surface, and so there is a cooling effect. The heavier the precipitation, the more air parcels get carried down, and so the cooling effect is even greater. This can see rain turning to snow as the temperature profile from cloud to surface cools sufficiently enough to support snow), and so we should be wary of this. Secondly, I have also seen some suggestions that the Euro4 tends to underplay snowfall potential. I personally can find no scientific evidence of this, but something perhaps to consider. The 0z ECMWF paints a halfway house between the two, and tends to follow the track of the updated Met Office yellow warnings - I would suggest that represents the best guess that I could make in terms of a potentially affected area at this juncture. That is not to say it will be the correct track, but based upon model consensus that is the best estimate. We can do some quick checking on this right now. Euro4 projection for 9am: Radar imagery from 9am: Observations - The intensity projection of the precipitation is pretty accurate. The system itself looks a tad further North, but that may be down to the fact that it is simply more expansive than projected, covering a larger area from North to South. It has advanced further Eastwards more quickly than projected too so far, though may slow down a little later. So it looks to me as though in terms of track, the halfway house solution of the ECMWF may be the closest to the truth. From here on in we can monitor the models as much as we like, but the more useful time will be spent looking at obs and radars. Good luck to all. SK