nick sussex

Any chance those weak lows near Svalbard could get sent packing allowing the Arctic high to ridge down or is that being greedy!

Still the UKMO looks the odd one out in terms of that upstream phasing of those two lows. Big differences at only 96hrs between it and the ECM. Great 120hrs from the ECM with that much more amplified ridge to the west, OMG this should be a great run and also those pressure rises to the ne.

Its been like that for several days, the 144hrs when it comes out should be okay though and it does have a pressure rise to the ne.

Much better upstream pattern on the GFS 12hrs run also pressure rises to the ne. A good start to the evening.

Thanks. In terms of model bias you will often see the GFS trying to remove blocking in the lower resolution output. What often happens is that this never makes it into the higher resolution. Blocking patterns have to evolve, you cannot for example have a Greenland block in the same position for weeks, equally Scandi highs also evolve. Generally when the atmosphere is predisposed towards higher latitude blocking you often get a retrogression signal, for example if people look at the archive chart for end of November 2010 and follow them through you'll see the shuffling from Scandi high to , Greenland block,then you'd get a briefish easterly again as the ridge gets flattened but then back to Greenland yet again. For Europe in these circumstances if you avoid a western based negative NAO during that process then you generally stay cold. I think people sometimes expect too much from numerical weather prediction, the models are often slow to pick up on signals or when they do they can often over react. Operational output is at its worst when a pattern change is imminent, its best to wait for the models to settle down once the block is in place and at that point we can see how the pattern could develop.

It always makes me laugh when people start worrying over a possible breakdown in the expected blocking pattern before its even arrived on the scene. Lets just get there and see how the models evolve. In terms of todays output good agreement in terms of the transition to higher pressure to the north and nw, I particularly like the ECM as its got more margin for error than its previous operational runs.

Go and take a cold shower! Enough of all this outrageous ramping! Yes the infamous early Canadian Warming, its funny the longer I've been on here the more cautious I get, perhaps its the bitter disappointment etched in the memory of past colder synoptics imploding. I will say again all we need to concentrate on is getting over the first hurdle, once we get the initial block in place we can see what wonders await after that!

Yes I think the possible initial easterly is a bit of a red herring. I think we're going to need some retrogression in the pattern to deliver what most people in here want. If we do see the initial block then if that cross polar flow develops favourably then we could see that troughing dropping south to the east, maybe after that we could see the flow veer back to the ne or east but thats well into the future. Personally I just want to see us clear the first hurdle, until I see that within 96hrs then I'm going to remain unusually reserved!

Theres always trouble with shortwaves! I wouldn't worry about the 850's the main hurdle is the transition from 168 to 192hrs, the trigger shortwave heads se as the ridge builds to the ne, its absolutely crucial that the energy splits from that main trough to the west. If you don't get enough energy heading under the ridge then I'll be manning the Emergency Trauma Helpline as hysteria breaks out in netweather!

Unfortunately Dan the 850's not quite cold enough to develop good convection off the North Sea. However at this timeframe we don't know exactly how much cold air will be advected into the flow as that trough splits, there is a chance though of some frontal backedge snow as that low drops south, we should still see lowish dew points brought in from the continent but any snow in that situation more likely away from windward coasts. Overall a very good ECM run but its going to be nailbiting stuff waiting for the trigger shortwave to be ejected off the main troughing to the west.

Emergency shortwave warning! Here we go with the ECM at 192hrs, if that heads se and the 192hrs verifies then given the profile to the north its a big change coming.

Its a shame we can't pin that quote to the thread! For newer members heres the verification stats for both the ECM and GFS in terms of anomaly correlations for the 12hrs runs. Day 6 ECM 0.867 GFS 0.832 Day 8 ECM 0.707 GFS 0.652 Day10 ECM 0.539 GFS 0.467 Given the steep drop off after day 6 would anyone like to hazard a guess as to just how woeful those verification stats would look like by day 12 and onwards for the GFS. Even if you are pretty sure of the overall pattern and expect higher latitude blocking to be in evidence given the teleconnective signals its impossible to say this far out temps in terms of 850s, what sort of flow you will have ie north/ne/nw which makes a big difference in terms of snow potential and also the exact location of any blocking. The GFS lower resolution is still largely seen as experimental by its own developers, and after day 10 you'd be better off pulling a scenario out of a hat, thats just as likely to be right as any of the nonsense the GFS comes up with in the latter reaches of its lower resolution output.

The UKMO and GEM look very similar at 144hrs they have a later phasing of low pressure in the west Atlantic, theres less margin for error though and we would need a shortwave ejected under the ridge afterwards. The much less risky option is the higher resolution GFS, here because of the stronger ridge ahead and earlier phasing and amplified trough any shortwave is likely to be ejected se'wards. It really depends how lucky you feel! The UKMO could develop better longer term than the GFS but it comes with that added risk.

So far so good on the GFS very similar to the 06hrs run. The UKMO I'm not liking as much as it takes far too long to phase those two lows, it eventually gets there but lower margin for error. We really need that phasing and amplification to drive lower pressure se ahead of the ridge.

What do you think? Looking at that I was wondering whether you had infiltrated the ECM ensembles! I should explain for people not acquainted with that chart, thats the ECM ensemble spread, from that we can see a clustering of solutions showing higher pressure in Greenland, high pressure stretching nw Russia towards Alaska this is the cross polar flow. It's a nice chart if you're looking for colder conditions in the UK.

It looks like the ECM 00hrs operational run is unlikely to verify as its out on its own regarding events in the east Pacific, we should expect to see a positive anomaly near the Bering Strait and a cross polar flow stretching from there towards nw Russia. The ECM ensemble mean has this aswell as a large cluster of ensemble solutions. If you take into account the comments from NOAA aswell as my new favourite toy those aviations forecasts from NOAA then the ECM operational run has got the wrong solution to our north. NOAA comments: THE MOST COMMON ELEMENT OF D+8 MULTI-DAY MEANS FROM AVBL MDLS/ENSEMBLE MEANS IS A STG POSITIVE HGT ANOMALY CENTER NEAR THE BERING STRAIT... WITH TELECONNECTIONS SUPPORTING RELATIVELY LOW AMPLITUDE FLOW OVER THE CONUS. INTERESTINGLY IT IS THE OPERATIONAL 00Z ECMWF THAT DOES NOT DEPICT THE PRONOUNCED ANOMALY CENTER WHILE IT WAS IN THE 12Z/18 RUN. THIS SUGGESTS LEANING MORE TOWARD THE 00Z GFS AND 00Z GEFS/ECMWF ENSEMBLE MEANS ESPECIALLY BY DAY 7. Aviation comments: HIGH LAT BLOCKING IS BEGINNING TO TAKE PLACE IN THE KARA SEA AREA AND IS LIKELY TO BRIDGE WITH THE EASTERN PACIFIC RIDGE...ALLOWING FOR DEVELOPMENT OF MORE CROSS POLAR FLOW AND COLDER TEMPS FOR MUCH OF CONUS.

They're unlikely to say much for the timebeing as theres still alot of uncertainty in terms of how the pattern could progress. An easterly flow which could occur at the start is unlikely to be too troublesome for the general public as theres not the depth of cold available to the east to cause good convection over the North Sea, its whether the pattern retrogresses with a trough dropping into Scandi and high pressure to the nw, thats when theres more potential for snow.

This from the NOAA aviation longer term discussion: RETURN TO WARMER WEATHER HEADING INTO SUN AND MON...BUT DO NOT THINK IT WILL LAST ALL THAT LONG THIS TIME. HIGH LAT BLOCKING IS BEGINNING TO TAKE PLACE IN THE KARA SEA AREA AND IS LIKELY TO BRIDGE WITH THE EASTERN PACIFIC RIDGE...ALLOWING FOR DEVELOPMENT OF MORE CROSS POLAR FLOW AND COLDER TEMPS FOR MUCH OF CONUS.

Model convergence continues this morning and we're seeing the foundations laid for a pattern change. The ECM still looks a bit more risky in terms of evolution: You can see here if you look to the west in the mid Atlantic with that troughing, you'll see that bump in the isobars thats the beginning of a weakish shortwave development. Then here you can see the forcing on this trough suggesting energy from this is going to split and and head east. Here you can see that piece of energy has headed east as the ridge builds to the north, the shortwave is that bump to the sw of England. At this point you can celebrate! The problem with the ECM is the low margin for error, hence I really don't want its evolution to verify! Now if you look at the GFS 06hrs run at 168hrs you can see already a stronger ridge ahead of that troughing and even though this later ejects a shortwave se this has a much higher margin for error as the shortwave really has to go under the ridge. Looking at the ECM ensemble maps still quite a variation at 168hrs but quite a few there show an altogether less stressful way forward. http://www.ecmwf.int...t!2009112700!!/ In terms of the extended ensembles those suggest to me that any easterly flow is likely to be shortish with the likelihood of troughing to the east and some retrogression thereafter, the main cluster of dew points and the temp profiles suggest a more onshore flow ie north/ne. http://www.weerplaza...?type=eps_pluim So far today we can tick off certain key factors that are likely to verify: Cross polar flow into the USA amplifying upstream pattern. Phasing of two lows one coming out of the Great Lakes region another moving north out of Bermuda. Some amplification of this newly formed trough. These are the uncertainties Degree of amplification of that trough which effects sharpness of the ridge ahead Pressure rises to the NE How quickly the limpet trough weakens and splits We should have a better idea on these 3 this evening, for the timebeing we have made some progress to a pattern change but still need to see whether it will be the traumatic looking ECM evolution or the less fraught GFS trend.

I think if we look at all the output this evening the importance of events off the eastern USA becomes apparent: As the cross polar flow develops in the USA this will force more amplification ahead, you can see that here: The low near the Great Lakes moves east and this will engage another low off the eastern USA coast: This phases into this large trough, the key is that we must see these phase and amplify sufficiently, as this amplifies it will help carve the ridge ahead. We actually do have broad agreement across the big 3 models at 144hrs. The ECM 12hrs, UKMO and now GFS 18hrs all have the same key features and phase those lows. The differences are really to do with the shape and depth of the troughing near the UK not the overall pattern. I think at this point its fair to assume that we at least have found some convergence between the models. After this its where the differences are likely to continue for a while as the models decide on how much that trough amplifies and whether we'll see pressure rises to the ne to help put some forcing on that limpet trough. So I think we've made a small step forward this evening but still a long way to go. I'll add these comments from NOAA this evening, they seem pretty happy with the consensus here: THE GENERAL FLOW PATTERN IS EXPECTED TO RE-AMPLIFY THIS PERIOD, WITH TROUGHING BECOMING REESTABLISHED OFFSHORE THE WEST COAST, RIDGING DEVELOPING ACROSS THE ROCKIES, WITH DOWNSTREAM TROUGHING ACROSS THE MIDWEST/GREAT LAKES/APPALACHIANS LURING A SLOW-MOVING DEEP CYCLONE FROM THE NEIGHBORHOOD OF BERMUDA TO ATLANTIC CANADA. ENOUGH SEPARATION EXISTS BETWEEN WEAKENING TROUGHING ACROSS NUNAVUT AND A SYSTEM CROSSING NEAR THE CENTRAL US/CANADIAN BORDER TO HELP ALLOW THE BORDER SYSTEM TO CLOSE OFF ALOFT. THE GUIDANCE GENERALLY AGREES ON THIS SCENARIO

Looks like the GFS is going on yet another route here! Really the differences between GFS runs at only 120hrs are laughable,compare this to the earlier 12hrs over the eastern USA, unbelievable! This does look closer to the UKMO so maybe this is a good sign! Here we see the phasing of those two lows upstream and we have a mini northern Scandi high popping up aswell!

Yes they both get there eventually but its far too exhausting! Indeed given some of the previous winter dramas in here it will be like last netweather member standing, whilst these hardy members are out sledging the rest will be treated for nervous exhaustion! I have a grading system for cold evolutions and this has sirens and red flashing lights! Why can't we just order the BOM for tomorrow at least that removes the troughing in good time, overall I'd say the UKMO at 144hrs is the best of the bigger models for that timeframe. Because the heights aren't so low to the north we might see a quicker evolution to cold and not have to put up with the masochistic GFS and ECM!

The ECM upto 192hrs is very similar to the mornings 00hrs run, it gets to some interest after that eventually but its painful to watch. Realistically so much has to go right here especially with that Azores high lurking that the chances of success just taking this run at face value is IMO close to zero. The margins are so narrow here that this run has more pitfalls than many colder evolutions I've seen on the net, for this reason we can only hope that the models plot a more straightforward evolution. There are some more positives upstream and with the continued Aleutian ridge but overall the ECM picks the most nerve shredding route to cold, indeed if we had sit through this evolution counting down the days I fear for the sanity of some members!

Lol! Sorry I nodded off after having to endure the GFS run! As Chiono mentioned the UKMO is the cleaner run in terms of how we move to a pattern change,the area to keep an eye on is the eastern USA we want those two lows to phase and this would seem likely as that cross polar flow into the USA amplifies the flow ahead this should help sharpen that upstream troughing. If you follow the UKMO on the NH you can see that cross polar flow developing into the USA and the pattern being pulled west to the north at the same time that more amplified flow kicks out a low which combines with the other low in the west Atlantic.

Great UKMO at 144hrs with phasing of those two lows off the eastern USA combining into an amplified trough, if that went further we'd see pressure rising quickly to the nw, as for the GFS 12hrs so far underwhelming!