nick sussex

An okay run from the GFS but still that trough refuses to tilt favourably and we have to wait for it to split and weaken. The GEM looks similar in pattern at 144hrs, the UKMO once again underwhelms me with a flatter upstream pattern and the troughing further east, although the UKMO in recent days have been modifying their raw data in the fax charts which suggests they think their operational output isn't up to much. We can only hope that this is the case as it looks like a long road to anything colder on that output. Having just checked the latest verification stats they show the UKMO performing woefully in recent days, indeed at this rate it could be keeping the NOGAPS company!

Welcome back WE, what you need is some good cold synoptics and snow to cheer you up. Lets hope this evenings outputs get a move on and we see that limpet troughing blasted into the galaxy with the help of a ridge to the nw and at least initially one to the ne aswell!

I'm trying to find a good example to explain this but in the meantime: http://www.theweatherprediction.com/blocking/ If you look at the first diagram Omega block, and lets use that to discuss the troughing. The left hand L lets say is our eastern USA trough, the right L is the limpet trough near the UK. We need to put some forcing on that trough to eject energy se'wards and at the same time develop a ridge to the nw to block the Atlantic. Basically the steeper ascent ahead of the left L will lead to more digging south of energy where the right L is, in this instance because we have high pressure to the east of the right L as the energy heads south/se this will force the ridge ahead nw. In culmination we develop a strong ridge to the nw and at the same time develop a ridge to the ne the trough then has nowhere to go but south or southeast. Remember also that to develop decent WAA warm air advection which helps build ridges ahead of troughs then we go back to the much talked about negative tilt to the trough. Think of it another way low pressure represents a bundle of energy if you have a positive tilt ie running ne/sw then the energy eventually falls over the top of a ridge flattening it, if you have the tilt nw/se then where does the energy go it falls back and underneath not over the ridge. You will never see an undercut of energy with a positive tilt trough, at worst you have to have at least neutral but really some negativity to the tilt. This is why I tend to drone on about tilted troughs in the winter, hopefully people can see why now!

Yes this makes a big difference to the outlook and for fear of turning into an eastern USA trough obsessive we really need the upstream pattern to remain amplified. As you mentioned this will carve out a better ridge to the nw but also serves to take energy se'wards towards Africa, this will help tilt the trough more negatively and then we can end up with two high pressure cells one to the nw and to the ne, in culmination this will help sink the limpet trough. There is still some uncertainty though with that trough in the west Atlantic so we'll have to see what the final solution is there.

More complications today with that eastern USA troughing imploding, the ECM still gets to some interest eventually but we really needed this troughing to hold out to carve more amplification ahead of it. The limpet trough near the UK takes an age to fill and refuses to tilt negatively, the GFS in lower resolution is a total waste of time, each output pulls out a different scenario. There seems to be alot of concern in here over whether an easterly will be toothless, I'd get there and then start worrying, its possible to get two high pressure cells one to the ne and one to the nw, Also the UKMO at 144hrs is very progressive and very underwhelming in terms of how it could develop so for the timebeing I'd be more worried about getting enough energy to head se and whether the trough will sink away. The ECM is like a stuck record playing the same old track at 240hrs for the last few days, we really need to see things get a move on and the better synoptics to be inching closer rather than remaining the pot of gold at the end of the rainbow!

The limpet trough is beginning to outstay its welcome! But as you can see from the vast differences between this GFS run and the earlier 12hrs one wonders why we even bother to look at the lower resolution output of the GFS. At least the models agree on the trough after that its very uncertain as to what might happen. I think its a case of being patient and seeing how the models deal with the upstream pattern and whether the trough does eventually sink se'wards or whether it just sits there and drives us all crazy!

Further info regarding that eastern USA troughing, this from the extended forecast discussion at NOAA: SOME NOTABLE SHIFTS IN THE GUIDANCE TOOK PLACE ALTHOUGH THE 12Z GFS SHOWED LITTLE CHANGE FROM THE PREVIOUS RUN. IT APPEARS THE 12Z ECMWF CONTAINED THE MOST DIFFERENCES FROM ITS 00Z RUN WITH A TRACK OF THE COASTAL LOW FURTHER OFFSHORE. ADDITIONALLY...IT APPEARS TO HAVE SLOWED IN ITS D4 TROF PROGRESSION THRU THE WRN U.S. WHICH COMPARES MORE FAVORABLY TO THE 00Z ENS MEANS. MEANWHILE....THE ECMWF TRENDED AWAY FROM ITS MORE ZONAL FLOW ALOFT BY D6/D7 OVER THE WEST WITH MORE AMPLIFICATION...ALBEIT WITH DIFFERENT PLACEMENT RELATIVE TO THE GFS. WHILE THE UKMET MAINTAINED ITS CONTINUITY...THE 12Z CMC CONTINUED ITS POOR RUN-TO-RUN CONTINUITY NOW TRACKING THE WRN ATLC LOW CENTER FURTHER OFF THE COAST. GIVEN ENOUGH SPREAD IN THE ENS LOW STAMPS FOR THIS LOW CENTER...NO CHANGE WAS MADE AS FURTHER WAFFLING SHOULD BE POSSIBLE IN FURTHER RUNS. THE PREFERENCES/PROGS WERE NOT ALTERED SIGNIFICANTLY RELATIVE TO THE MORNING ISSUANCE. Until theres agreement on the upstream pattern then we won't know really what impacts that will have on the troughing near the UK.

If there was a 264hrs chart the shortwave as it heads east/se will help pull the main trough nw at the top allowing the high to ridge further west and nw. Anyway this is all conjecture because if the ECM has got that upstream trough modelled incorrectly then the rest of its output is wrong. The GFS keeps the amplified eastern USA trough and that follows on from its earlier 06hrs aswell as the 00hrs ECM so we need to see what happens with that. I'll be happier when the key synoptics come into the range of the UKMO.

The ECM 12hrs run is vastly different upstream from its earlier 00hrs run , especially if you look at that eastern USA troughing. This is important for the downstream amplification, here the ECM flattens that trough and then throws a few crumbs out at the end to keep us all interested. The 240hrs chart is nailed on for an easterly at 264hrs as that shortwave is likely to head under the block and there are already lower heights in the Med but really I think the models are pretty much clueless as to what they do with that troughing near the UK. But we have to keep that upstream amplification for long enough to help carve the jet se towards Africa, the key is to get enough energy heading se to allow the ridge to back west over the top. Overall a messy picture and certainly not worth getting the ear muffs and woolie scarves out until we see what happens upstream and with that limpet trough near the UK.

I missed the GFS 00hrs run but apparently it was rather underwhelming. The GFS 06hrs run looks similar in pattern to the ECM 00hrs which once again weakens that trough sinking it se'wards. At the moment its a bit of a stalemate, we've yet to see what happens with that troughing near the UK in the more reliable timeframe, there are quite a few variables at play, one of them being troughing in the eastern USA which helps the downstream pattern. Because we're trying to get that trough to dig further south that eastern USA trough is crucial, the ECM amplifies this trough which carves a ridge ahead of it and amplifies the upstream pattern. .

What a surprise as soon as the GFS hits the lower resolution output it decides to suddenly change the tilt of the trough. Earlier it took a different route but the end result was still the jet heading towards Africa so this signal does look strong. Certainly the trough near or to the west of the UK looks nailed on, its just what happens after that which is still uncertain.

Given how bullish GP is about the outlook expect Sledgerama to be inundated soon by netweather members! Personally I'd like to see a ne first then northerly and thats something that this mornings ECM hinted at. We can get both so lets hope the ECM delivers a nice run when it comes out.

It all goes downhill in the lower resolution part of the GFS, the earlier timeframe was fine. The UKMO however looks progressive with the pattern further east, normally the GFS 12hrs would be doing this but the UKMO makes even more of that low running east into the main trough.

Crucial part of the run coming up, this looks very good so far with the trough pivoting and about to split. Also the pattern is further west, another good sign.

This NH view of the GFS 06hrs run at 192hrs gives a good idea of what the pattern is likely to be around that time. Different models will probably have that troughing near the UK a little further east or west. If you look here you can see the PV is split into separate pieces. The jet stream off the USA has dug well south into the Atlantic. The key here is really how far south that jet stream digs and how much energy from the jet goes into the southern arm of this. This troughing is likely to spawn small features at the base which will follow the isobars anti clockwise, in undercutting scenarios one of these will eventually be forced east or se, generally as this happens the trough will weaken, if you're looking for the undercut you want to see this troughing become more elongated. A crucial factor is the tilt of this troughing, you have to have preferably a negative tilt to this ie running from nw/se, this is especially important when you're trying to develop a ridge ahead of this trough. For newbies I recommend you look up WAA warm air advection, this process helps to develop high pressure to the east of troughing this is a key component of many synoptics that we need to deliver colder conditions to the UK. In terms of this evenings output in a nutshell west is best, the further west the troughing is the better chance of a successful outcome if you're looking for a change to colder conditions, also the jet stream off the eastern USA, the more this digs south again the better chance. In this type of set up the ensembles as I mentioned earlier follow the operational, they don't lead the way. Undercutting scenarios are very difficult to pin down at later timeframes, hence you will often find the ensembles dragging their feet and only coming on board relatively late. I wouldn't worry about FI too much for the timebeing, keep an eye on the tilt of the troughing and where its located, the rest will likely follow if those two things are favourably placed.

Its quite interesting if you view the GFS 06hrs run from the NH, you can see how the jet stream digs further south off the east coast of the USA this helps pivot the trough and force some ridging further north ahead of it. Overall theres very good agreement on troughing near the UK, this is likely to verify but still question marks on how far east the pattern will be and how much energy goes into the southern arm of the jet. In terms of the ensembles of both the GEFS and ECM I wouldn't read too much into these because undercutting scenarios are poorly handled and even more so by the lower resolution ensembles.

JS I can understand why you made that post and it's easy to become impatient when you see that trough just sitting there to the west of the UK. To answer your points. 1. Continental Europe cools rapidly at this time of year under the right synoptics. 2. Proper Scandi highs can appear at anytime during the winter. 3. Models do not model Scandi highs at all well,but if you look historically at the 240hrs charts of both the GFS and ECM thats more likely to be followed by a build of pressure from the east. 4. In terms of the stratosphere cooling rapidly, bear in mind there are other factors at play that will effect that, too long to mention here but the strat thread gives a more detailed explanation. 5. The Meto long range is just that and they're not exactly jumping ship to mild zonal dross. 6. The CFS IMO is a pile of rubbish that needs to find its way to the nearest landfill site. I think most in here would love to see a quick change to colder conditions but its not going to happen like that, the troughing will sit there for a while but if theres one positive to take its the PV split, I'd join you in getting worried if we saw one large deep PV limpeted to Greenland. Hopefully we'll see some changes past 240hrs to a colder outlook as that troughing by then should start to weaken and the output should look better in the coming days! Hope I haven't jinxed things now!

Looking at the upstream pattern heres something to factor in to the output which could effect troughing near the UK: From the morning discussion at NOAA: MOST OF NOT ALL THE VARIANCE IN THE MEDIUM RANGE FORECAST BEGINS TO DEVELOP IN THE DAY7-8 PERIOD AND IS A DIRECT RESULT OF PERSISTENCE OF THE UPPER TROUGH IN THE WEST...COUPLED WITH AN EMERGING CLUSTER OF SOLUTIONS FOR UPPER TROUGHING ALONG THE EAST COAST. The amplitude of that troughing in the eastern USA will effect the downstream pattern, no mention yet by NOAA of how amplified any troughing might be over there, but certainly something to keep an eye on if it verifies. Also the track and depth of that low exiting the eastern USA around 120hrs makes a difference, this interacts with the main trough to the west, preferably a weaker feature with a more southerly track.

Interesting control run from the ECM: http://www.weerplaza.nl/15daagseverwachting/?type=eps_pluim If you look at the rainfall totals its looking likely that any low pressure won't make much inroads into continental Europe. Todays ECM operational run gets to the same end point as yesterdays 12hrs, so the ECM at least wants to keep putting energy into the southern arm of the jet. This really is key because if it keeps doing this you will eventualy see an undercut,

As Steve M mentioned the good thing about this cold tease is that its not a case of hoping that something well into FI survives intact. Here its all down to events within 144hrs, theres no chance of getting close to the ECM if we don't see that ridge holding in the east. Generally the models are poor at handling undercutting scenarios but you still need certain things in place to begin with.Theres no grey here, its black or white you either get the undercut or the energy piles over the top sinking the high

Unfortunately it all goes downhill after that with that low powering up! At the moment the ECM seems out on its own with its stronger ridge within 144hrs. Still I'd rather have the ECM on its own than the GFS given its track record with high pressure in Europe. We'll see tomorrow morning whether theres any mileage in the ECM.

The GFS 18hrs has pulled troughing further west than the earlier 12hrs. Still not as good as the ECM , this looks more like the UKMO so perhaps this might give us a clue as to how that could develop if it went further than 144hrs.

Thanks I've come out of hibernation ! In terms of the BOM although its based on the UKMO model it has a lower resolution so best take that into account. I should add a note of caution to the ECM, to get to 240hrs you need a good starting position within 144hrs re how it holds troughing further west with less energy into Scandi. The UKMO isn't as good although much better than the GFS, we'll have to see whether those pull the pattern further west tomorrow.

The ECM at 240hrs is the best output of the season so far in terms of wintry potential. I'd gladly take that if offered as that trough disrupts further you'd see more energy transferred into Iberia with the ridge backing west. Unfortunately its at 240hrs so has a very low probability of verifying. There are some key things though to watch in future outputs and these are within 144hrs. The starting position of any ridging to the east in a nutshell the further north and west has big effects later, also that low heading out of the eastern USA the further south this is at interacts with the main troughing, and finally tilt of troughing. For newbies this means the orientation is nw/se this helps to develop the ridge to the east by means of WAA warm air advection, the key thing is to get enough energy heading se into Iberia and the Med as this happens pressure fills out to the north and nw. This may well be a false dawn and just a crumb by the ECM to keep us interested but given the current stratosphere profile I can't see any route to cold at least initially unless it comes from the east, after that things may look different.

Redemption possibly here comes the negatively tilted troughing! The 192hrs chart isn't too far from what the BOM cannon fodder model showed this morning, its essential we get this negative tilt to help ridging extend nw.