pyrotech

The jet stream is driven by exactly the same symnotics that have the greatest uncertainty, The polar Vortex being disrupted will influence that rather acutely and so it could end up much further South and weaker than shown, which will make the outcome accross the troposphere very different. I expect huge uncertainty and inconsistancy in the models especially beyond 4-5 days to continue.

Alot to be decided yet, that warm sector looks alot different on the Surface pressure charts. The cold front occludes with the warm front, watering down the temperature. This could still become a snow Event right accross wales from the west Coast to the East Borders. Plenty of factors yet and looking at latest GFS it seems to think the same. Nothing is nailed for Monday yet.

I am looking forward to your alps info tomorrow. Regarding the NAE yes less percipitation, but in my opinion i would rather have less but as snow than more but as rain, which is what it shows. Evaporative cooling is only good if its very marginal and then turning it to snow, but i would have thought that and the other variables required were already factored in. Its a little on preference i suppose, but for some thats an earlier and longer spell of snow that brings the magic for some a little sooner. For you in Swansea and myself in Cardiff evaporative cooling might have been enough later in the evening to give us something white and so in that respect you have a valid point.

Hi again Jackone Looking at the NAE is see a fairly large upgrade between rain or snow at 12z Saturday, Look at the 12z NAE at 48hrs and Mid and East Wales is all rain, on the 18Z 42hrs its changed to a large snow area. Thats a positive change and lets hope it improves further, although we do know percipitation Rain/snow should not be worried about to much betond 24 hours, but it shows to me that they are upgrading and its a trend i hope continues. Got a mate just gone today sking in the French Alps, I think he might get more than he bargained for too.

TEITS i agree thats what the charts show now, but we should make sure newcomers are aware that all thats important at that timescale is likelyhood of cold and likelihood of percipitation, we know that even saturdays event will change for Snow/Sleet/Rain and extent North or South.Further into the week the will it snow IMBY will change even more, personally i expect upgrades as the cold air digs in and it being denser than the Modified Atlantic air, however to be accurate it could alter either way. Very positive run from GFS and some cross model agreement makes next week much more likely.

Difference by Wednesday between 6z and 12Z GFS 850MB temps Most of us on here would have banked this run, if we had never seen the ECM, after the rocky start to winter these alone are very good charts and a huge back track by GFS, So even if ECM walked all the way to this chart i would be happy. Although i still believe GFS will walk more to ECM anyway. So much potential in Fi also. SSW not had full effect by any means so perhaps much more to come.

Gfs is also well known for finding a trend and dropping it closer to timeframe and then bringing it back at last minute. I am certainly not worried by this change in GFS . I personally find the 06z the worst run anyway but the Gfs is very volatile and normally seems to have covered every possable outcome in fi before it gets to reliable timeframe. Its a useful tool for searching for trends but on its own its unreliable. I suggest the trend over last few days is cold, it has now dropped that only to bring it back again soon.

I can not say thats not written somewhere, but generally a SSW increases the likelihood of a Cold outcome for the UK but does not guarantee a cold outcome. It just increases the chance. I have now added a quote from Met office, if any one needs a link let me know - "We normally expect our weather to come in from the west – with a flow of relatively mild air coming in off the Atlantic. When an SSW brings easterly winds this tends to alter our weather patterns slightly, weakening areas of low pressure and moving our jet stream further south. This leads to high pressure over the North Atlantic, ‘blocking’ that flow of mild Atlantic air and dragging in cold air from the continent to the east. Exactly how cold it might be depends on the details of where the air comes from. SSWs don’t always result in this outcome – but a cold snap follows more often than not, so the SSW greatly increases the risk of wintry weather."

Historically really does not matter here as the atmosphere is not in an average state, we do have an SSW ongoing propogating through the layers, When it reaches the troposphere it will effect blocking and shortwaves. Blocking becomes more likely, shortwaves shallower. FACT However that does not mean a shortwave can not form and spoil this for us, but i would suggest that it very likely GFS without the extra baloon data has missed a key part of the puzzle. If GFS is correct then blocking is again over Russia and we would possibly be in a pre xmas state and flooding an issue.

Shortwave, = low pressure outlier is when a operational run, or control run is against the other runs. Gfs might run 20 different scenarios with different starting data by day four they are normally going in very different directions, an outlier is one that goes totally away from most of the other runs(members) Toppler, this is where we get a brief Northerly that is disrupted by high pressure spoiling it.

Hi Ian, yes i understand that, but is it the thickness and WB thats giving doubts about anything further South, i would also expect the line to move south as the day goes on and increase with elevation. Its just looking at GFS and your comments the two factors above are what i saw as main forecasted difference between the outputs

Not quite sure why GFS is so keen to bring in shortwaves, i am certainly not saying its wrong as it may yet be right, however as a SSW get underway and influences the troposphere, blocking is more evident and shortwaves will have more dificulty in forming and certainly less deep. So its with reason that i say this is a less likely outcome. I would suggest that as the effects propogate down through the atmosphere, anomalies like this will become evident, for now i will stick with the ECM, as GFS goes against the known effects of a SSW event. ?But we shall see

Looking at the Meto surface pressure charts and ECM the difference is the thickness, 528 Dam runs Very top of NE Wales to Norfolk diagonally just about clipping Birmingham so if data is correct that must be the reason. Alot can change in 3 days thou. Plus im guessing thats the difference they see, it seems the most obvious one to me.

Yes surprises can happen, especially if we get a layer of dry air that would allow some evaporative cooling, i would expect Meto to have taken this into account thou. Saturday looks interesting to me as the band moves thru, looks like a possible North of the M4 hill snow event, ( not good for me they built the dam M4 a few miles to far North) Seems Meto do not agree. Upper air is borderline and if Low is slightly further North then unlikely. But i suggest what Meto see now is most likely event with slight upgrade or downgrade as we move forwards. However if the GFS and ECM have that right, which Meto think they do in general, then there is huge hope that FI is also correct somewhat. It stands to reason that if that low was completely out of place, that all the run after would be wrong, but it seems that far the consistancy is now getting agreed, just the fine details are being debated by various models.

Why are some people so worried about Ensembles in FI, huge agreement to 15th on them of COLD. Percipitation also with -5 to -8 uppers Then a total mess afterwards but thats Fi , and alot to play for, nothing to say the op after that is somewhat correct or not, but even then and throughout the mean stays below zero, a huge positive but yes uncertainty. But for the reliable frame it looks very good even though we need things on track by thursday to get there.

Caution is the word of the day the mogreps says westerly influence and Ecm. moving in that direction. That does not mean game over,but is a warning as mogreps was first to ut us zonal rather than cold five weeks ago. i hope its wrong but this has possible issues for areas where water table is still high as we could repeat that pattern. i am not being negative just realistic. To much inconsistancy at the moment but right now the new trend is wrong way, but that can change. Mogreps later and 12z Ecm will hopefully change the trend.sorry done this on phone

There is no bias in the models, they are number crunchers and at 3 days they are pretty good. problem we have is that most of our weather is driven by factors that make it west based. Unfortunately alot of the time those factors are in place, so the models only need a few data inaccuracies or programming errors ( there would be many bugs in programs of that size) to send them towards a zonal picture. The best news is, what ever bugs or inaccuracies it does not control what will happen. Basically if there was a reset button for the weather here, it would reset to zonal due to these overwhelming factors, Now and then other factors overwhelm them.

By the 13th many changes, 2m temps better, -5 850mb temps further west, and chances of sleet and snow increased. Prior the two lows are less deep and so allow the cold air to push westwards Everything to be positive about, although just one run the trend in semi reliable frame is better

Hi again to all Here is what is causing a headache for forecasters regarding next weekend, obviously what happens then will determine what happens after This is what meto are quoting "the models are having a hard time with Thu & Fri forecast in speed of Atlantic front. This crucial, slower front =cold" So we need to be watching how that evolves The NAO is forecast to go negative later next week, before slightly positive before. I would envisage that as it moves briefly positive that some models will continue that theme rather than the more likely return to negative. That will of course send the models on very different runs So just watch how that front behaves and look to see if a model tries to amplify the NAO or not, if it does it probably on the wrong track

Trouble is everyone is jumping on each run, How often have you been told look for trend. right now no one knows how the vortex will end up, with that how the blocking will sit. Further more that will have an effect on the NAO, which is reliably set to go negative after a brief weak positive step early this week. The Meto have given two different forecasts today, one by Chris Fawkes and one by Matt Taylor They normally do 5 days forecast, as we know thats the normal time for reasonable model anylasis to be somewhat correct. They both decided to talk about cold and possible snow into next weekend, yes with some uncertainty. However they would have to have had a briefing from senior forecasters, so the model we do not get to see must be throwing something interesting into the mix. Fergie on here, a respected member and official forecaster said over the last week that we will see models flip, they have and will continue to do so. Why because the polar vortex will effect so many other conditions. The trend is colder, many runs, ensembles and accross models, with as always some anomalies thrown in. I would expect something to be decided about next weekend around Wednesday, but right now there is alot to be happy about. The GFS 18z may come up with a blow torch scenario or freezer scenario but its not to be worried about either way, its unable to correctly forecast these vast scientific equations with out the split in vortex completed and stabilised to a uniform consistant phase.

Sorry never saved end of first line, differences between ECM Fax and GFS The Models are struggling with this senario which should be no surprise to us on here. However the GFS seems to be behind the ECM on here. Its not a light the cigar time, but certainly the trend is for colder and the models just not sure how to get there. Scandi high looks most likely evolution and that is very positive.

The signals are there, without doubt. Problem is how the vortex splits and where blocking forms and builds. Some places in NH will see a very cold period, i will put my name on that, problem can be if the building blocks are in wrong place we can easily lose out. I will try to explain it We have a truck traveling along M4 we are going to ask it to stop The truck is the PV, like the truck we know it will stop or split in case of Pv. We know somewhere on NH blocking will form due to disrupted PV, same with truck it will stop between where it is now and somewhere between its current position and mile ahead. With the truck we now have unknowns 1. thinking distance ( time to apply brakes) time SSW takes to disrupt vortex 2 tyre tread ( grip to stop truck) orientation of vortex when split 3. weight of truck - The amount of warming propogated to surface 4, drivers urge to stop ( pressure on brakes) Is it a constant wave, or multiwave So with the truck we know it will stop, we can estimate between point A and B , where we know it will not be emediate so we move our starting point slightly away from A As we see the truck begin to stop we can get a better idea, however we still can not predict fully, because the driver has ability to change all the constants by easing on or off the brakes. That can be a second or third wave, so as you can see until that truck begins to stop its dificult to predict stopping distance, even then other drivers can influence changes along the way. Thats why models are struggling, but two models showing cold may mean that they have some constants ruled in now and getting suggestions on where this will end up, but i see both models using differnt formulas for this truck right now, however they both get it to within a few metres. If it stops in wrong place blocking could cause a zonal cold spell or blocked Zonal for us with Low pressure stuck to our west driving mild air and rain, if it stops in right place we get the cold we dream of.

I agree that they are the coldest runs and maybe outliers However they are the runs based on actual changes and reading, without modification. So maybe in this unusual senario they are closer to the mark, i say this because we look at the other members, which assume different readings due to inaccuracy over an area. So just may be the changes happening in the atmosphere are showing and due to PV are fairly acute, but modification would make them more or less acute. This is where i would expect the operational to be closer to the mark even as an outlier, thats if the changes are happening already in a way that the data can be collected for the computers. With such wide variations happening i would expect the computers to under estimate as its against the norm, We could see the outlier operational also under estimated, if it is then again the final outcome would be very different, and not nessisarily in a good way. For the models an inch wrong today is a mile by tomorrow. But the pub run by gfs along with ECM 12z is very good, but better if they got there same way.

Whats important here with these fantastic charts by ECM, after the dismal charts by GFS is the inconsistency of the models. This is why seasoned members have said stay patient. The Ecm has a fair chance of verifying, however it has a chance equally of being very much out. The SSW and resulting disruption to polar vortex will throw out many possabilities as its ongoing situation, The ECM is a more accurate model but it would be interesting to see what Meto make of this run. I think it would probably cause some chatter like on here, but like us they will want some consistancy now either by ECM or accross models. I am as excited as anyone by the ECM run but without more consistancy i am worried about the fall, i would take it as one possible outcome that may resolve as something similar. Its nice to see the models manage to get to that position, but we are along way from lighting the cigar. Although having written that, looking at the way it gets there, perhaps i will start looking for my lighter.

Some one said Hello Ian, I must admit, I am a little suprised by your apparent dismissive attitude to solar activity. I understand the arguments that this may be more appropriate in another thread but at the end of the day, everything is inter related in this science. How many times have people mentioned the strat over the last week or so in this thread? This has been mentioned because of it's obvious effects on our weather and is understood more than solar activity but I would argue that the sun probably affects our weather more so. solar flares might be huge compared to earth, but they have very little energy in comparrison to what leaves the sun every second. The energy that does reach us is taken in the ionsphere and not effect the troposphere at all, Cumalatively there is some effect but anyone who thinks one flare will make or break our forecasted possible cold spell is living in fi. CME'S will not have huge effect either if they were to occur. CME and flares will effect the very outer parts of the atmosphere, beyond stratosphere with an aurora visible. Long term solar cycles can cause cooling and warming but we are really looking at a longterm trend. The current run of very active sun until cycle 23 was in my opinion a part of climate warming. But flares are common and alot of study has been done without evidence that a flare can cause any pattern change. Does the sun effect our weather ? of course it does, its the main factor, every day, but not with a single solar flare.