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Model Output Discussion - cold spell to end Autumn

Paul

Please only post model discussion in this thread. 

For more general chat and banter, or moans and ramps loosely around the models, please head to the banter thread:
https://www.netweather.tv/forum/topic/86721-model-moans-ramps-and-banter/

For general weather chat including about the snow/cold chances around the country, please go to the regional threads:
https://www.netweather.tv/forum/forum/142-regional-discussions/

Thank you!

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7 minutes ago, johnholmes said:

re this comment

Maybe the experts could refresh us on how sliders are modelled in regards to their biases? This might give us a hint of what to expect in a few days? 

No model has any bias, all are run with the mathematics to deal with the laws of physics as applied in meteorology. Lows and highs appear/disappear on this basis.

Indeed John. Though for some the laws of physics need not apply when viewing any model output when it shows something they’re not too keen on.

From years off model watching purely from a rank amateurs perspective these sort of set ups are never resolved until +24-48hrs, the genral theme of a low dropping on a SE axis is relatively straight forward once within these timescales but the boundary between rain and snow less so with so many variables that come into play. A great learning curve watching this unfold though.

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We know for certain that models have bias’. NOAA forecasters often reference the gfs/gefs ones when making their text assessments. They surely not intentional but they exist within the model.

as snowballz comments above, trying to rewrite the algorithms to avoid them would be a bigger job than just taking them into account when assessing the output. 

Joe B on weatherbell often shows the bias of the ec46 to bring the trough into the western coast of the USA. He says it’s to do with feedback loops because of the rockies but whatever it is, it happens quite a lot. 

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TBH though, Joe B could arguably do with having a few of his own algorithms rewritten?:D

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MORE ON SLIDERS AND WHAT TO LOOK OUT FOR

Further to my post late last night describing a slider event which I experienced back on January 9th 1968 (now buried on page 151) I see that there continues to be considerable confusion as to how a slider behaves. Now, whilst I am certainly not a technical expert, I do have considerable experience of slider events and I must have seen well over 100 of them in the last 55 years since I started studying weather maps every day. This included over 10 of them in the epic 1962-63 winter alone (which I described in my post on page 134).  I shall endeavour to demonstrate what to look for once any model run has shown one developing on their charts. I shall use the most current GFS 6z run as an example which is still churning out whilst I am writing this post.

Now I fully appreciate that next week’s sliders projected on most of the recent model runs may not materialise at all. It is possible that the LPs around the south of Greenland or in the mid-Atlantic may not break away and slide south-east and they can easily deepen and take a much more northerly route from west to east or even south-west to north-east – ie: not a slider event at all, with much milder conditions spilling across most or all of the UK. At this stage, this appears to be the least likely scenario, fortunately for the coldies on here. Even those models which show this flatter and milder pattern for next week generally return to renewed mid-Atlantic amplification and colder conditions later on in the run.

Some runs have shown successive slider events and others just a single event. Almost every run is at least slightly different with varying routes for the slider and these changes should be expected right up to T+24. This is not really due to major changes, although the models are generally struggling to nail down some of the broader pattern changes, let alone any detail. It’s because just a tiny change in exactly where the LP breaks away can make a big difference on the path that it takes. With the jet stream also changing direction slightly this can influence the path. Just minor changes in the position and strength of HPs and ridges adjacent to the LP can also impact on any changes down the line.

So, to keep things simple (for me as well as many of the less experienced readers), I shall pick things up from where one of these sliders has been predicted on the current GFS 6z model run (I started writing this 1130). First of all, let’s focus on the run from T+108 through to T+144 starting off with the surface pressure charts:

GFS 6z, December 5th -  Surface Pressure Charts:

                            T+108                                                 T+114                                                 T+120        

        gfs-0-108.png?6?6            gfs-0-114.png?6?6           gfs-0-120.png?6?6

                                      T+126                                                T+132                                                 T+144

        gfs-0-126.png?6?6            gfs-0-132.png?6?6           gfs-0-144.png?6?6

Going through this run you can see the slider breaking away from the LP south of Greenland and taking an east-south-easterly path, reaching Northern Ireland by T+120 and deepening only very slightly. It continues on this route and crosses northern England and has moved out into the North Sea by T+132 having continued to deepen very slightly. At this stage, it looks like much of central and southern England will be under the milder (or less cold) air to the south while Scotland should get some heavy snowfall (see later).  Now if we jump to T+144 the slider phases with the larger LP over Scandinavia and it deepens a little more and engages with some much colder air there and sweeps this across the UK in its rear. Having done its job as a slider, it then changes its behaviour and moves south into the southern North Sea maintaining central pressure.

Now let’s look at what is at least partially responsible for these changes, the meandering and buckling jet stream.

GFS 6z, December 5th - Jet Stream Charts:

                           T+108                                                   T+114                                                  T+120    

         gfs-5-108.png?6            gfs-5-114.png?6           gfs-5-120.png?6  

 

                            T+126                                                  T+132                                                  T+144

         gfs-5-126.png?6           gfs-5-132.png?6            gfs-5-144.png?6       

On this run at T+108, the GFS shows a branch of the jet stream running south-east from Greenland, across Iceland and through the UK. This is already being pushed away eastwards by a more powerful and more direct west to east flow seemingly set to blast right across the UK. By T+120 the jet is already showing signs of buckling, weakening slightly and moving further south. Remember, all the time, the slider LP is on the northern and cold side of the jet stream. The remnants of the jet that ran south-east from Greenland despite fragmenting and weakening are still having an impact in helping to push the main stream further south (it’s much more complicated than this not least because we are dealing with a 3 dimensional jet profile but I’m keeping it simple so that we can see what impacts the small changes in strength and direction of the jet have on the pressure patterns). From T+120 through to T+144 the jet has buckled further and, although broader and stronger, it’s meandering favourably for the UK now pushing well to the south of us. By the time the slider low has phased with the LP over Scandinavia, this allows the whole system to move southwards.

Next I move on to the 850 temperatures and I feel that these can give some of biggest clues to a slider's behaviour - this seems to be quite widely misinterpreted.

GFS 6z, December 5th- 850s Temperature Charts:

                             T+108                                              T+114                                                      T+120

         gfs-1-108.png?6?6           gfs-1-114.png?6?6            gfs-1-120.png?6?6 

   

                             T+126                                              T+132                                                      T+144

         gfs-1-126.png?6?6           gfs-1-132.png?6?6            gfs-1-144.png?6?6

We see that at T+108 the UK is widely under -8s with -4s to -6s further west. It looks like the area of 0s to +4s or even higher is about to push right across the UK but this is misleading. Focus on the narrow strand of +4s looping to the north-west. This is in the area where the slider LP is just beginning to breakaway east-south-eastwards. Look at the shape of these positive 850s, the northern arm is actually being lifted or squeezed out. By T+114 that northern strand is fragmenting and by T+120 you can see it being pushed away south and south-eastwards to the extent that in only 12 hours just southern England is likely to see positive 850s rather than the whole country. The slider is doing its job. It “does not engage with the mild air to the south” whilst it remains on this east-south-easterly track. It is actually starting to engage with the colder air ahead of it to its north and east. The only way this could change would be if the jet stream takes a more northerly route pushing the slider LP on to a more north-easterly path, in which case it would no longer be a slider and would take on the properties of a more typical Atlantic LP sucking increasingly milder air into its circulation.

As we have seen above, GFS project the jet stream to take a more southerly route. So what looks like a bad evolution for coldies is actually a very good one – perhaps a few of those posters who view each chart as they are churned out and start to moan about the apparent warmth about to swamp the UK can take note of this. Waiting for at least the next few charts and referring to the changes in the jet stream might lead some to change their view very quickly!  I say this in relation to this type of set up but it equally applies to any situation where a broader pattern change is indicated and seems likely.

Moving on to T+126 we see how far south of the UK the above 0c 850s have already been pushed. This shows that the segment of mild air will not get caught up in the slider system. Yes, it will drag the less cold air across the south but unlike a more typical Atlantic LP there will be very little mixing in the centre of the slider. It’s not all good news “at this stage” as we are left with mostly -2s to -4s over much of the UK with only the far north of Scotland under sub -8s. What we can see is that there is now a much wider area of sub -2c 850s not only ahead of the slider but also coming in from the west behind it. The reason why we only have extremely marginal 850s at this stage is due to the slider taking an “east-south-easterly” path – just a very small change to a more direct south-easterly path would maintain as well as suck in rather lower 850s (sub -4s or lower) from the north and east. Alternatively if the whole system was just a little further south (eg: taking a track say through the Midlands) a lot more cold air would be retained throughout with sub -6s and lower more generally (except in the far south). Please note that I am going by the 850s that GFS have been showing on this run which may actually be slightly higher or lower than those predicted even if this precise pattern verifies (pretty unlikely).

Going through T+144, we see that with the LP now phased with the LP over Scandinavia, that it’s starting to drag in rather colder air behind it – nothing remarkable at this stage but going in the right direction. A larger part of the UK is once again under -4s and there is a pool of sub -6s developing over north-west England. Remember some of these uppers are now being sourced from Scandinavia and north-west Europe with a much more continental influence. This will likely engage drier air with much lower dew points as well as far less modification from warmer seas. This should gradually result in favouring most marginal precipitation events to fall as snow with only around -4c 850s (or even slightly higher). Looking forward to T+192 (not shown) it looks like GFS wants to rinse and repeat the pattern.

Now a quick look at the surface temperatures for the same period.

GFS 6z, December 5th - 2m Surface Temperature Charts:

                            T+108                                                   T+114                                                  T+120        

             gfs-9-108.png?6                gfs-9-114.png?6               gfs-9-120.png?6

 

                             T+126                                                  T+132                                                 T+144

          gfs-9-126.png?6             gfs-9-132.png?6           gfs-9-144.png?6 

We start off with temperatures over most of the mainland UK between 0c and -4c. The milder air only gets into the far south and far west for just a few hours with up to 8c there. Most of England is closer to 4c and the far north and much of Scotland remain closer to 0c throughout. Towards the end of this period most parts are closer to 0c with night frosts returning. These values (+ the 850s) will be reflected in which areas get snow rather rain or sleet.

GFS 6z, December 5th - Precipitation Charts:

                                        T+108                                                  T+114                                                 T+120   

          gfs-2-108.png?6?6              gfs-2-114.png?6?6           gfs-2-120.png?6?6  

 

                              T+126                                                  T+132                                                 T+144

         gfs-2-126.png?6?6              gfs-2-132.png?6?6            gfs-2-144.png?6?6  

Now I selected this particular slider scenario as it doesn't really produce much in the way of snowfall at this stage of the run. I could have chosen one from another run which takes a path further south and leads to much greater snowfall but I did not wish to mislead anyone. The purpose of this exercise has been to point out the basics of what to look for and what to follow with any of these slider scenarios. The charts above start off at T+114 with the legacy of the earlier northerly and some snow showers in parts of the country. There is some snow in the east and north ahead of the slider with rain further south and west. This does give an idea of how much more wintry the precipitation might be with a very minor shift southwards in the pattern. By T+126 most of any snow in the east has turned to rain but there are significant snowfalls indicated for the far north of England and Scotland. Then look at what is going on further north. The slider is engaging with the colder air on its northern flank and dragging it southwards again in its rear. There is some widespread and quite heavy snowfall to the north of Scotland and the northern isles would be prone to large accumulations in this particular scenario. By T+132 the area of snow over Scotland is pushing southwards. The rain further south is clearing away too. By T+144 snow showers are shown down exposed west, north and east coasts.

GFS 6z, December 5th - Snowfall Accumulation Charts:

                              T+108                                                  T+114                                                T+120    

          gfs-16-108.png?6             gfs-16-114.png?6            gfs-16-120.png?6 

 

                              T+126                                                   T+132                                               T+144

           gfs-16-126.png?6            gfs-16-132.png?6             gfs-16-144.png?6 

Snow accumulation charts are notoriously unreliable and this exercise actually reveals some of this. We start off with the residual accumulations shown from any snow from the earlier northerly. Given the showery nature of that air stream with possible disturbances and troughs probably embedded in it, possible larger accumulations might have been seen in some inland areas away from the more favoured exposed coasts. So, the starting position is almost certainly pretty inaccurate. Let's focus on the changes shown as the slider passes through. At T+120  around 2 cm + is shown in England and Wales across all but the far south with greater accumulations further north. Based on the path of the slider, I feel that this area of snow ahead of it, is shown rather too far south, although it is possible that there is a short period of snow over a wider area prior to it turning to rain. Much of any lying snow is shown to melt later in the period but only in the Midland southwards. Meanwhile the snow cover further north is shown to deepen. There is no point in my analysing the snowfall any further on one particular run around 4 to 6 days out. Even if this model run comes anywhere close to verification the marginal temperatures and the intensity of the precipitation will probably not even be certain in the nowcasts! 

Now remember that this is just one example of a great many scenarios. If we do see a slider it could take any path across the UK, although given the ”usual” south-east trajectories, I would favour somewhere between the Scottish borders and the English Channel. Some sliders move too far south and miss the UK altogether and move into France.

For those of you who want to view these changes in more detail and look for other potential sliders and haven’t done this before, here is the link to the Meteoceil charts:

http://www.meteociel.fr/modeles/gfse_cartes.php

I hope that some of you have found this very simple analysis useful and will have a slightly better handle on what to look out for.

I think that I have said enough about this subject for now. I am in real danger of being dubbed “Mr Slider” with my obsession in the same way that @nick sussex is dubbed “Mr Shortwave”!

 

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1 minute ago, Ed Stone said:

TBH though, Joe B could arguably do with having a few of his own algorithms rewritten?:D

Whatever about model bias, humans certainly have their own bias! :rofl:

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1 hour ago, MattStoke said:

People commentjng as if the ECM is spot on with the slider low. You really think a model will be spot on with such a complicated setup this far out? Ridiculous! It will be days before that one is resolved. The huge difference in the handling of it by the various models over the past few days should tell you that. Come on, people!

Spot on to demonstrate the last four runs from GFS in ascending order. What does that tell you? *HUGE* uncertainty it will not be till Friday until the models truly have a good handle of it.

BE6A22A9-7CD8-4638-9BF8-3C6B4E0FC225.thumb.jpeg.e10eefbefbc1c291d2fa19d825f21a66.jpegAEA45ABD-AE82-4F07-8731-7D9D34135D3C.thumb.jpeg.443f843fb672f72eca8270ef06112d4e.jpegB7AC20A3-E351-4500-8777-28E85F24BCE2.thumb.jpeg.a72bebf529a84aa94405832ba1d3600a.jpegA28EA0D5-68DD-4733-837D-324A4771EEB7.thumb.jpeg.694b3366ce44e3ece7892adff657633f.jpeg

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Just now, Daniel* said:

Spot on to demonstrate the last four runs from GFS in ascending order. What does that tell you? *HUGE* uncertainty it will not be till Friday until the models truly have a good handle of it.

BE6A22A9-7CD8-4638-9BF8-3C6B4E0FC225.thumb.jpeg.e10eefbefbc1c291d2fa19d825f21a66.jpegAEA45ABD-AE82-4F07-8731-7D9D34135D3C.thumb.jpeg.443f843fb672f72eca8270ef06112d4e.jpegB7AC20A3-E351-4500-8777-28E85F24BCE2.thumb.jpeg.a72bebf529a84aa94405832ba1d3600a.jpegA28EA0D5-68DD-4733-837D-324A4771EEB7.thumb.jpeg.694b3366ce44e3ece7892adff657633f.jpeg

Greav post Daniel

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Absolutely fantastic Metoffice UK 6-30 days update today -  every year I would always hope December would be cold throughout with the chance of snow as it adds to the festive feeling! 

With what the models are showing for the foreseeable and the background signals it really is an exciting time of model watching. (If cold weather is what you are after)

Looking forward to seeing all the pictures from different areas of the UK and Ireland this month.

Hoping the GFS and ECM start to look similar today and tomorrow with regards to the upcoming cold spell this week. 

Most folk have a bias on here when posting thoughts on different outlooks but why would they not? (It’s a weather enthusiasts forum after all) 

Thank you to everyone who posts on here all year round or during particular seasons - I have been a member since 2009 and still learn new things every year! 

 

 

 

 

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I agree with Joe b regarding mountains holding and moving the slider's as they cross from the west that's why I think a line from northern Ireland,Wales and SW England will be furthest the slider will get to . Could be wrong but that's my guess

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1 hour ago, johnholmes said:

re this comment

Maybe the experts could refresh us on how sliders are modelled in regards to their biases? This might give us a hint of what to expect in a few days? 

No model has any bias, all are run with the mathematics to deal with the laws of physics as applied in meteorology. Lows and highs appear/disappear on this basis.

At the risk of causing some dissent another point worth trying to remember. That is compare 00z to 00z and 12z to 12z, not the most recent, that is until the time period gets below, usually, about T=96. Around this time then the more recent data increasingly tends to outpoint this method. 

I quite often see the comment that the models have a “mild bias”

 

I don’t really understand how this can be since it appears to me they deliver us, in terms of percentage returns, many more “snowmageddon” runs than the actual weather ever ends up delivering us.

 

One would hope that if the models are programmed with the right algorithmns, they would deliver us the same proportion of the all the weather types we end up getting as they project from however many days out. But, to me, they have a “cold bias”. I cannot help but think there is more excitement to be had from model watching over the years than from watching out of the window!

 

 

Let’s hope that this time the weather delivers us what the models, on so many runs, have projected for us in the way of snow.  It will help  avoid the “cold bias” getting worse!

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im not sure the last run from the GFS had this slider crossing the midlands the ECM has been consistantly modeling this on the same path for the past 3 days if you look back a few days the GFS had storm caroline smashing in to northern france which was also wrong its only starting to catch up to the euro models as always

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gfs-0-102.png?6
gfs-0-96.png?12

 

06z then 12z. Slightly lower on this run with a little more of a ridge ahead of it. Could try for something akin to the 00z UKMO run?

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gfs-0-96.png?12

Heights further north and a better south north alignment of the disrupting low.
gfs-0-108.png?12

Low slight further North, therefore boundary is further north.

gfs-1-114.png?12

Edited by frosty ground

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A correction west on this run as it stands here:

gfseu-2-114.thumb.png.6f01d3c9cbfc25774409c4f866648772.pnggfseu-2-120.thumb.png.4f9c427dbdc4bce2dfb3fa8aafde50bb.png

 

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GFS much better for more snow

UKMO rain for pretty much everyone @120-

so no resolution yet

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UKMO not great at 120 for slider #1

D11E85F3-0BFD-4076-B6C0-09C3531A2664.thumb.gif.3ca7d670a4f16c82300f53817089f6fa.gif

Should we be worried about the PV lobe? :cc_confused:

Edited by karlos1983

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gfs-2-123-3h.png?12
Very snowy for Manchester North, Everything is slightly sharper than the 0z run and a correction south for the Snow/Rain line.

 

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gfs-0-114.png?12 gfs-1-120.png?12 gfs-14-126.png?12
Hmm not quite eh?

That is one long trough going on right there! At least the sliding portion is weaker so less milder air drawn into the situation. With heavy precipitation and the wind kept from the SE across the northern half of the country, quite a large portion of the UK sees a substantial fall of snow although it looks a bit mixed with rain at times at lower levels outside Scotland. 

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Liking the look of the 12z, could be very snowy from the Midlands north on Sunday on this run. :rolleyes::)

gfs-2-120.png

gfs-2-126.png

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1 minute ago, karlos1983 said:

UKMO not great at 120 for slider #1

D11E85F3-0BFD-4076-B6C0-09C3531A2664.thumb.gif.3ca7d670a4f16c82300f53817089f6fa.gif

In fact, not just for the slider, UKMO is flattening the pattern

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GFS 12z and UKMO both differing again this afternoon, still 5 days out currently.

Models are bound to continue to chop and change as this is unusual setup for the UK.

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