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Model output discussion - the beast arrives


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Icon followed the GEM & UKMO in that is disrupted the energy east. 850hPa still very marginal but it does undercut the precipitation after a short period of rain/sleet along southern areas. I think we are all in agreement that whatever outcome, be it enegry going east or west uppers will be marginal along southern areas. 

Edited by MattTarrant
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Evening Guys  Officially signing off for the winter from my window - still all but 5 inches on the ground - have a great year ! Current   favourite   See

First rule of GFS model watching. Never trust a dartboard low....... But especially in this type of set up.

See you next Saturday then, Steve?

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Well what a week we have in front of us, a polar low spreading across the north sea into tomorrow with heavy disruptive snow and that's just the tip of the iceberg because it's a very severe wintry week for many and looking at the 00z the north at least stays in the deep cold through the weekend and much of next week too!..very exciting end to winter and start to spring!:cold::)

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15 minutes ago, Herefordweather said:

GFS appears slightly better to me? Low further south and edging across the channel?

Yes a small improvement from the GFS. Looking at next Saturday, the cold upper air still covers much of the country.

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2 hours ago, nick sussex said:

Still no resolution to the low this morning .

Don’t expect any answers to at least tonight.

In the medium term the ECM has a very slack Atlantic pattern , it looks quite messy but interesting with the PV shredded.

 

Don't mean to be selfish but I'm concerned for us snow starved southerners.

The gradual push of milder southeasterly coming from the low to our SW I'd rather it went bit south or more to southeast so we hold on to cold for bit longer.

Bit greedy I know but that's not to say this not an exciting weather event.

Easterly flows always fill me with dread.

Much prefer a northeasterly so I'm waiting for 12z in hope that things shift in more favourable position.

But I don't think it will.

But good to see our east coast and Easterly crew already seeing wintry weather as they to waited awhile to.

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We almost get 499 dam thickness on the Gfs 6z on wednesday and some fabulous sub zero maxima of minus 3 or 4 c with heavy drifting snow and severe wind chill making it feel like minus 10 to minus 15c in the gale force easterly winds!..can't believe it's so quiet in here!:cold::D

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I think it's just natural frosty - people forget about the exceptional stuff that is here and locked in, and just focus on the breakdown instead! Enjoy the next 5 days - it may be a long wait until we get anything like it on our shores.

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35 minutes ago, MattTarrant said:

I don't envy forecasting this low, GFS is not really budging (maybe a little further east at around 80 - 86hr but minimal really). 

It does develop a secondary low in the channel however to halt to push of mild air northwards on Friday.

The 06z shows a complex setup with regards to snowfall, as mentioned many times, T850s are not the best guide in this setup, rather air temperature and dewpoint are. Unfortunately I can't seem to find wet bulb temperature anywhere but here is where the border between rain and snow should be judging by the 06z output. Each timestep is 3 hours from midday Thursday

Anywhere north of the black line is very likely to be snow, south and more likely sleet/rain...

image.thumb.png.2e93810d0390cf97eda73fb50338a692.png image.thumb.png.812db056bc754259d6843ecc07f8394c.pngimage.thumb.png.d3da690caf02df29ff7b1afacbf3f276.pngimage.thumb.png.66e73b6b72728eaf6992762ec6a45acd.pngimage.thumb.png.1bb64fbee0f353d9944e6e6e9594831d.pngimage.thumb.png.3e988b9f05d4aa492865c604244475c5.pngimage.thumb.png.09d7aecd4c3b993792296bca4f2ce39b.pngimage.thumb.png.36d08e0d0a7ebe9cd8fb92a9c93942a1.pngimage.thumb.png.b8534e004336ad7a84ed239c8c2e4c0d.pngimage.thumb.png.52904fb6175bb12411d3de25e3eb210e.pngimage.thumb.png.d2f713f54daf04f88f66d79952606462.pngimage.thumb.png.a08507957121727b3f18df295297cd2b.pngimage.thumb.png.2098292f993b66be7810cb01dad2b5fc.pngimage.thumb.png.8960ed5c76473f9e7696eb1186f0bb4c.pngimage.thumb.png.803d340432316beaaf42e1c4be6fa6f9.pngimage.thumb.png.05773890abf93cb086ba77d32bfa4f86.pngimage.thumb.png.611cc8f67524e628f9b8a611272abebd.png

So yup, fine margins, if the northwards movement of milder air is held back even a small bit I'd expect consistent snow even down here,

The mild air starts making notable inroads on saturday for this run unfortunately but lets see what the ens show.

Edited by Quicksilver1989
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1 minute ago, Quicksilver1989 said:

It does develop a secondary low in the channel however to halt to push of mild air northwards on Friday.

The 06z shows a complex setup with regards to snowfall, as mentioned many times, T850s are not the best guide in this setup, rather air temperature and dewpoint are. Unfortunately I can't seem to find wet bulb temperature anywhere but here is where the border between rain and snow should be judging by the 06z output. Each timestep is 3 hours from midday Thursday

Anywhere north of the black line is very likely to be snow, south and more likely sleet/rain...

image.thumb.png.2e93810d0390cf97eda73fb50338a692.png image.thumb.png.812db056bc754259d6843ecc07f8394c.pngimage.thumb.png.d3da690caf02df29ff7b1afacbf3f276.pngimage.thumb.png.66e73b6b72728eaf6992762ec6a45acd.pngimage.thumb.png.1bb64fbee0f353d9944e6e6e9594831d.pngimage.thumb.png.3e988b9f05d4aa492865c604244475c5.png

image.thumb.png.09d7aecd4c3b993792296bca4f2ce39b.pngimage.thumb.png.36d08e0d0a7ebe9cd8fb92a9c93942a1.pngimage.thumb.png.b8534e004336ad7a84ed239c8c2e4c0d.pngimage.thumb.png.52904fb6175bb12411d3de25e3eb210e.pngimage.thumb.png.d2f713f54daf04f88f66d79952606462.pngimage.thumb.png.a08507957121727b3f18df295297cd2b.png
image.thumb.png.2098292f993b66be7810cb01dad2b5fc.pngimage.thumb.png.8960ed5c76473f9e7696eb1186f0bb4c.pngimage.thumb.png.803d340432316beaaf42e1c4be6fa6f9.pngimage.thumb.png.05773890abf93cb086ba77d32bfa4f86.pngimage.thumb.png.611cc8f67524e628f9b8a611272abebd.png

So yup, fine margins, if the northwards movement of milder air is held back even a small bit I'd expect consistent snow even down here,

The mild air starts making notable inroads on saturday for this run unfortunately but lets see what the ens show.

I believe they are the original planning sketches for the M4? ?

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52 minutes ago, Frosty. said:

Well what a week we have in front of us, a polar low spreading across the north sea into tomorrow with heavy disruptive snow and that's just the tip of the iceberg because it's a very severe wintry week for many and looking at the 00z the north at least stays in the deep cold through the weekend and much of next week too!..very exciting end to winter and start to spring!:cold::)

Is that a polar low? I know it has been described as such but I always associated with Arctic airmasses, in showery air streams with hpas sub -40C and they don't have frontal systems as such. This one is over land and under very high pressure. 

I'll be interested in what the synoptic experts think.

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A very pleasing FI on the 6z unless of course you like a warm March! :rofl:

Hopefully the effects of the SSW will continue to be felt though next month with repeated wintry attacks.

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8 hours ago, Tracy Flick said:

NWP is traditionally purely Newtonian physics (no historical data use), but some companies are trying Artificial Intelligence ('Neural Networks', the self-learning way).

How I see these 2 approaches:

Using pure Newtonian physics (which is the traditional MetO way):  In the case of fluid dynamics, the balance of forces and the conservation of momentum, energy and mass are expressed through Navier-Stokes partial differential equations (mass, energy and momentum are conserved in everything, even car crashes... read about Newton's Laws for more).  But these NS equations are difficult to 'solve' both numerically and analytically... in fact we don't even know how to generally prove that physical solutions exist to them at all.  The best we can do is therefore create a simplified version of the NS equations and then use computational techniques such as 'finite difference methods' (see Wikipedia) to solve the many partial differential equations simultaneously: these iterate the evolution over increments of time and space (the space increments are the grid spacings that are often referred to.)  The starting point (t=0) is observed data combined with reanalysis data.  Obviously smaller increments of time and space give better results but are more computationally expensive.

These Newtonian NWP models should in theory be fine with unusual atmospheric states because the laws of physics are still always true... however, the approximations introduced by the physicists and programmers may mean that accuracy is only guaranteed for certain ranges of atmospheric states.  By making observations of unusual states we can test the validity of these approximations and improve them if necessary.

An example: If there's an experimental observation such as eg 'high sea surface temperatures in an ocean leads to warmer summers' , this will 1st be checked for good statistically correlation.  Then this will be checked on the computer model to see if the simulation output matches the experimental observation.  If it does, great, but if not it means something is either lacking from the code or something is in error in the code.  Then the possible sources of error are identified eg maybe evaporation needs to be modelled less simplistically, so they will try that.  This will mean increasing the complexity of the approximate partial differential equations, or adding new ones.

Nonlinear systems (eg the atmosphere) require exponential increases in computing power for linear increases in forecasting accuracy... I suspect there is therefore probably no real desire anymore to simply improve results by buying better computers because the forecasting improvement is increasingly tiny for each extra dollar spent.  It is surely instead more efficient to improve the formulation of the differential equations that have to be simultaneously solved... a question that could be asked is eg "how can I better model turbulence on the edges on tropical storms?" ... answering this question will lead to modifications of the set of partial differential equations.

GFS I think uses finite difference methods to solve the partial diff equations but they want to advance to finite volume methods.  ECMWF use spectral methods... these involve writing the solution as a sum of basis functions as is done in Fourier analysis.

 

The AI Neural Network way: 

Some private companies are it seems now trying to somewhat blindly get forecast results using Artificial Intelligence.  See https://blogs.microsoft.com/ai/hows-the-weather-using-artificial-intelligence-for-better-answers/

Basically this method I think (or at least one possible implementation) uses historic observations with known outcomes to 'train' the software.  Then given enough training it should be able spot patterns and give a correct output (the forecast).

This is similar to spectral methods in that we are trying to find weights to give to each element of a set of functions... BUT this time the desired weighting on a function is evolved towards using nothing but historic observations as a guide, whereas the ECMWF find the weights by 'simply' plugging the sets of functions into the partial differential equations.

For an introduction to weather with neural networks and practising using it yourself:   https://www.amazon.co.uk/Neural-Network-Programming-Java-Souza/dp/178588090X

As computing power increases AI might become more important in forecasting, BUT weather has sensitivity to initial conditions so perfect AI forecasting can surely only be achieved with an infinite amount of training data(!)... that's to say AI methods must have at least the same fundamental limitation as classical physics methods.  I suspect that it is with unusual atmospheric states that AI will seriously struggle.

Maybe combining AI and traditional NWP in some way is a way some might go in the future.

 

TLDR of my take:  AI uses purely historic weather observations, whereas traditional NWP uses purely Newton's Laws of physics, with approximations. 

I'm sure someone at the MetO is playing with AI but they surely aren't using it yet.

 

This is fascinating, thank you.

So is it the case that the AI system would look at a given weather situation and all the inputs this entails, and then construct a probability-based forecast?   (So to put it very crudely, if input 'x' has, 9 times out of 10, produced output 'y,' the prediction will therefore be 'y'.  Obviously the real model would have to balance huge quantities of such outputs.)  

And is AI really purely based on historic data?  Surely a system could also improve on its own accuracy by comparing its earlier forecasts to the actual weather that then occurred.  So real time weather data could inform two sets of modelling, retrospective and predictive...

  

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29 minutes ago, Weather-history said:

Is that a polar low? I know it has been described as such but I always associated with Arctic airmasses, in showery air streams with hpas sub -40C and they don't have frontal systems as such. This one is over land and under very high pressure. 

I'll be interested in what the synoptic experts think.

I was just going to ask the very same thing...squinting at the various models trying to figure it out with my very novice  eyes !!:)

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Well the 6z looks stunning for most of this week for most of the uk with the most severe wintry spell for years and although the deep cold eventually becomes confined to the far north and even they lose it for a few days..through low res it comes back with further snow events!..the last days of Feb and possibly the first half of March look predominantly great for coldies, especially further north!:):cold-emoji:

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27 minutes ago, Man With Beard said:

Going to be a strange week in here. Most of the country will be probably spend most time in the regional threads to discuss snow prospects within the following three hours. The only area not really included is the South West, so their attention will turn to Thursday/Friday which is the only hope down there of a snowfest.

It most certainly is not game over for the South West by any stretch. 100 miles north or south is nothing for a system to move even 36 hours before an event happens, and that's when no block is in place at all. There is a reason why the Met Office have a weather warning in the most severe category over the South West (at low likelihood at the moment simply because of how far out it is).

My personal opinion, from looking at the historical records we have, I feel that if anything, this low will not be further south but instead will be further west, before possibly splitting in the way shown on the UKMO.

North of the M3/A303, I don't think this will make a major difference - you will get an all snow event on Friday.

I'm not entirely sure what that would mean for areas south of that. I think the main factor will be the shape of the low. If it is extremely rounded on Friday morning, like the GFS shows, then it will slowly suck up air from the English Channel and out to the Atlantic. However, if it starts to lose its shape as it hits the block like this GEFS member:

gens-3-1-102.png

then the flow turns more easterly for the whole of the south and the 0C uppers line might stay right on the coast, probably resulting in snow for all except just maybe the very fringes of the coast.

gens-3-0-102.png

This is an entirely reasonable evolution from the current trends - much more likely than a sudden shift of track to the east or south.

Hi MWB - far SW (Cornwall/S devon) still have things to be looking at over the next two days, especially Wed if the wind veers after the short wave low passes through N England. 

Wrt Thu/Fri, yes very small margins, the slider back in Dec ended up going through the midlands/west country rather than NW england/east anglia and i think that was picked up at about T36.  I feel the Fax charts are now the best guidance for that system as we are in the range (<T84) of the modified charts now so should have an idea of Exeter's thinking.  My opinion from an IMBY view, is rain south west of a line Bristol to Brighton, but living in Exeter has given me a glass half empty take on weather when it cones to snow/Tstorms.  As long as it's not freezing rain!

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