General Model Output Discussion Thursday December 24Th:

[snowlover2009]

basically convection is when the warmth is drawn out from a warm sea and rapidly builds heavy/beefy showers due to very cold upper temperatures!

so massive snow showers?

oh my terrible bbc 5 day forecasts. just looked. double digits in the south undertsnadable, but i would have thought this would have been to the extreme south, then snow in the midlands, but know snow for wales? then north of england has a mix of everything and then scotland also has a mix of everything. i understnad the gradient of everything, but everything is just one complete mess with the new update of the 5 dayer. but i realise the 5 dayers are the worst things to look at, but i don't know how they can show them. terrible and no one who understands weather that much will be looking at the 5 dayers and believing them, although they are correct with the snow, but a massive mess with one spot in midlands snowing, then one spot not too far in double digits and heavy rain lol.

[Craigers]

so massive snow showers?

oh my terrible bbc 5 day forecasts. just looked. double digits in the south undertsnadable, but i would have thought this would have been to the extreme south, then snow in the midlands, but know snow for wales? then north of england has a mix of everything and then scotland also has a mix of everything. i understnad the gradient of everything, but everything is just one complete mess with the new update of the 5 dayer. but i realise the 5 dayers are the worst things to look at, but i don't know how they can show them. terrible and no one who understands weather that much will be looking at the 5 dayers and believing them, although they are correct with the snow, but a massive mess with one spot in midlands snowing, then one spot not too far in double digits and heavy rain lol.

ignore those mate! its terrible and BBC never get forecasts correctly, i found looking at updated charts and radar helps to forecast what could happen etc . Net weather is so much more accurate than watching the BBC weather.

[Barb]

just hoope that forecast changes for Cardiff. Even if its 10c in Cardiff it would only be 6c here and thats not going to be low enough for the snow!

Well the low further North/South by 50 miles could mean the difference between 10c and heavy rain and 1c and blizzards. 50 miles this far out is nothing, even 6 hours out it could easily change by that much.

[kold weather]

Swilliam, the reason I say that is notice the upper high change orientation between 216-240hrs and more to the point notice the way the eastern flank is changing from a WNW-ESE to a NW-SE direction.

That is a classic sign that the PV is heading southwards, also notice how the heights on the ECM change:

http://www.ecmwf.int/products/forecasts/d/getchart/catalog/products/forecasts/medium/deterministic/msl_uv850_z500!Geopotential%20500%20hPa!216!Europe!12!pop!od!oper!public_plots!latest!chart.gif

http://www.ecmwf.int/products/forecasts/d/getchart/catalog/products/forecasts/medium/deterministic/msl_uv850_z500!Geopotential%20500%20hPa!240!Europe!12!pop!od!oper!public_plots!latest!chart.gif

The rise in pressure over the UK also shows that the whole lot is coming south, as is the little secondary depression on the normal pressure charts.

Finally along with the first point, notice how heights decrease over Iceland and the southern edge of the PV pushes into veiw. What would IMO evolve from that pattern is the PV holds close to where it is, the Greenland high contniues to change angle and a strong northerly is forced southwards. The high over the UK sinks southwards in advance of the LP as it tries to come in. The low to the west tries to attack the UK from the SW but isn't able to nudge in...eventually we are left in a strongly cylconic flow from the north. Its a very similar set-up to presently BUT the huge difference is we still have a powerful Greenland block which stops the LP from getting too far north.

Of course thats a very long way out and IMO will change but IMO the 12z ECM is a slightly more risky pattern but has a FAR greater possible reward then the 12z GFS..

As for the 12z GFS by the way, its not stable at all really, it takes *4* seperates Greenland highs to keep us in the cold pattern, each upper high only lasts 48-72hrs on the GFS run and each time we need a top up it has a risk obviously.

[MR EXTREMES]

mmmm nice

[Bolton67]

mmmm nice

Deffo looking like the winter of 78/79.

The exact same weather, snowy and cold period before xmas then mild for about 2 days before the real winter set in.

New years day 1979 saw about 8 inches falling in bolton, with the match between Bolton V Everton being called off at half time.

worse snowfall i saw. started about 1pm by 4pm 8 inches had fallen.

[MR EXTREMES]

Deffo looking like the winter of 78/79.

The exact same weather, snowy and cold period before xmas then mild for about 2 days before the real winter set in.

New years day 1979 saw about 8 inches falling in bolton, with the match between Bolton V Everton being called off at half time.

worse snowfall i saw. started about 1pm by 4pm 8 inches had fallen.

indeed might be worth having a read in the tec model output gp has placed his thoughts and it looks promising.

Edited December 24, 2009 by grab my graupel
Supplied link :)

[I remember Atlantic 252]

low seems to be tracking further south at 72, I feel that will be the trend, most of the snow in the south, midlands northwards dry but still FI yet

Edited December 24, 2009 by mark forster 630

[kold weather]

Yep the 18z is quite a bit further south with regards to the low on this run, the snowfall now is as far south as probably the M4 region, though further west is even better in this situation according to the models.

Certainly a better trend for the Midlands south on this run.

[Barb]

More south this time. It'll be chopping and changing every run. I think Midlands/Wales is the prime zone for this one.

Somewhere is in for a very major dumping that's for sure...

[swilliam]

what is convection? sorry i have heard a dozen times, does it have something to do with snow potential or something.

Convection is when air is warmed at the surface, usually in the winter by passing over warm water such as the North Sea, in the summer by heating of land by the sun, causing the air to rise like the smoke and hot air from a fire. This causes clouds to form and eventually rain or snowfall.

In winter in lowland places we generally get three types of snowfall, convective, frontal and those caused by troughs in the general flow (usually due to some warm air aloft or low level convergence of wind flow). Convective snowfall is obviously more likely the closer to the source of the warming - in this case the North Sea. It will depend on the wind direction and strength where the convective snow actually falls. For the East of England (including Leeds) the best direction is the Eastern quadrant. The nearer the coast you are the more likely you will get some precipitation, However because you are close to the source of warmth (the sea) the more likley it will be rain rather than snow (which is why the upper air temperatures matter so much in this situation). For inland areas such as West Yorkshire it is also necessary for the the wind to be strong as well as in the correct direction. The convective activity dies down as the showers move away from the source of the warmth. Therefore a strong wind is needed to get significant precipitation a long distance away from the coast.

This can change when troughs are present which enhance the shower activity. What matters is the orientation and movement of the troughs. If the movement is at right angles to the length of the trough then usual most places get a spell of snow with the length depending on the speed of movement. These troughs are usually due to some upper air feature. Sometimes though the trough is aligned along the direction of flow and this can be mean the trough is virtually stationary. In this setup some places get many hours of snowfall whilst either side not many miles away nothing falls at all. This is what occurred last Thursday/Friday so that some places got a lot of snow whilst other places not much.

Frontal snowfall has two main variants. The most common is warm air replacing cold and in this situation we get a spell of snow followed by rain and a thaw. This occurred yesterday although a lot of us down South and in the Midlands missed out on the snow first due to the upper air temperature profile. Sometimes the warm air pushes forward and then is rebuffed by the cold air and we get a long spell of snow and then the cold air wins and the rain does not arrive. This gives us (in lowland England) our biggest snowfalls. This is what is being projected for early next week with warm air pushing up from the South but strengthening NE winds pushing it back. How far the warm air gets determines where the snowfall will occur. The setup next week is complicated by the fact we are not very cold to start with so it may start as rain and then change to snow. All the discussion on this thread is to do with where the warm air will get to which is why there was much mention of snow in the South or North. At this stage it is far to early to know either how cold it will be when the main precipitation starts where it will get to. This will only be approximately known 24-48hrs beforehand so you should not worry about this till then. The other main frontal situation is when the cold air is initially introduced which can lead to rain followed by snow. This rarely gives us a good snowfall as evidenced by last Saturday night when a front of this nature passed through us all.

So on this thread we are discussing all the different model runs and what that might mean for cold weather and what type and where we might get precipitation. However this is not usually done from an in my back yard (IMBY) perpsdective. This is mainly because the detail such as this will always change beyond the 72hr mark and the models are running out to 384hrs. So I think if you are interested in the will it won't it snow in Leeds next week type of information you should use some of the other threads on this excellent site. I believe there is a will it snow thread and also a North of England cold spell discussion. You should use to these to get your local iformation. Use this thread to see what the general evolution of the synoptic pattern is likley and possible.

Hope this helps

Cheers

Stewart

[03jtrickey]

Hope this helps

Cheers

Stewart

Great post Stewart! Deserves to be archived and put into one of the NetWeather guides

[West is Best]

Stunning synoptics these. Although I like the 18z track we don't want the whole thing to slide too far south. These events are always knife-edge. But what a great position to be in. Unless you're driving. Or running. Or elderly. Or infirm. Etc. etc.

[kold weather]

EVerything is further south, from the LP to also the upper high that eventually give us a colder spell in FI...so will be interesting to see if it gives any sustained cold spell down the line on this run...somewhere would get nailed on this run though thats for sure...

A very cold run at the surface thanks to the snow cover on the ground.

From my point of veiw, just shunt it just a little further south and I'll be happy, I suspect Evap cooling would drag the snowfall level down quite quickly though as the cold air cuts in.

[swilliam]

Swilliam, the reason I say that is notice the upper high change orientation between 216-240hrs and more to the point notice the way the eastern flank is changing from a WNW-ESE to a NW-SE direction.

That is a classic sign that the PV is heading southwards, also notice how the heights on the ECM change:

http://www.ecmwf.int...atest!chart.gif

http://www.ecmwf.int...atest!chart.gif

The rise in pressure over the UK also shows that the whole lot is coming south, as is the little secondary depression on the normal pressure charts.

Finally along with the first point, notice how heights decrease over Iceland and the southern edge of the PV pushes into veiw. What would IMO evolve from that pattern is the PV holds close to where it is, the Greenland high contniues to change angle and a strong northerly is forced southwards. The high over the UK sinks southwards in advance of the LP as it tries to come in. The low to the west tries to attack the UK from the SW but isn't able to nudge in...eventually we are left in a strongly cylconic flow from the north. Its a very similar set-up to presently BUT the huge difference is we still have a powerful Greenland block which stops the LP from getting too far north.

Of course thats a very long way out and IMO will change but IMO the 12z ECM is a slightly more risky pattern but has a FAR greater possible reward then the 12z GFS..

As for the 12z GFS by the way, its not stable at all really, it takes *4* seperates Greenland highs to keep us in the cold pattern, each upper high only lasts 48-72hrs on the GFS run and each time we need a top up it has a risk obviously.

Hi Kold,

Yes I agree with you in general - certainly about the reloads on the GFS evolution which I mentioned earlier. As for the ECM it does look like the natural evolution for a massive reload of the Northerly due to the PV appearing like that. But I also think it possible to get the SE flow that I mentioned - particulalrly if the PV starts heading SE rather than S. I think this is rather an usual situation so there are not many analogues to look at - may be something for Christmas evening if, as is likley, I get bored with the telly.

I expect everything will be differnet by tomorrow anyway but very interesting model watching at the moment.

Some excellent previous posts BTW.

[rich1]

I feel there has been a tendency lately from a few to maybe over analyse and pick holes in the output when in years gone by we would be drooling over charts like these

its a different kind of winter this year, cant remember a spell like this before xmas fo so long, and we didnt get that much here. Was speaking to my dad, he says he doesnt remember anything like this since he moved south in 1981, the sheer depth of cold, the persistence of lying snow before xmas etc. Up north its even better

so if cold runs are being shown, this winter isnt what we have been used to, built upon last winter which for some areas brought a lot of wintry weather, perhaps the rules of 'expect it to all go t!ts up and downgrade or go wrong' arent the same any more

wondering if we ended up getting a winter that topped the 80's, would be interesting to see what people would think of future cold spells in the models on forums like these in the coming years ...

Edited December 24, 2009 by grab my graupel
Rephrased:)

[kold weather]

Not a great run in FI however it has to be said, as I said for long term prospects this run is most certainly a huge downgrade on the 12z run...so a better run with regards to the LP position but a far from stunning run later down the line...

The whole lot topples in FI and we lose the -ve AO totally by 240hrs...MEGA progressive run on that front!

So one big snowfall possibly for the Midlands (and for my location, probably a bucket load of rain followed by sleet!) then a brief cold spell followed by a return to the normal of recent winters barring 08-09...

12z GFs IMO was far better...18z remains cold at the surface with probably an inversion high...but CACK synoptics.

On my scale earlier on potenial for what can happen in terms of the evolution in the cold spell...this gets only a 20%...

Edited December 24, 2009 by kold weather

[swilliam]

EVerything is further south, from the LP to also the upper high that eventually give us a colder spell in FI...so will be interesting to see if it gives any sustained cold spell down the line on this run...somewhere would get nailed on this run though thats for sure...

A very cold run at the surface thanks to the snow cover on the ground.

From my point of veiw, just shunt it just a little further south and I'll be happy, I suspect Evap cooling would drag the snowfall level down quite quickly though as the cold air cuts in.

The HP in FI never properly connects with the arctic high - hence a likely toppler in far FI. Still lots of cold and wintry weather though. I still think the battleground situation next week looks marginal even though this run probably gave one of the better alignments. This may change (the marginality) nearer the time though.

[TEITS]

when in years gone by we would be drooling over charts like these

I don't get that excited.

Just a quick post before I call it a night.

Im fairly confident now of a LP tracking S although obviously the exact track is uncertain. Im also confident of the following NNE/NE/ENE,ly but whether this turns into a rprolonged spell is still too early to say, although the signs are very promising.

Finally a quick word about the model output tomorrow/Boxing Day. Last year I followed the models on these days and the output was very strange indeed and contrary to what was shown previously. Now on the 27th normality resumed in the model output. This could of been coincidental but I think this was down to data issues. I doubt upto +144 will be affected but as the timeframe increases the kaos theory comes into play with these models. Rather than a butterfly flapping its wings it could be more like a flock of geese!!

[swilliam]

Not a great run in FI however it has to be said, as I said for long term prospects this run is most certainly a huge downgrade on the 12z run...so a better run with regards to the LP position but a far from stunning run later down the line...

The whole lot topples in FI and we lose the -ve AO totally by 240hrs...MEGA progressive run on that front!

So one big snowfall possibly for the Midlands (and for my location, probably a bucket load of rain followed by sleet!) then a brief cold spell followed by a return to the normal of recent winters barring 08-09...

12z GFs IMO was far better...18z remains cold at the surface with probably an inversion high...but CACK synoptics.

On my scale earlier on potenial for what can happen in terms of the evolution in the cold spell...this gets only a 20%...

Agreed but I think we have to expect some of these as well as the wonder synoptics (and Nick S I am not moaning just commenting so please don't bar me!)

I don't get that excited.

Just a quick post before I call it a night.

Im fairly confident now of a LP tracking S although obviously the exact track is uncertain. Im also confident of the following NNE/NE/ENE,ly but whether this turns into a rprolonged spell is still too early to say, although the signs are very promising.

Finally a quick word about the model output tomorrow/Boxing Day. Last year I followed the models on these days and the output was very strange indeed and contrary to what was shown previously. Now on the 27th normality resumed in the model output. This could of been coincidental but I think this was down to data issues. I doubt upto +144 will be affected but as the timeframe increases the kaos theory comes into play with these models. Rather than a butterfly flapping its wings it could be more like a flock of geese!!

I bet that does not stop you looking at the models though (and using it as an a reason when we don't get what we want)!

[J10]

Not a great run in FI however it has to be said, as I said for long term prospects this run is most certainly a huge downgrade on the 12z run...so a better run with regards to the LP position but a far from stunning run later down the line...

The whole lot topples in FI and we lose the -ve AO totally by 240hrs...MEGA progressive run on that front!

So one big snowfall possibly for the Midlands (and for my location, probably a bucket load of rain followed by sleet!) then a brief cold spell followed by a return to the normal of recent winters barring 08-09...

12z GFs IMO was far better...18z remains cold at the surface with probably an inversion high...but CACK synoptics.

On my scale earlier on potenial for what can happen in terms of the evolution in the cold spell...this gets only a 20%...

There really is a trade off, for here this is a better run in the short term, with (potentially) a major snow event, but in the longer term, nowhere near as good synoptically as the Greeny High never really dominates as per the ECM 12Hz as a example.

If it meant the major snow event,from an IMBY point of view, I'd take this run, but in the real world, perhaps we would not only miss out on the initial snow, but then be left with the worse FI as well.

[nick sussex]

Not a great run in FI however it has to be said, as I said for long term prospects this run is most certainly a huge downgrade on the 12z run...so a better run with regards to the LP position but a far from stunning run later down the line...

The whole lot topples in FI and we lose the -ve AO totally by 240hrs...MEGA progressive run on that front!

So one big snowfall possibly for the Midlands (and for my location, probably a bucket load of rain followed by sleet!) then a brief cold spell followed by a return to the normal of recent winters barring 08-09...

12z GFs IMO was far better...

I think you're looking at this from a glass half empty perspective, i just wouldn't bother with the lower resolution part of the output, lets just get over the first hurdle and worry about the longer term later, this is one gfs run out of 4 a day,the ecm has been just as erratic so for the timebeing at least we have generally good agreement before FI.

You've been on here a long time KW and you know only too well that reading too much into every FI is liable to lead to a rather emotionally draining experience!

IMO a great days model output, a good way to welcome Xmas in, so Happy Xmas all.

[J10]

I don't get that excited. :lol:

Just a quick post before I call it a night.

Im fairly confident now of a LP tracking S although obviously the exact track is uncertain. Im also confident of the following NNE/NE/ENE,ly but whether this turns into a rprolonged spell is still too early to say, although the signs are very promising.

Finally a quick word about the model output tomorrow/Boxing Day. Last year I followed the models on these days and the output was very strange indeed and contrary to what was shown previously. Now on the 27th normality resumed in the model output. This could of been coincidental but I think this was down to data issues. I doubt upto +144 will be affected but as the timeframe increases the kaos theory comes into play with these models. Rather than a butterfly flapping its wings it could be more like a flock of geese!!

Indeed Christmas Day charts last year showed some of the biggest downgrades possible.

[Frosty.]

Not a great run in FI however it has to be said, as I said for long term prospects this run is most certainly a huge downgrade on the 12z run...so a better run with regards to the LP position but a far from stunning run later down the line...

The whole lot topples in FI and we lose the -ve AO totally by 240hrs...MEGA progressive run on that front!

So one big snowfall possibly for the Midlands (and for my location, probably a bucket load of rain followed by sleet!) then a brief cold spell followed by a return to the normal of recent winters barring 08-09...

12z GFs IMO was far better...18z remains cold at the surface with probably an inversion high...but CACK synoptics.

On my scale earlier on potenial for what can happen in terms of the evolution in the cold spell...this gets only a 20%...

The 18z still gives us a pretty potent N'ly blast for much of next week and into the new year with a partially successful reload before the high builds in and destroys everything but it's only one possible outcome of many so let's hope we get a nice christmas morning surprise.

Finaly I would like to wish everyone on netweather a very merry christmas :lol:

[kold weather]

Yep Jackone, however the 18z wouldn't give me any real snowfall bar back edge stuff, of course thats not to say a decent part of the country couldn't get decent snowfall from that but my location wouldn't get much at all from that, esp given the airflow will cause modification of the flow thanks to the thames valley.

and I think the GFS is really downgrading the start of Jan synoptic pattern from something that could evolve into something...into something that is quite frnakly bog standard!

As I also have said before, in terms of the evolution of the pattern, that run is really one of the worst possible evolutions we can have, the mid latitude high saves us but I'd rather not have to depend on that sort of set-up.

Very 96-97 to me from that run...

Nick, your quite right but thats relaly rather ruined that xmas cheer from the 12z run, esp as I said before the LP wouldn't give my location much other then maybe sleet...profiles look just the wrong side of marginal, west of London would get *nailed* on the run.

Edited December 24, 2009 by kold weather