Model Output Discussion 26/11/2012 18Z onwards

[The PIT]

I don't think there'll be a widespread chance of snow between now and Sunday, any precipitation looks to be limited to eastern coastal counties, the far north of Scotland and perhaps a few north facing coasts in the west. Sunday onwards, yes, though too much uncertainty over how widespread atm, as Atlantic fronts try to attack the cold from the west.

Looks like Sheffields staying dry but cold http://www.sheffieldweather.co.uk/Five%20Day%20Forecast.htm will be interesting what the GFS thinks later today.

[TAFKAP]

I woke up this morning to check the model thread and the mood was downright suicidal. I almost feel as if the late night model watchers must have The Smiths and The Cure playing in the background with maybe a bit of Leonard Cohen for good measure.

So then I had a look at the ensembles. Now this is an IMBY perspective I know but apart from a brief mild blip the op shows below average temperatures. Far better than last year and so much better than the horrors of the mild garbage endured during the 'even larger teapot' era (m.odern w.inter just in case filter blocks it like last time! Stop moaning and enjoy the possibilities. Scintillating times ahead with many pants wettingly good model watching to come.

[SMU]

Steve Murr, on 26 November 2012 - 22:24 , said:

Every single person posting in this thread has got it wrong in the last 30 Mins-

wait till the morning.

You will see this sort of chart starting to develop,

the slider is coming. GFS RIP.

Morning people-

The gfs 06z is one of many corrections South & west I expect & expected last night-

watch that 0c isotherm slide SW past ireland- Those in the NW cherishing the 06z run im afraid theres a HIGH chance it wont reach you..

Watch the 12's for more confirmation of the UKMO & BOM...

The UKMO Raw has done fantastic so far...

S

[Summer Sun]

Having a had a look through the precipitation charts Monday could be an interesting day with cold air in the east and rain moving in from the west

A cold start

With rain moving west to east snow is possible

Moving further in to the morning it stays very cold

Later in the day and it remains cold enough for snow for some

Just wondering as well what does the numbers stand for on these charts 1,2, 4, 32 ect?

[Paul]

Just wondering as well what does the numbers stand for on these charts 1,2, 4, 32 ect?

It's the precip rate in mm/hr

[chionomaniac]

posted into technical thread

[The Northern Ramper]

Morning people-

The gfs 06z is one of many corrections South & west I expect & expected last night-

watch that 0c isotherm slide SW past ireland- Those in the NW cherishing the 06z run im afraid theres a HIGH chance it wont reach you..

Watch the 12's for more confirmation of the UKMO & BOM...

The UKMO Raw has done fantastic so far...

S

When you say North West, how far North West are you talking?

[snowmadchrisuk]

Well the Arctic High / Split Vortex remains in play this morning , We remain in a Cold setup , one which could provide Snow chances at very short notice , Blocking to the NW still seems very likely , and I haven't seen it mentioned this morning but look at all that Cold air just to our East , When the block shortly rebuilds we shouldn't have many issues with 850's. Just a case of letting the new pattern change setup ... r.e. Snow Monday morning , North of the M4 could get quite a dumping as it will likely be heavy wet Snow. Look at it this way , the pattern is setting up for a very Cold outlook and we get a bonus Heavy Snow dumping why were waiting ... Excellent start to winter in my eyes .

Edited November 27, 2012 by snowmadchrisuk

[joggs]

encouraging mo update,sorry mods

[Summer Sun]

It's the precip rate in mm/hr

Thanks Paul

[SMU]

When you say North West, how far North West are you talking?

It would be IMPOSSIBLE to pinpoint somewhere now & foolhardy-However from what the 06z is showing you would expect incrememnts west & south- its how that goes is the key...Who knows- Wales, Cornwall, the atlantic??-to early to tell.S

[johnholmes]

for those with Extra I recommend you click on the link below and watch how the GFS version of msl-Td via Extra shows the pattern of surface features and Td over the next 384 hours.

It gives a very good idea of the instances where much milder air tries to move over the Uk and largely fails.

Remember it is the GFS version and it may be somewhat different if we had the same facility with ECMWF out to 240 hours but worth watching.

If you have not got Extra then if you want to have a better insight into winter weather then I urge you to contact Paul asap.

[nick sussex]

Regarding the track of the low and alluding to SM's comments before anyone gets deflated or elated there is likely to be changes in the track right upto close to the time.

We should also bear in mind if the Atlantic low phases with a shortwave to the north then its likely to be a damp squib for the majority of people as this will pull the system too far north.

So IF the slider low does verify then this is where people must take each output with the view that because of the set up its essential to be on the eastern flank of this to get the snow, this will likely go a bit to the east and to the west with the snow band therefore changing between outputs.

Historically once the low is beaten back and begins to disrupt then we do often see corrections more south and west, however at this point we don't know where that starting point will be and where any corrections might leave the different regions.

Again I will stress if the angle of attack isn't favourable because of that phasing then realistically its game over and then you have to wait for the low to sink away, if the upstream pattern is sufficiently amplified you might get some snow on the backedge as this happens.

Edited November 27, 2012 by nick sussex

[Weathizard]

Regarding the track of the low and alluding to SM's comments before anyone gets deflated or elated there is likely to be changes in the track right upto close to the time.

We should also bear in mind if the Atlantic low phases with a shortwave to the north then its likely to be a damp squib for the majority of people as this will pull the system too far north.

So IF the slider low does verify then this is where people must take each output with the view that because of the set up its essential to be on the eastern flank of this to get the snow, this will likely go a bit to the east and to the west with the snow band therefore changing between outputs.

Historically once the low is beaten back and begins to disrupt then we do often see corrections more south and west, however at this point we don't know where that starting point will be and where any corrections might leave the different regions.

Again I will stress if the angle of attack isn't favourable because of that phasing then realistically its game over and then you have to wait for the low to sink away, if the upstream pattern is sufficiently amplified you might get some snow on the backedge as this happens.

So to summarise so you, based on past experience, back the slider low? Some sort of middle ground, or do you think the low will phase with the shortwave and cause in your words 'a damp squib'? Interesting to know where you would lie your money if you were pushed, as I find your posts very interesting and informative and most importantly, unbiased.

[Snowman31]

Well the Arctic High / Split Vortex remains in play this morning , We remain in a Cold setup , one which could provide Snow chances at very short notice , Blocking to the NW still seems very likely , and I haven't seen it mentioned this morning but look at all that Cold air just to our East , When the block shortly rebuilds we shouldn't have many issues with 850's. Just a case of letting the new pattern change setup ... r.e. Snow Monday morning , North of the M4 could get quite a dumping as it will likely be heavy wet Snow. Look at it this way , the pattern is setting up for a very Cold outlook and we get a bonus Heavy Snow dumping why were waiting ... Excellent start to winter in my eyes .

Quick question that i've always wondered. Why are snow events so frequently forecast just N of the M4? seen it many times over the years.

[bluearmy]

http://www.meteo24.fr/fr/home/meteo-france-et-europe/passion-meteo/ensemble-de-previsions.html?eps=london

looks like we'll be sweltering middle next week as mean maxes reach 5c. look at the clustering in fi. this is london and likely to be too far south in battleground britain. if i was oop north, i'd be waxing my sledge.

[Buzzit]

Hi All

Trying to stay away from cold / snow posting at the moment as I don't think I can take a full seasons worth in here haha

Just want to make a point about the GFS ensembles (focused on London)

Last nights 18z ensembles showed quite a lot of divergence between the ensemble mean and the control / op on the 850s from 1-4 December

This mornings 6z shows much better agreement between the ensemble mean, control & op for 1-4 December

For my own sanity during winter I tend to start FI from the point of this 850 divergence IMBY

Overall I dont see anything wrong with charts like these for this time of year - we are certainly going to start December with well below average temperatures for time of year.

[nick sussex]

So to summarise so you, based on past experience, back the slider low? Some sort of middle ground, or do you think the low will phase with the shortwave and cause in your words 'a damp squib'? Interesting to know where you would lie your money if you were pushed, as I find your posts very interesting and informative and most importantly, unbiased.

Thanks.

OMG answering this question could get me in alot of trouble in here!

OK at this point I wouldn't like to make a call on whether we'll get phasing or not as theres too much uncertainty with the shortwave. However if that shortwave clears the se into the Low Countries in good time then thats likely to lead to the ridge extending further sw and causing the low to disrupt further west.

In this situation the band of snow will also move that way, generally once the models reach a tipping point and realize the block is too strong then this often gathers momentum, at this stage its very rare for them to correct eastwards it will generally be inching sw with each output.

But again I should say until we get an idea of where the correction might start from then its difficult to plot where any corrections might impact in terms of snow.

Hope that helps.

[Weathizard]

Thanks.

OMG answering this question could get me in alot of trouble in here!

OK at this point I wouldn't like to make a call on whether we'll get phasing or not as theres too much uncertainty with the shortwave. However if that shortwave clears the se into the Low Countries in good time then thats likely to lead to the ridge extending further sw and causing the low to disrupt further west.

In this situation the band of snow will also move that way, generally once the models reach a tipping point and realize the block is too strong then this often gathers momentum, at this stage its very rare for them to correct eastwards it will generally be inching sw with each output.

But again I should say until we get an idea of where the correction might start from then its difficult to plot where any corrections might impact in terms of snow.

Hope that helps.

Thanks for that Nick, although I have to say you dodged the question with a politician like stance haha, to be fair though, I'd say the same, its extremely difficult, when such minor movements can make such a large difference in the output, thats why I think we need to cut the models a bit of slack at the moment, because even for EXPERIENCED, seasoned forecasters, this is a very tough call. Hopefully after todays 12z runs we'll have more of an idea, but I fear it will not be sorted for a while yet.

[john mac]

IMO I feel that the 12z models will start to agree more with eachother. In the last 24hrs there have been signs of things starting to piece together. I expect the models to continue showing a cold theme and as nick says we will have to wait with the finer details until later in the week. All in all though very encouraging signs.

[johnholmes]

I suggest to all those wanting to know what will happen to any precipitation band to think how hard it is to predict rain in summer 72 sometimes less than 24 hours ahead. At the risk of boring some of you, will it snow has about SEVEN or so other variables that need to be right for snow to fall.

[Frosty.]

Just had a read of the latest metoffice update, have to say it looked astonishingly like the Gfs 06z op run with northern and eastern britain (NE Britain) coldest with wintry showers and snow in places, cold elsewhere too but a slightly milder interlude followed by generally cold and unsettled weather, for northern britain it could become a persistently cold outlook with little or no mild weather for several weeks, not surprising really given the strong northern blocking and a mainly southerly tracking jet..wintry times ahead but very complex and messy synoptics.

[The Northern Ramper]

It would be IMPOSSIBLE to pinpoint somewhere now & foolhardy-However from what the 06z is showing you would expect incrememnts west & south- its how that goes is the key...Who knows- Wales, Cornwall, the atlantic??-to early to tell.S

Would those north of this Wonga low stay in the cold air then? With a nw/nnw feed?

[snowmadchrisuk]

Quick question that i've always wondered. Why are snow events so frequently forecast just N of the M4? seen it many times over the years.

Its because when a Short wave comes in from the SW it drags with it some Milder air wraped around it's core.. As the low moves in depending on it's track it drags that Milder air across Southern England . When there has been a Cold snap before hand the North of the M4 can hang on to that Colder air a little longer as the Milder air doesn't quite reach it , In the case where a low pressure moves NE then the Milder air will reach it eventually , but the PPN can be in front of the milder air and so initial falls of Snow are likely M4 North.. In the Case of 6z the low is moving East/SE so the Mildlands North will hold on to the Colder air for longer.

[offerman]

I find it amazing how just a motorway can be the boundary on so many occassions to rain ro snow. It literally is that one side can be rain and the other snow.

I`m in Frome and the local weather variables are probably some of the most localised on the planet. Micro climate between Frome, Westbury, Sheppton Mallet ,Trowbridge Bath, huge weather variations can happen here and anyone on this thread who lives nearby knows what i`m on about. the charts might just bring some more of this soon.

here`s hoping for further improvement in them.