Model Output Discussion - 23rd Jan - 12z onwards

[stewfox]

GFS about as bad as possible to get after the initial cold plunge with mosts low lying areas likely the wrong side of marginal and just to rub it in it looks like going on to produce some FI deep cold. (Pass the sledge hammer)

 

 

UKMO much. much better and I would rather have that and worry about FI later than GFS jam tomorrow.

 

 

That's the same chart as SM posted 2 minutes earlier but a totally different take on it. No wonder some of us get confused.

[Eugene]

At 210 and we're still in a cold northerly.  I would happily take either UKMO or GFS given what we've had to endure so far this and last winter.  They're both showing a significant cold spell with chances for all for some of the white stuff!  Maybe not the best for my location, but some places could do very well.

 

 

 

You'll be enduring more cold and mainly dry weather in Chelmsford.

[snowking]

If people will insist on analysing the 850mb temperatures every single run here's something to bear in mind when viewing the output for the upcoming period - -6c is a baseline figure to put snow/rain discrimination against!

 

Those pretty colours that we see on the 500mb charts are not just there to increase the artistic merit of the Meteociel output, they are there to indicate how high or low 'heights' are. To try and break this down to laymans terms as much as possible, lower heights result in the rate of increase of temperature as you move down the atmosphere to the surface (the lapse rate) decreasing. The reason we usually wish for -6c 850mb temperatures is that as an average figure this would result in temperatures at the surface (and numerous other factors) being conducive to snowfall. However, when we have such unusually low heights progged, that changes. So we may find all of a sudden that whilst the 850mb temperature might be only -4c, it is still conducive to snowfall.

 

If you want to concentrate on figuring out at this range whether atmospheric conditions are conducive to snowfall then stop focussing on 850mb temperatures (so long as they are below 0c) and start concentrating on Dew Points, Wet Bulb temperatures, and 850-1000mb thicknesses, because these will be the more determining factors - and you will not get any broadscale accuracy out of those at this range, never mind specific local detail.

 

As an example of this:

 

 

Ohh no, how will it possibly snow when the 850mb temperature is not below -6c...

 

 

Ohhh hang on, so despite the 850mb temperatures only being -4c (still conducive of snowfall) all the other factors are still supportive of snow.

 

NONE of these variables are yet within accuracy range, but it hopefully goes to show the fruitless nature of attempting to forecast snow at this ridiculous timeframe and based upon 850mb temperatures alone as many seem determined to do.

 

Everybody take a couple of days off and come back on Tuesday for snow forecasts. Until then lets focus on the synoptic evolution.

 

SK

Edited January 25, 2015 by snowking

[SMU]

SK -that's the 06z 850 thicknesses - was awaiting the 12z to see if they were sub 1290 D

 

S

[winterof79]

I don't know what has changed, but all of a sudden today I can understand why the mod's and so many seasoned model watchers get frustrated by knee jerk reactions. 

 

I am clueless as to why some people are already dismissing this run as being a downgrade or saying words to that extent. 

 

we have a northerly feed and as others have stated previous to this disturbances within the pattern are now showing. 

 

it's simply a variation on a theme and theme is for a cold spell to develop something which I didn't think was going to happen a few days ago, so I need to maybe take a backseat for a bit and take some of my own advice.

You have seen the light my friend.Welcome to assessing the output over time not every GFS run.Cold spell imminent enjoy.

[Continental Climate]

This is, in my opinion, a perilous setup and I urge caution at the moment. The main reason is that the likelihood of it all getting shunted east remains high with the jet barrelling out of the US:

 

 

 

 

In the absence of upstream blocking or any significant amplification that jet is very likely to encourage further cyclogenesis. I know nothing much is showing yet, but it's only a matter of time before a jet like that generates further low pressures with our name on them.

 

I hope that's wrong but without upstream signals I'm not going to get excited by this until T24 to T36.

 

(I'll leave aside the other issue about poor uppers as it has been covered above but, yes, they do matter imho.)

You are right west they do matter and in my mind they are the main indicator we have on whether its going to snow but anyhow like you said its all been covered before. 

 

Anyway big differences in the output at 144   Which one will be right? UKMO is notoriously unreliable at 144 so i will maybe go GFS on this one. Either way it all looks a bit marginal but hopefully the right side of marginal 

 

And yes the screaming jet is a worry as well!!

Edited January 25, 2015 by Continental Climate

[Day 10]

If people will insist on analysing the 850mb temperatures every single run here's something to bear in mind when viewing the output for the upcoming period - -6c is a baseline figure to put snow/rain discrimination against!

 

Those pretty colours that we see on the 500mb charts are not just there to increase the artistic merit of the Meteociel output, they are there to indicate how high or low 'heights' are. To try and break this down to laymans terms as much as possible, lower heights result in the rate of increase of temperature as you move down the atmosphere to the surface (the lapse rate) decreasing. The reason we usually wish for -6c 850mb temperatures is that as an average figure this would result in temperatures at the surface (and numerous other factors) being conducive to snowfall. However, when we have such unusually low heights progged, that changes. So we may find all of a sudden that whilst the 850mb temperature might be only -4c, it is still conducive to snowfall.

 

If you want to concentrate on figuring out at this range whether atmospheric conditions are conducive to snowfall then stop focussing on 850mb temperatures (so long as they are below 0c) and start concentrating on Dew Points, Wet Bulb temperatures, and 850-1000mb thicknesses, because these will be the more determining factors - and you will not get any broadscale accuracy out of those at this range, never mind specific local detail.

 

As an example of this:

 

 

Ohh no, how will it possibly snow when the 850mb temperature is not below -6c...

 

 

Ohhh hang on, so despite the 850mb temperatures only being -4c (still conducive of snowfall) all the other factors are still supportive of snow.

 

NONE of these variables are yet within accuracy range, but it hopefully goes to show the fruitless nature of attempting to forecast snow at this ridiculous timeframe and based upon 850mb temperatures alone as many seem determined to do.

 

Everybody take a couple of days off and come back on Tuesday for snow forecasts. Until then lets focus on the synoptic evolution.

 

SK

Great post SK

[Geordiesnow]

I can understand some still being underwhelmed by the GFS because it faffs on when the cold sets in and a lot of it does turn quite marginal and its just messy ether way you look at it, it does get there in the end and that has been the trend from the GFS so you can't rule it out by any means.

 

I still think the UKMO is the best, cleanest and safest set up, although i prefer to see it on WZ before reserving judgement but too me, it does not look to bad of a run too me, questionable just how cold those upper air temps will be mind but I would bank that run personally, especially if I live in the West. 

 

See what the ECM does tonight would not be surprised if it downgraded the FI potential but I am more interested in what happens in the 96-144 hour period, hopefully it will keep its cleaner set up. 

[snowking]

SK -that's the 06z 850 thicknesses - was awaiting the 12z to see if they were sub 1290 D

 

S

 Well spotted!

 

Have updated 850mb chart to the 6z to demonstrate the same point

[Nick F]

I have refrained from posting much in here over the weekend, mainly because I'm working and other commitments but also I don't really have much to add to the furore and knee-jerk reactions with each run over details such as how cold it will be, will it snow, etc ... as to to be honest, these details can't be firmed up on yet, so no use getting wound up by certain models suggesting it not looking so good IMBY.

 

In the broadscale picture, coldies should be very pleased, we have a slow-moving or almost stationary long-wave pattern setting up for the last few days of January and into early February - whereby the UK remains in a cold polar flow between a ridge to the west and a long-wave trough sitting over mainland Europe.

 

Yes, it may be marginal for some for a few days between the intial cold NWly blast on Weds/thurs and the colder Nly arriving sometime over the weekend, but I'd be very suprised if its just the north that sees snow, lower elevations in the south could see some snow too at some point from Wednesday - where and whether it will setlle and how long it'll hang around for can't be firmed up on yet.

[Cloud 10]

A nice wintry run from the GFS this evening and hi-res ends with one of Chiono's Star Wars fighter vortex shapes.

 

[snowice]

If people will insist on analysing the 850mb temperatures every single run here's something to bear in mind when viewing the output for the upcoming period - -6c is a baseline figure to put snow/rain discrimination against!

Those pretty colours that we see on the 500mb charts are not just there to increase the artistic merit of the Meteociel output, they are there to indicate how high or low 'heights' are. To try and break this down to laymans terms as much as possible, lower heights result in the rate of increase of temperature as you move down the atmosphere to the surface (the lapse rate) decreasing. The reason we usually wish for -6c 850mb temperatures is that as an average figure this would result in temperatures at the surface (and numerous other factors) being conducive to snowfall. However, when we have such unusually low heights progged, that changes. So we may find all of a sudden that whilst the 850mb temperature might be only -4c, it is still conducive to snowfall.

If you want to concentrate on figuring out at this range whether atmospheric conditions are conducive to snowfall then stop focussing on 850mb temperatures (so long as they are below 0c) and start concentrating on Dew Points, Wet Bulb temperatures, and 850-1000mb thicknesses, because these will be the more determining factors - and you will not get any broadscale accuracy out of those at this range, never mind specific local detail.

As an example of this:

Ohh no, how will it possibly snow when the 850mb temperature is not below -6c...

Ohhh hang on, so despite the 850mb temperatures only being -4c (still conducive of snowfall) all the other factors are still supportive of snow.

NONE of these variables are yet within accuracy range, but it hopefully goes to show the fruitless nature of attempting to forecast snow at this ridiculous timeframe and based upon 850mb temperatures alone as many seem determined to do.

Everybody take a couple of days off and come back on Tuesday for snow forecasts. Until then lets focus on the synoptic evolution.

I think you have just saved the mods from reaching for the paracetamol? Hopefully this excellent post on 850s will put this to bed once and for all!

Edited January 25, 2015 by snowice

[J10]

A direct comparison for the GFS shows for Wednesday night into Thursday shows colder 850Hpa temps with sub -7c air now covering more of the UK than before.

 

Heights are also better with sub 1290m covering more of the UK than before.

 

Thicknesses also better sub 519 dam line covers more of the UK.

 

All of this is especially true for the very early hours of Thursday am, and less so towards 9am.

 

Things continue to be very marginal indeed over the weekend (and probably morseo on Saturday compared to 6Hz run but on a final positive note  there is better amplification in this run compared to the earlier run into next week.

 

Still miles off ECM though. Normally you would take this run but given what ECM is offering. you would still take ECM.

[Mark Bayley]

If people will insist on analysing the 850mb temperatures every single run here's something to bear in mind when viewing the output for the upcoming period - -6c is a baseline figure to put snow/rain discrimination against!

 

Those pretty colours that we see on the 500mb charts are not just there to increase the artistic merit of the Meteociel output, they are there to indicate how high or low 'heights' are. To try and break this down to laymans terms as much as possible, lower heights result in the rate of increase of temperature as you move down the atmosphere to the surface (the lapse rate) decreasing. The reason we usually wish for -6c 850mb temperatures is that as an average figure this would result in temperatures at the surface (and numerous other factors) being conducive to snowfall. However, when we have such unusually low heights progged, that changes. So we may find all of a sudden that whilst the 850mb temperature might be only -4c, it is still conducive to snowfall.

 

If you want to concentrate on figuring out at this range whether atmospheric conditions are conducive to snowfall then stop focussing on 850mb temperatures (so long as they are below 0c) and start concentrating on Dew Points, Wet Bulb temperatures, and 850-1000mb thicknesses, because these will be the more determining factors - and you will not get any broadscale accuracy out of those at this range, never mind specific local detail.

 

As an example of this:

 

(SNIP)

 

SK

 

To further illustrate SK's point (with the 12z) charts. Looks marginal on the 850hpa for Friday (potential snow event). Dewpoints support snow away from the coasts 

[Ice Day]

Apologies if already done, but Tomas Schafernaker tweeted this earlier this afternoon:

 

Off to BBC for another pm/evening shift. #Countryfile forecast later & already scratching my head about the icy blast later in week #snow

 

Plenty of uncertainty, clearly.

[The Post-modern Winter]

You are right west they do matter and in my mind they are the main indicator we have on whether its going to snow but anyhow like you said its all been covered before. 

 

Anyway big differences in the output at 144   Which one will be right? UKMO is notoriously unreliable at 144 so i will maybe go GFS on this one. Either way it all looks a bit marginal but hopefully the right side of marginal 

 

And yes the screaming jet is a worry as well!!

 

[West is Best]

If people will insist on analysing the 850mb temperatures every single run here's something to bear in mind when viewing the output for the upcoming period - -6c is a baseline figure to put snow/rain discrimination against!

Those pretty colours that we see on the 500mb charts are not just there to increase the artistic merit of the Meteociel output, they are there to indicate how high or low 'heights' are. To try and break this down to laymans terms as much as possible, lower heights result in the rate of increase of temperature as you move down the atmosphere to the surface (the lapse rate) decreasing. The reason we usually wish for -6c 850mb temperatures is that as an average figure this would result in temperatures at the surface (and numerous other factors) being conducive to snowfall. However, when we have such unusually low heights progged, that changes. So we may find all of a sudden that whilst the 850mb temperature might be only -4c, it is still conducive to snowfall.

If you want to concentrate on figuring out at this range whether atmospheric conditions are conducive to snowfall then stop focussing on 850mb temperatures (so long as they are below 0c) and start concentrating on Dew Points, Wet Bulb temperatures, and 850-1000mb thicknesses, because these will be the more determining factors - and you will not get any broadscale accuracy out of those at this range, never mind specific local detail.

As an example of this:

Ohh no, how will it possibly snow when the 850mb temperature is not below -6c...

Ohhh hang on, so despite the 850mb temperatures only being -4c (still conducive of snowfall) all the other factors are still supportive of snow.

NONE of these variables are yet within accuracy range, but it hopefully goes to show the fruitless nature of attempting to forecast snow at this ridiculous timeframe and based upon 850mb temperatures alone as many seem determined to do.

Everybody take a couple of days off and come back on Tuesday for snow forecasts. Until then lets focus on the synoptic evolution.

I think you have just saved the mods from reaching for the paracetamol? Hopefully this excellent post on 850s will put this to bed once and for all!

 

No it won't but some of us know better than to argue right now! Snow in the UK at warmer than -5C 850hPa is the exception rather than the rule.

Edited January 25, 2015 by West is Best

[Catacol]

I don't know what has changed, but all of a sudden today I can understand why the mod's and so many seasoned model watchers get frustrated by knee jerk reactions. 

 

I am clueless as to why some people are already dismissing this run as being a downgrade or saying words to that extent. 

 

we have a northerly feed and as others have stated previous to this disturbances within the pattern are now showing. 

 

it's simply a variation on a theme and theme is for a cold spell to develop something which I didn't think was going to happen a few days ago, so I need to maybe take a backseat for a bit and take some of my own advice. 

 

Totally agree. It beggars belief doesnt it? We are an island in the atlantic with the jet stream aimed straight at us, and a semi permanent line of atlantic systems that wing their way towards us most of the year. Continental heat or cold are relatively rare in summer/winter.

 

And we get a forecast of around a full week of northerly air at the peak time in winter, slightly against expectations (well at least of mine though the vortex relocation to Siberia was always going to introduce a rogue variable into the output) with low dew points, low thicknesses and low 850s. And the draw off the ocean is hardly slack so showers and streamers will be possible, let alone the potential for areas of thundersnow in the instability.

 

AND STILL PEOPLE ARE SOUNDING GRUMPY???

 

Good grief. I reckon we have a mix on here sometimes: some obvious trolling, some that will only ever be happy with another 1963 setup, and some who just dont understand what is potentially on the horizon and still have the front to post their less than informed opinions.

 

Sigh. I have one piece of advice for those learning. The MetO are pretty good at medium range forecasting - by their own stats I think they reckon to be close to 90% now even in seasonal terms. Learn to translate their carefully balanced and non sensationalist language designed to avoid tabloid hysteria - and take a look at what they are saying. Snow is there. IF has tweeted the S word on more than one occasion today. If you base your initial thoughts initially on what the Met are saying you will rarely be too far away.

[inghams85]

CC uppers are very important but in this setup with the dew points and thicknesses shown it will be snow to low levels!!! SK has just shown everyone why so can we please put this to bed!!!

You are right west they do matter and in my mind they are the main indicator we have on whether its going to snow but anyhow like you said its all been covered before. 

 

Anyway big differences in the output at 144   Which one will be right? UKMO is notoriously unreliable at 144 so i will maybe go GFS on this one. Either way it all looks a bit marginal but hopefully the right side of marginal 

 

And yes the screaming jet is a worry as well!!

[lorenzo]

Thanks to Sebastiaan for sharing this chart a couple of winters back, dug it out from the archives, may be useful this week..

 

[Frosty.]

Apologies if already done, but Tomas Schafernaker tweeted this earlier this afternoon:

 

Off to BBC for another pm/evening shift. #Countryfile forecast later & already scratching my head about the icy blast later in week #snow

 

Plenty of uncertainty, clearly.

Yes it looks severe, I'm itching for this cold spell to start, it looks like being a corker according to models and latest met office update.

[Jason M]

No it won't but some of us know better than to argue right now! Snow in the UK at warmer than -5C 850hPa is the exception rather than the rule.

A good point IMHO. I suspect there are quite a few sceptics who are wary of posting at present. The fact that the debate is actually occurring is probably informative in itself as to where things stand.

That said, from the output so far today IMO I think we currently sit the right side of marginal whereas yesterday it looked more iffy. Will it change again tomorrow? who knows!

For me though, the biggest plus point today is that there are signs that we won't just return to zonal in 10 days time. GFS 12Z has quietly further extended the cold spell this evening and the GEFS will be interesting again.

[IDO]

 

 

As an example of this:

 

 

 

Ohh no, how will it possibly snow when the 850mb temperature is not below -6c...

 

 

 

Ohhh hang on, so despite the 850mb temperatures only being -4c (still conducive of snowfall) all the other factors are still supportive of snow.

 

NONE of these variables are yet within accuracy range, but it hopefully goes to show the fruitless nature of attempting to forecast snow at this ridiculous timeframe and based upon 850mb temperatures alone as many seem determined to do.

 

Everybody take a couple of days off and come back on Tuesday for snow forecasts. Until then lets focus on the synoptic evolution.

 

SK

 

I may be missing something but it looks like it will not be snowing at sea level with those variable. The WBZ shows at 100m plus. Posting this site's snow chart for that time period:

 

Over 100m in the central belt for snow needed?

 

Any snow is shown on hills and mountains. I am aware that with a higher altitude the chances of snow increases even with marginal variables but I always look at sea level so that covers nearly everyone. 

[SMU]

Just one to add -

 

When we saw the PPN in the cold air last week we were really scratching around with dewpoints @ between -0.5 & +0.5 / +1 making it very marginal-

 

The depth of the cold front clearing through weds night has a clear deep polar signal with a strong cold dewpoint line all the way from the south coast to the North! - its clearly visible here

 

http://modeles2.meteociel.fr/modeles_gfs/runs/2015012512/78-101UK.GIF?25-12

 

This very dry air will help quickly reduce surface temps to freezing-

Theta chart

http://modeles.meteociel.fr/modeles/gfs/runs/2015012512/gfs-6-90.png?12  10c &

uppers

http://modeles.meteociel.fr/modeles/gfs/runs/2015012512/gfs-1-84.png?12 -6/-7

 

At T90 a band of SNOW - not rain & sleet looks to slide across the south.

 

In terms of probability of falling & possible settling snow- its the south of englands best chance this winter ( also the showers piling into the NW as well- which of course will be snow )
ENJOY-

S
 

Thanks to Sebastiaan for sharing this chart a couple of winters back, dug it out from the archives, may be useful this week..

 

temps 850 seb.png

 

He needs to lower it a few DAM for the UKMO 144!!!

Edited January 25, 2015 by Steve Murr

[Long haul to mild]

A good point IMHO. I suspect there are quite a few sceptics who are wary of posting at present. The fact that the debate is actually occurring is probably informative in itself as to where things stand.That said, from the output so far today IMO I think we currently sit the right side of marginal whereas yesterday it looked more iffy. Will it change again tomorrow? who knows!For me though, the biggest plus point today is that there are signs that we won't just return to zonal in 10 days time. GFS 12Z has quietly further extended the cold spell this evening and the GEFS will be interesting again.

Indeed we should always be cautious in the run up to any potential cold spell, especially in marginal setups, as the disappointment for some will be overwhelming if it all goes wrong!

However, the mere fact that we have some kind of cold spell guaranteed (albeit unknowable snow amounts at present) compared to the zonal tedium we have been used to in recent winters/times is in itself cause for a small celebration; also the fact that it doesn't look like being a 24hr toppler for once.