Model Output Discussion: 12Z (20/11/12) And On...

[nick sussex]

here we go - one of the biggest snow events for the UK

http://modeles.meteo...996-2-6-0-0.png

quote john kettley that eve- SOME HEAVY SNOW NOW FALLING IN THE CHANNEL ISL-

Uppers

http://modeles.meteo...996-2-6-0-2.png

-2 C from the continent..........

S

Oh did you have to remind me of that trauma!

Yes it was lovely seeing all that snow fall just to the west of London! The sum total in Wimbledon where I was living at the time was a few rogue snowflakes blowing about in the wind. Grrrrrrrr!

Yes thats a good example of frontal snow with not so cold uppers thanks to that lovely feed of lowish dew points off the continent.

Edited November 20, 2012 by nick sussex

[kold weather]

850s are around -4 or below for most the UK from 150 right out to the end of the run almost. We can get snow with 850s of just -1 so why people are worried bout the lack of cold I have no idea.

Because in those set-ups we've had bitter air either over us at the lower levels or nearby in France...this time we don't have anything close to that. Your likely going to need -7/8C given the type of set-up aloft to get anything beyond wintry at this time of year given the SSTs and the latent relative warmth still being pumped up over W.Europe...it will go, but it'll take a bit of time.

Do note though right at the end of the run that Europe is starting to create its cold pool...sadly this set-up is going to be wasted trying to develop a cold pool of our own...but it'll be really good if we get a reload a little down the line because we won't have had to deal with a never ending trough disruption that completely moderates the cold out of the system.

Edited November 20, 2012 by kold weather

[Geordiesnow]

With all due respect Geordie, have you not read dozens of posts saying, "Once the blocking is in place, the cold uppers will follow"?

And with all due respect, this run(and the ECM) shows that even if you get set ups which may look favorable on paper for a cold/snowy set up, if the cold uppers are not there then its not going to be the snowy charts what people may expect. I mean, in most circumstances, the 18Z should be a very snowy run once the ridge has started but unfortunately, its not, its just cool and showery(wintry showers).

In terms of "once the blocking in place, the cold uppers will follow" talk, then yes in general this is true but the orientation has to be favorable to have this and a retrogression into Greenland would be the most likely to deliver the colder upper air temperatures. I think its a tad complacent and mis-leading to suggest if we keep the blocking, we will get the cold uppers following because any blocking may orientate in a NW/SE'ly direction which may deliver SE'ly winds with potentially milder upper air temperatures for example!

As strange as it sounds, the January 1984 charts are snowier and colder than tonights 18Z despite a massive PV over Greenland and westerly winds!

That said, its all about trends and the models do want to set up an Atlantic ridge at least, how the rest plays out is quite questionable indeed and it will all depends how strong that Atlantic ridge will be.

[Tim Bland]

here we go - one of the biggest snow events for the UK

http://modeles.meteociel.fr/modeles/reana/1996/archivesnh-1996-2-6-0-0.png

quote john kettley that eve- SOME HEAVY SNOW NOW FALLING IN THE CHANNEL ISL-

Uppers

http://modeles.meteociel.fr/modeles/reana/1996/archivesnh-1996-2-6-0-2.png

-2 C from the continent..........

S

This was a very different set up though. Preceding this we had v cold surface temps from an easterly and this front moving in from the west hit the colder lower air. Dew points / wet bulbs etc would have been v low.

I'm happy with the current runs though. Would rather have marginal snow events as per GFS 18z than sub -5c air under high pressure any day. Drive up a hill inland or keep a lookout overnight and you will see some snow with -4c uppers IMO

[Zakos]

From this

To this

Not fantastic but a significant upgrade. This easterly continues to be upgraded, interesting. a few more upgrades and it could prove to be a quite significant event.

In my opinion, the GFS at times is "reverting to default" by putting to much bias in reforming the PV.

We have a split in the PV just within the high res output. As soon as the low res starts, it begins to reform again. Coincidence? I think not.

As hard as it is I would largely ignore the output beyond 180 hours, especially on the GFS as IMO it has a bias to weaken the block and reform the PV too quickly

Edited November 20, 2012 by Zakos

[Matty M]

Again i reiterate we had the exact same problem in 2010, no cold pool for the initial eastery on November 20th, the synoptic evolved and in a few days we got the cold incursion.

If we get the right synoptics for a sustained period the cold will come.

Let us just hope we see this upper level pattern continue to be progged in the medium term. A fantastic output so early in our winter season.

[Andy H]

With all this talk about if its going to be cold enough, maybe somebody can explain something to a thicko here?

How did we get all the snow in certain parts of the country a couple of weeks back? I know I went pretty quick, but was still snow!

[Matty M]

Again i reiterate we had the exact same problem in 2010, no cold pool for the initial eastery on November 20th, the synoptic evolved and in a few days we got the cold incursion. The similarities between what's progged and 2010 are uncanny. Take a look for yourself, it is quite amazing!

If we get the right synoptics for a sustained period the cold will come.

Let us just hope we see this upper level pattern continue to be progged in the medium term. A fantastic output so early in our winter season.

[SMU]

This was a very different set up though. Preceding this we had v cold surface temps from an easterly and this front moving in from the west hit the colder lower air. Dew points / wet bulbs etc would have been v low.

I'm happy with the current runs though. Would rather have marginal snow events as per GFS 18z than sub -5c air under high pressure any day. Drive up a hill inland or keep a lookout overnight and you will see some snow with -4c uppers IMO

possibly- however the same could be said if we were sustaining cold in this scenario after 10 days-

Remember the most important part of the atmosphere for us is the column of air between the cloud base to the surface- & what the lapse rates are like in that column....

however that discussion is for another thread so no more off topic posts from me-

Lets get to saturday & see where we are- the 18z is ropey as no blocking over svalbard- thats where to be casting your gazes over the next few days as well as western greenland...

S

[ScottSnow]

From a northerly you can snow from -6 to -8c. but obviously the colder the better.

Some great synoptic charts I wouldn't get too hung up about the depth of cold at this stage.

Yeah I generally find in this part of the world you need lower 850s off an easterly. I think its down to modification from the sea. February 10th 2010 is one example of this although there was snow in the higher parts of the region.

I would think on past experience that the current synoptic charts being shown would not produce lasting snow here.

[SMU]

With all this talk about if its going to be cold enough, maybe somebody can explain something to a thicko here?

How did we get all the snow in certain parts of the country a couple of weeks back? I know I went pretty quick, but was still snow!

X2 reasons-

Cold surface air fed into the system from the North-

Evaporational Cooling in the heavy PPN bands over south wales etc-

THe uppers were probably -2 IIRC so it was borderline but good enough....

S

[AWD]

The FAX chart for Thursday shows some very strong winds ahead of a cold front;

http://www.wetterzentrale.de/pics/brack1.gif

Aside from any potential cold spell, there is a lot of potentially serious weather in places this week.

[andymusic]

X2 reasons-

Cold surface air fed into the system from the North-

Evaporational Cooling in the heavy PPN bands over south wales etc-

THe uppers were probably -2 IIRC so it was borderline but good enough....

S

close Steve - and with great respect of course - that missed south wales virtually altogether - somerset/gloustershire and wiltshire had all the fun out of that one - yes a wrap around east north wind and cooling did that!

[chionomaniac]

First snow risk for some is less than a week away on the 18Z GFS. And it wouldn't take much upgrade from the northerly to produce more.....

[SMU]

Ive just scanned the 18z Ensembles & STILL after all this time they are not worth looking at- they add no value-

So before we get any 'ensembles' are ropey posts-

IGNORE THE 18Z ENSEMBLES- just as tomorrows 06z.

Follow the UKMO, ECM & JMA in this scenario- bin the rest even the mighty NOGAPS!

S

PS andy- yep it was bath etc so the SW- but wales was ballpark

Edited November 20, 2012 by Steve Murr

[phil nw.]

The FAX chart for Thursday shows some very strong winds ahead of a cold front;

http://www.wetterzen...pics/brack1.gif

Aside from any potential cold spell, there is a lot of potentially serious weather in places this week.

Yes we must not overlook this event for Thursday.

Charts courtesy of NW Extra

strong gusty winds ahead of that front along with heavy rain in a short space of time especially in the west moving across the country through the day.Hence the concern for flooding following yesterday,s and today,s rain.

More on this in our Severe Weather thread.

Edited November 20, 2012 by phil n.warks.

[ScottSnow]

First snow risk for some is less than a week away on the 18Z GFS. And it wouldn't take much upgrade from the northerly to produce more.....

Yes I'd certainly expect Scotland and higher parts of northern England to see some snowfall from the initial northerly and the uppers seem to be getting upgraded (colder) for the easterly as we come nearer to the reliable period.

[Dave Kightley]

Ive just scanned the 18z Ensembles & STILL after all this time they are not worth looking at- they add no value-

So before we get any 'ensembles' are ropey posts-

IGNORE THE 18Z ENSEMBLES- just as tomorrows 06z.

Follow the UKMO, ECM & JMA in this scenario- bin the rest even the mighty NOGAPS!

S

PS andy- yep it was bath etc so the SW- but wales was ballpark

Extremely heavy too. It helped with the time of day also. What makes me think that with -4 uppers overnight would produce at least marginal snow?

[Andy H]

X2 reasons-

Cold surface air fed into the system from the North-

Evaporational Cooling in the heavy PPN bands over south wales etc-

THe uppers were probably -2 IIRC so it was borderline but good enough....

S

I would imagine this was borderline? And could quite easily of been rain? Sorry just trying to get my head around why people are so worried about will it or will it not snow next week? When at the end of the day, it's going to be cold, and from what I can work out its not a couple of days cold shot! So the longer the cold lasts, the chances of snow increase?

[SMU]

I would imagine this was borderline? And could quite easily of been rain? Sorry just trying to get my head around why people are so worried about will it or will it not snow next week? When at the end of the day, it's going to be cold, and from what I can work out its not a couple of days cold shot! So the longer the cold lasts, the chances of snow increase?

yes because the column of air has had all the air mixed out- also Easterly flows are surface cold-

what MAY make a difference isnt the uppers, but the track of the cold-- Over the north sea & it may get mixed- over the continent will be better!

night all

S

[Snowy Easterlies]

Ive just scanned the 18z Ensembles & STILL after all this time they are not worth looking at- they add no value-

So before we get any 'ensembles' are ropey posts-

IGNORE THE 18Z ENSEMBLES- just as tomorrows 06z.

Follow the UKMO, ECM & JMA in this scenario- bin the rest even the mighty NOGAPS!

S

PS andy- yep it was bath etc so the SW- but wales was ballpark

but if they were showing mega cold we shouldn't ignore them right ? the control run is horrible, i hope that doesn't end up happening otherwise i might need to get the prozac even at this early stage of the season.

[Thundery wintry showers]

With an easterly flow that has sustained for a while, it also depends on how far north you are- over Scotland and northern England the track over the North Sea is relatively lengthy so 850hPa temperatures of -1 to -3C would support surface temperatures near 5C, but over south-east England, with a short sea track, such a temperature profile will often support falling and lying snow with surface temperatures near or below zero.

Another issue with that setup is that the combination of cold surface air and mild upper air tends to produce sheets of stratocumulus stuck underneath an inversion and days on end of dry cloudy weather with nothing more than a bit of drizzle or "snizzle" depending on the temperature- with that setup you need troughs/fronts to generate snowfalls as you won't get many snow showers off the North Sea.

I think there is truth behind both sides of the 850hPa debate- some are being too dismissive of the possibility of marginal snow events and lamenting the lack of deep cold too much (as far as I can see, the GFS, and the ECMWF ensemble mean, both point towards marginal snow events early next week especially for central and southern areas) but it is also important for people not to be misled into believing that we'll be getting widespread snow cover before November is out, as this appears highly unlikely at the moment.

Edited November 20, 2012 by Thundery wintry showers

[danm]

BBC Weather already hinting at a possible significant change to cold weather for the end of the month.

Darren Bett in this video explains: http://www.bbc.co.uk/weather/features/20419248

[Isolated Frost]

For those wondering about the lack of cold uppers- the relative quick pace of the transition to the sinking trough into the Bay of Biscay means that there is no substantial buildup of cold uppers in Scandinavia and Russia. This means that a gradual reduction of upper air temperature should occur as the subtropical ridge moves northwards into Iceland- leading to colder air flow moving SE from Russia into the continent. It will be a very slow process towards any meaningful cold upper air in the UK.

In the meanwhile- the synoptics shown are conductive to gradually colder weather over the next 7 days. The air flow looks like turning from NNE to E as the high pressure slips towards Scandinavia.

[weather eater]

Indeed so: I assume the model thread isn't solely about stuff into the MR or longer-range.. ? If so, as you note, aspects around Thursday look bothersome. To cite UKMO Ch Forecaster: "The frontal zone has the potential to collapse into a narrow band containing intense line convection. The low level shear across the front looks unusually large with gradients of over 70kn ahead of it and 25kn behind. Hence it is likely to be a particularly squally feature. MOGREPS-R shows a relatively strong signal for gusts >60kn on Thursday in the west on Thursday..."

Will there be any snow (patently not) because if there isn’t not many people on here care, that is until the water inundates their houses.

Leaving aside the snow question, if the 18z is close to the mark it’s going to feel pretty raw across the SE by the middle of next week with a fair wind chill, there might not be any snow but it will sure feel like winters arrived.

Edited November 20, 2012 by weather eater