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

[Liam J]

Looking much closer to the current time frame here and ignoring the long range predictions for a while it really is looking like a pretty violent spell coming up during Thursday morning as the cold front sweeps in from the west. A few charts from the 18z NMM highlighting the potential for some very hazardous conditions developing.

Surface pressure

A well defined squall line, potential for convection along the boundary line.

Very strong upper level winds which could potentially reach down to ground level as straight line gusts, the risk is there for this front to break down into line convection.

Deep layer shear is modelled off the scale.

Low level Shear

Keep an eye in the sky for funnel cloud development!

Edited November 21, 2012 by Liam J

[LomondSnowstorm]

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.

I'd certainly agree with that, while easterlies are for this part of Scotland anyway the best setup for delivering snowfall the margin for error is huge. The basic rule of thumb we use is -8C for proper snowy convective easterlies, otherwise we are really at the mercy of mixing of the localised maritime airmass from the North Sea. Certainly the uppers seen in the operational GFS pub run would be insufficient here for anything other than a sleety mix in a straight easterly flow, though with a more northerly component or with shortwave development -5/-6C could suffice.

I agree with Steve also that once deep cold is established and provided a largely continental flow (or at least not immediately maritime) we can get snowfall at uppers close to 0C. The biggest snowfalls of 2009/10 here came when uppers were around -3C and -1C respectively, but because the flow wasn't straight off the North Sea and surface temperatures were sitting at around -5C immediately before the fronts arrived marginality wasn't as much of an issue.

My argument is that, while we appear to be getting decent agreement on the broad strokes of this pattern, the smaller details are really up in the air as they always are at this kind of range. For anyone looking for a specific potential snowfall event to focus on I'd suggest that the northerly around +144 has a lot of potential:

http://cdn.nwstatic.co.uk/gfsimages/gfs.20121120/18/144/h850t850eu.png

The ECM flagged it up about 3 days ago but even last night the GFS wasn't giving it much of a chance:

[samadamsuk]

Thanks for the heads up Liam/all

Some pretty intense lift on that squall http://www.wetter3.de/vertikal.html

Latest GFS showing a reconfiguration in about 10 days with the blocking high tightening up around greenland introducing some very cold air from the NE, but the question will be I suspect how long the low in the med and the the low heights in the west atlantic can hold off interfearing and placing us below the polar front with the block to the north ..

Cheers, Sam

[summer blizzard]

GFS18z is certainly the best run yet for maintaining cold uppers although i can't stress enough how we need to keep the high away from Scandinavia so that we can drop a trough into Scandinavia and back the cold west quickly.

Looking at the JMA, GFS and ECWMF it seems that each disagrees with the positioning of the trough south of the UK and that it what is making the difference between the snow and drizzle scenarios.

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...atures/20419248

You can tell some forecasters love to go into deeper detail than the TV forecasts.

[Met4Cast]

Righto, maybe THIS will cheer some of you up.

[Harve]

Righto, maybe THIS will cheer some of you up.

Did you notice later on, -10c 850hPa temperatures reach Scotland and Northern England in a slack flow?

This could deliver wintry showers to eastern England and rather low minimas further north.

I estimate that the UK spends 10 days with average hPa temperatures of -5/6/7c across the bulk of the country.

[Met4Cast]

Did you notice later on, -10c 850hPa temperatures reach Scotland and Northern England in a slack flow?

This could deliver wintry showers to eastern England and rather low minimas further north.

I estimate that the UK spends 10 days with average hPa temperatures of -5/6/7c across the bulk of the country.

Yep, massive upgrade in terms of cold uppers across the UK later next week! Still firmly out in FI but good to see the models starting to toy with this idea

[IDO]

Short term the 0z op is great for cold uppers (and snow potential from T172) but paradoxically medium to longer term it breaks the block and the PV re-organizes:

http://cdn.nwstatic....384/npsh500.png

With a more zonal look:

http://cdn.nwstatic..../384/hgt300.png

The Alpha jet now parking to the north of the UK. Still chilly though. Obviously only one run but another option to the stack of potential outcomes. About 20% support from the ensembles for this progressive return to a quasi cool zonal setup:

http://www.meteociel...&mode=6&ech=384

However the 0z op is in the colder 10-15% 850s (London) ensemble members throughout most of the run:

http://cdn.nwstatic..../t850London.png

So little support for the cold uppers from later next week. From as early as T120 there is a scatter of about 7.5c 850s suggesting the progress of the colder uppers has minimum confidence at the moment. At the end of FI the uppers vary from +6c to -8c with the mean at -2c. The waiting game continues and maybe we will know more by the weekend. That BBC weather feature: http://www.bbc.co.uk...atures/20419248 had a lot of "ifs" and "maybes", highlighting that things are very uncertain, though cool to cold still most likely, but duration wise, possibly showing a more transient nature.

Edited November 21, 2012 by i'm dreaming of...

[xioni]

Another non-cold EC op run unfortunately, this one actually has above normal temps for next week despite the blocking

I suspect the 00z ensemble from EC will also make a move but still plenty of time for EC to flip back to colder solutions

[xioni]

Still goes cold from Thu though so it's more of a delay but delays are rarely good

[shotski]

Still goes cold from Thu though so it's more of a delay but delays are rarely good

To be honest it looks like an upgrade to me, the arctic high looks more intense on this run and its pushing the cold our way.

[Ian Brown]

Very interesting output this morning, the GFS goes for a link up with the High to our West with emerging pressure rise over Scandi prolonging North Easterlies over the UK, whereas the ECM goes for retrogression to Greenland and lowering of heights over Scandi.

[IDO]

Great post. Perhaps all the moaners could kindly read this before posting more garbage.

Hi. Could you post some charts that show the cold will last the 10-14 days to enable the cold uppers to reach the UK. Referring to what happened in 2010 is really unhelpful as this was a very rare occurrence and should not be used as an example of what will likely happen.

ECMW has excellent potential but that is all at the moment. Even at T240 the cold has not hit us yet, though it maintains its hint of a strong block. The December CFS 850 anomalie has the UK with average 850s: http://modeles.meteociel.fr/modeles/cfs/run/monthly/cfs-1-12-2012.png. Two Buzz: http://www.theweatheroutlook.com/twoother/latest.aspx, also highlighting a breakdown possibility around 10-14 days.

Its too early to be confident either way, other than high confidence in a pattern change, but having to rely on the block to stay for two weeks plus to enable a constant flow of cold uppers in our neck of the woods is a big ask, possible but not probable. So when realists post their lack of confidence I suspect it is from experience of these setups and to call them "garbage" is rather condescending.

FYI my views are that we are heading to a cold setup till about mid December with an above average chance of snow for many, probably.

[MKN]

Ecm looks better long term with a decent Greenland high in place at 240. Earlier on though 850s are milder than the gfs.

[Duane S.]

Broad pattern is there and firmed up, GFS and ECM are different but both very good and nice the see the UKMO back on board.

In fact, fairly stonking charts this morning. In fact, the charts starting to show how daft some of these 'omg look at the lack of low 850s' posts have been over recent days, even from more some experienced posters.

Be objective but for goodness sake let's enjoy what's setting up here before our very eyes. I await the 1st "sos!! the pv is starting to reform at +384!" post...

[bluearmy]

Please please please give up on the disrupting trough and initial flow to bring any lowland snowfall. Its all falling into place next week. Watch for developments to follow as we enter dec.

Good runs this morning

[Gibby]

Good morning. Here is the review of the 00zs from GFS, UKMO, GEM and ECM for today Wednesday November 21st 2012.

All models continue the unsettled theme between now and the beginning of next week as Low pressure areas to the NW continue to dominate the UK weather. All areas will see spells of very heavy rain with flooding becoming an issue for some areas on occasion through this period. With time Low pressure migrates further South and SE to end up to the SE of the UK with a rather cold NE flow setting up across the UK, strong in the South by early next week.

GFS then shows High pressure to the North and West next week with Low pressure over Southern France with a cold and raw North toNortheast flow continuing to blow across the South and East of the UK. There would continue to bring rain at times with some sleet or wet snow on the higher ground. In the NW there will be less in the way of precipitation as a ridge of High pressure lies jut to the NW. In FI today the rather cold conditions persist especially in the South as the pressure pattern remains very similar for some time. Later a ridge is shown to slip down across the UK from the North with lighter winds and frost and fog developing overnight. Itt looks like a Northerly could be on the cards after the end of the run.

The GFS Ensembles show that the operational was very much on the cold side later but not without support. There are a few mild options shown but in general the ensembles are firming up on a prolonged spell of below average uppers across the UK starting from the weekend.

The Jet Stream continues it's trend from last night of blowing NE across the UK for the next few days before ridging strongly North through the Atlantic all the way to the Arctic later next week with the returning arm sinking South to the West of Ireland to Spain and North Africa.

UKMO for midnight on Tuesday shows Low pressure centred over Eastern England with a cyclonic flow over the UK. There would be a continuation of a lot of rain with temperatures gradually on the slide as winds settle more towards the North later.

GEM shows Low pressure in a belt from Spain to Denmark at the same time with a rather cold and raw NE flow over Britain with rain at times. This veers Easterly and strengthens further for a while later in the run before the High pressure to the North sinks down over the UK to bring a quieter spell of weather with cold and frosty weather with fog, slow to clear

.

ECM shows Low pressure centred over SW England with a wet and breezy day shown for Monday. This model too shows Low pressure then sinking SE to Europe with a rather cold NE flow developing through the middle of next week before a ridge slips South over Britain later with drier and rather cold weather persisting temporarily before a cold front moving South on Day 10 is likely to bring very cold weather with snow showers in the days that follow the expiry of the run.

In Summary a rather cold spell is on the way. All models show variations on a theme but the general pattern will become of Low pressure to the South or SE and High to the North, West and possibly NE. A cold NE flow looks likely and while the period of the runs do not suggest anything very cold there are increasing signs that a blast of very cold Arctic winds could be on the horizon towards the outer stages of the outputs preceded by a spell of Anticyclonic frost and fog briefly. In the shorter term it cannot be ignored that there will be a lot of rain for many parts of the UK with flooding an issue in the next 7 days chiefly in the South.

[weather eater]

Good runs this morning and I’m sure the GFS 00z will have made some of you perk up a bit, but I have to say I think the ECM will be nearer the mark, firstly because it has better verification stats than the GFS and two, because it ties in better with the prognosis of heights building to our NW.

Edited November 21, 2012 by weather eater

[bradymk]

Great output again this morning. Both GFS and ECM going for some very cold options but down different routes. Personally, I like the ECM as it builds a nice Greenland High with a Northerly by the end of the run. The next few frames would of been good!

Just for some fun this morning aswell, latest CFS temperatures for next 4 weeks. That is one cold December.

[nick sussex]

In typical model fashion now that we have got cross model agreement on the upstream phasing of those lows we now find disagreement as to where that shortwave drops south.

The ECM seems resolute here with dropping it south to the west, and thats two runs in a row. The GFS has the best option in terms of cold in the higher resolution and the UKMO is a halfway house.

Although the ECM eventually gets low pressure into the heart of Europe it does throw alot of milder air north into Europe beforehand.

I'm hoping the ECM might be wrong here but when it does this two runs in a row within 144hrs you really can't ignore it.

Thereafter it develops a typical retrogression pattern with troughing developing to the east, the only thing to be wary of in these situations is not to have the pattern too far west.

If we get lucky then that troughing and high combo should pull down some much colder uppers and snowfall.

Indeed even in the earlier timeframe a finger of much colder uppers gets funnelled sw out of Scandi,the GFS looks better for this, the ECM has already started its retrogression and because of this any colder uppers at that point generally only skirt with the UK before being funnelled into the Continent.

We should know this evening whether the other models will follow the ECM with that shortwave.

Edited November 21, 2012 by nick sussex

[phil nw.]

Posts that snipe at others have and will be deleted.

If anyone has a different view then express it by referring to outputs.

If you are a glass half empty type then you can use the moaning thread.

Thanks.

[chris55]

Great runs this morning, up to day 6 we have some broad cross model agreement (with only small differences)

http://www.wetterzen...cs/Rukm1441.gif

http://www.wetterzen...cs/Recm1441.gif

http://www.wetterzen...s/Rtavn1441.png

where we go from this point is open to all sorts of cold options, GFS this morning is just one possible outcome, but goes to show small shifts here and here CAN get cold air in pretty quickly. ECM at day 10 also shows much much colder air coming into the mix, although not making it to the UK on this run, it soon would.

http://www.wetterzen...cs/Recm2402.gif

Very interesting model watching at the moment!!!

Edited November 21, 2012 by chris55

[SMU]

Yes A good haul this morning from the top models-

The best news is the Allignment of the Arctic high- alligning the cold directly west South West into Scandi-

the UKMO very fast with this - ECM a few days lag behind.- GFS in th middle.

Again a few days before we get total clarity, but I feel the interest has perked up a little today- just the ECm that was a little disapointing in the mid term, a good recovery at the end with the -20c isotherm getting into the low countries...

Looking good for the start of December now.

IGNORE THE 06Z RUN!

S

[SMU]

PS the BOMB run today as ive renamed it is the form horse- I always said those Aussies had the best model!!

[nick sussex]

I think the BOM at 240hrs is the best case scenario with a good shape to the Greenland high.

The pattern is not too far west and you'll get disturbances running sw into the UK.

http://www.meteociel.fr/modeles/bome_cartes.php?&ech=240&mode=0&carte=1

The core of the high is ideally placed.