Model Output Discussion - 18z - 25th Nov onward

[Catacol]

I am not 100% sure but I don't think these update over the weekend so the one you see is from Friday, you are better off using the ECM/GFS 500MB over the weekend.

Dated 25th November.

[Met4Cast]

Just going to post these GFS Ensembles from 2009 to show you how useless the model can be at times

[Iceaxecrampon]

FAX for Friday....

http://www.netweathe...;type=fax;sess=

I see the 528 DAM line is skirting East Anglia - any expert opinion on this chart as I am only a simple mountaineer

edit: - apologies - click on +120hrs

Ian

Edited November 26, 2012 by Iceaxecrampon

[SMU]

Morning All-

ITS ALL ABOUT THE PHASING OF 2 SHORTWAVES.........

Well for those people that are new, & for those that have been here for years - what can you say- a rollercoaster of epic proportions.

We are getting down to the finite detail of the models today & in terms of whats been shown is the at this particular time the model divergence is at T72-96. Over the last 24/36 hours we have been monitoring the development of the canadian low that has been shown with various permutations - right from being a fish storm, to an inland runner -

If we cast our minds back to 10 days ago we were talking about 3 stage retrogression & the fact of how hard it is to get the upper air cold pool to reach the UK, well we have overcome the first 2 hurdles, however the final phase as ever is the biggest jump to leap- almost the beachers brook!

The handling of the shortwaves is critical & the inflection of the jet.

http://www.meteociel...2-102.GIF?26-06 UKMO phases the Polar jet with the the subtropical jet moving north & quickly develops it-

Which brings together 2 systems into a deeper low-

Because the 2 systems morph into one they have the net effect of tugging the one off the east coast back west.

The ECM doesnt get the phasing- as well as the UKMO, however its a signal thats better than 12z yesterday-

Today

http://www.meteociel...H1-72.GIF?26-12 Better, not as good as UKMO.

http://www.meteociel.../ECH1-96.GIF?12 No Phase- fish storm

Im not inputting the 06z GFS.- as it has the worst atlantic performance going.

What does this difference mean>?- EVERYTHING- Why?> Because this track will determine the track & intensity of the atlantics push East- remember the block over the Svalbard area isnt fully established & isnt rock solid at this side, so could be brushed aside by a reinvigorated atlantic.

Take these 2 charts across a 400 Mile piste-

NOGAPS

This is an early phaser & pulls the low west of greenland-

GFS

No real phasing - south of the tip

Its SO important to get the energy up the western side of greenland- because that big huge chunk of glacier will decelerate & difuse a deep low.

In the 90's every single low went under that tip- named by Ian & some the corridoor of death ( well it was for the UK)-

Since then though we have had a lot of blocking supported by the SSTA in that region.

TIMINGS & TELECONNECTIONS-

Broad brush teleconnections are put in the background here- yes the AO is going to go negative, & so on & so forth, however we will not be able to pin this on such big features- even the stratosphere is the king here-

Weather is about TIMINGS-

If we look at the supserstorm Sandy that went out to sea then recurved west back into the east states it was a set of unique paramenters & timing that allowed everything to fall in place for a 50 year storm.

We are not looking at those sorts of %ages, its a plain & simple issue of model interpretation of mesoscale features within the jet streak & their interactions.

We cannot slate the models really for this-

To put it into perspective the UKMO through GFS & ECM may be different at T 24 over the SE states by 2-5 M/S across the 200 MB jet.

That timing & speed difference will put 2 shortwaves in 2 different locations by T72,-- Huge miles no..- maybe 100-200 miles difference.

This difference snowballs the WHOLE NH pattern,

So can anyone forecast this. NO. not me, not anyone on here, Not the NOAA,- its all best guess, & because of the significant change then it makes it a coin flip.

I guess there is some merit in the UKMOs consistency, however it will only take one adjustment of those phasing lows & the UKMO will be significantly different come tea time.

I for one have enjoyed this rollercoaster, sometimes its almost as good as the actual event, The model thread has been well moderated & opinions have mostly been well considered.

Will we know this afternoon the full outcome. NO- tomorrow 12's 100%, however we will have less divergence today.

One thing we havent considered is the middle ground *could* be ok- if the low gets up western greenland but sends energy SE below it then would could get a 78/79 type scenario where the jet straddles the UK with snow on the eastern flank.

Also the ECM this morning suggestive of longer term cold- just because it adjusted NW on that low enough to get the energy west of greenland-

Gripping stuff, remember no model can be beaten up on this- its about MACRO scale changes around T24-48

see you for the 12's

S

[SMU]

Exeter lean towards the block being a temporary phase ahead of return to mobility, albeit their detailed 6-10d prognosis (not copied below, but abstracted version of their latest (00z run) analysis is shown here, incl to trend period) expresses obvious uncertainties (including as emphasised in some posts here already):

"4. Trend for Days 10-15 : Still much greater than average level of uncertainty through this period. Most likely scenario is for rather cold unsettled cyclonic W to NW to displace the cold/blocked type at least for time but with an increasing probably that blocking becomes re-established with the UK within the cyclonic limb of the block.

5. Discussion : Slightly better agreement between MOGREPS and EC ENS to break down the block and allow more atlantic mobility by around day 8-9 and EC still supports this breakdown longer into the trend period but still a big spread in degree of cyclonicity in terms of track, depth and orientation of fronts. No major change of story suggested for the 6-10 day period From as early as this weekend a major divergence of weather types is present in the ensembles. The majority of MOGREPS and NCEP members retain the cold block over the UK, with any breakdown only partial and only affecting the far W/SW. EC deterministic, along with about 50 % of the EC ensemble have a full return to Atlantic mobility and much milder conditions by as early as Sunday. Interestingly, the zonal spell may only be temporary with EC deterministic trending colder again by day 10. Clearly a good deal of uncertainty is present in this period and a compromise has been made in allowing a partial breakdown of the block to take place later in the weekend and early into next week. Such a scenario brings a risk of heavy rain to the SW and a risk of sleet and snow along the leading edge..."

Ian your post at the end is exactly the same of mine- but without the benefit of the MOGREPS model. A tricky navigation of cold weather forecasting- with high potential for snow, however we could be at the margins for it- no surprises there for the UK-

S

[Paul]

Dated 25th November.

At the weekends those maps are entirely automated, during the week they're manually adjusted by forecasters to take into account model bias and the like, so the weekend output is generally less reliable.

[london-snow]

Exeter lean towards the block being a temporary phase ahead of an equally temporary return to mobility, albeit their detailed 6-10d prognosis (not copied below, but abstracted version of their latest (00z run) analysis is shown here, incl to trend period) expresses obvious uncertainties (including as emphasised in some posts here already):

"4. Trend for Days 10-15 : Still much greater than average level of uncertainty through this period. Most likely scenario is for rather cold unsettled cyclonic W to NW to displace the cold/blocked type at least for time but with an increasing probably that blocking becomes re-established with the UK within the cyclonic limb of the block.

5. Discussion : Slightly better agreement between MOGREPS and EC ENS to break down the block and allow more atlantic mobility by around day 8-9 and EC still supports this breakdown longer into the trend period but still a big spread in degree of cyclonicity in terms of track, depth and orientation of fronts. No major change of story suggested for the 6-10 day period From as early as this weekend a major divergence of weather types is present in the ensembles. The majority of MOGREPS and NCEP members retain the cold block over the UK, with any breakdown only partial and only affecting the far W/SW. EC deterministic, along with about 50 % of the EC ensemble have a full return to Atlantic mobility and much milder conditions by as early as Sunday. Interestingly, the zonal spell may only be temporary with EC deterministic trending colder again by day 10. Clearly a good deal of uncertainty is present in this period and a compromise has been made in allowing a partial breakdown of the block to take place later in the weekend and early into next week. Such a scenario brings a risk of heavy rain to the SW and a risk of sleet and snow along the leading edge..."

Morning Ian,

So is it right for me to say after reading your update that it looks to be heading for a battleground type situation where eastern areas is more favored to keep the cold in place?

If so buckle up its going to be one heck of a ride i remember that these situations can delight or disappoint potential for large amounts of snow.

[beng]

http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=namk&MODELL=nam&MODELLTYP=1&BASE=-&VAR=prec&HH=66&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD=

One good sign is that the NAM (which is a high resolution model for short term forecasting) looks to be developing the American East coast low.

[Catacol]

At the weekends those maps are entirely automated, during the week they're manually adjusted by forecasters to take into account model bias and the like, so the weekend output is generally less reliable.

Thanks - I assumed that once the date was updated that meant forecaster input had occurred. Best wait for the forecaster updated one today then, before getting bullish about a medium term blocked atlantic!

[Rollo]

I would like to thank Steve Murr for that graphic description of the current state of affairs-we will all be put out of our misery sooner rather than later.

[bobbydog]

a point some people should remember- the models are a guide to forecasters, not THE forecast.

for example- even if the UKMO model is unwavering in its output, it doesn't necessarily mean that the Met Office forecast will follow it to the letter. the fax charts ARE a forecast based on the forecasters' opinion of all the models and the vast array of tools available to them.

people say "don't take one run as gospel" well, - don't even take any one model as gospel... the forecasters don't.....

edit- point proven by latest Meto update.

Edited November 26, 2012 by bobbydog

[xioni]

I posted the 5 day cfs mean chart earlier - here is the slightly longer range one - there is nothing remotely zonal or mild on the horizon from this up to date mean analysis. In fact this is reverse zonality at its best!

This chart might now show a zonal setup but it's not particularly cold either. The pattern to the west of the UK is quite flat/mobile (green lines)

[karlos1983]

FAX for Friday....

http://www.netweathe...;type=fax;sess=

I see the 528 DAM line is skirting East Anglia - any expert opinion on this chart as I am only a simple mountaineer

edit: - apologies - click on +120hrs

Ian

what happens to that Low Pressure to the west of Iceland between 96 and 120? it just disappears......

[Iceaxecrampon]

Welcome to netweather, maximus!

Are you sure? - this site drives me bonkers

[Jaffusmaximus]

Are you sure? - this site drives me bonkers

I have an addictive personality and now chart watching has become my latest vice since I cannot afford to bet on the horses.

[IDO]

UKMO now following (as usual) GFS and ECMW:

UK Outlook for Saturday 1 Dec 2012 to Monday 10 Dec 2012:

Wintry showers are expected to affect some eastern areas on Saturday with snow on hills, possibly to low levels in the north. Elsewhere, largely dry and fine, but cold with the risk of frost, icy patches and overnight freezing fog. By Sunday, rain may spread into the far west, preceded by snow, mainly over higher ground. There is a lot of uncertainty for the remainder of the period but it is likely that northern and eastern areas remain cold with wintry showers for some. Elsewhere, it will probably become milder, at least for a time, but also more unsettled, with rain and hill snow accompanied by stronger winds. Later in the period, colder but drier conditions may become re-established across the UK, with frost and icy patches for many.

Updated: 1107 on Mon 26 Nov 2012

Hopefully the second shot will be more fortuitous.

[AWD]

Taking Ian's post above in context, that isn't a bad forecast for December as a month.

Stepping back from wanting an imminent cold & snowy spell and looking at the bigger suggested picture, a second bite of the cherry might be better for the UK in the long term.

Both the GFS & ECM bring cold uppers and snowcover into Europe from this weekend onwards. This would cool the continent down quite a bit after recent mild air over there. This therefore would help the UK in the future with any potential Easterly or North Easterly flow.

http://www.meteociel.fr/modeles/ecmwf.php?ech=168&mode=0&map=1&type=0&archive=0

http://www.meteociel.fr/modeles/gfse_cartes.php?ech=168&code=0&mode=1&carte=1

For the UK, a brief cold spell, followed by cold zonality with depressions travelling on a NW/SE axis would also provide snowcover for northern hills and possible transient snow events to lower levels.

If what Ian has said above comes to fruition, and as long as I am reading what he has said correctly, then basically the Meto favour a brief blocking spell followed by a return to cool/cold zonality followed by a possible return to blocking. If this does happen, and blocking does reappear, not only will we be in winter proper by then, but the cold air over the continent and to the north would be much more accessible to the UK, making it easier & quicker to tap into it.

Anyway, I'm starting to speculate now, but what I'm trying to say is a collapse of the block this weekend might necessarily be a bad thing as it could provide a stronger block further down the line, at a better time in the season, with the colder air much nearer.

There are many teleconnective analogues and Stratospheric charts that could support this as well as some NWP FI output.

At work, so can't go further as the boss wants me! .

[nick sussex]

In a strange sort of masochistic way I'm quite enjoying this latest model drama! It's certainly more compelling viewing than anything on the tv at the moment.

Going back to SM's post he sums things up very well , thanks again SM for all your in depth analysis.

It really is impossible to say exactly how this will turn out and so we just have to wait and see what happens off the eastern USA.

In the meantime I'm teleporting myself over to the eastern USA and will be trying to find as much info as possible on what could happen!

[SLEETY]

UKMO now following (as usual) GFS and ECMW:

UK Outlook for Saturday 1 Dec 2012 to Monday 10 Dec 2012:

Wintry showers are expected to affect some eastern areas on Saturday with snow on hills, possibly to low levels in the north. Elsewhere, largely dry and fine, but cold with the risk of frost, icy patches and overnight freezing fog. By Sunday, rain may spread into the far west, preceded by snow, mainly over higher ground. There is a lot of uncertainty for the remainder of the period but it is likely that northern and eastern areas remain cold with wintry showers for some. Elsewhere, it will probably become milder, at least for a time, but also more unsettled, with rain and hill snow accompanied by stronger winds. Later in the period, colder but drier conditions may become re-established across the UK, with frost and icy patches for many.

Updated: 1107 on Mon 26 Nov 2012

Hopefully the second shot will be more fortuitous.

How can you say that when even the pros admit their is lots of uncertainty about how things unfold

[Dean E]

It's the same mostly every cold spell. Within a few days of the pattern change, the models start to throw in "twists" to the mix and we all start to question what is going on. From past years, I would expect a few more episodes like this up to the event. I even remember the November/December 2010 event had one of these episodes at one point... Then the ECM went out on its own with the GFS getting a grip and following straight behind.

I think people need to stay firm to the overall output of the models over the past few days and get a sort of "average" for now, until the models get a better grip on the situation.

The 6z is a clear example of someone who thinks they know a way to a place, decide to ignore maps and end up having to ask someone for directions... In other words, don't trust it

SM

[Hammer]

If as highlighted that Canadian low that moves up possibly to western side of Greenland is a feature that may well have significance later in the period for UK. Other than a search of the internet are we able to see what the weather sites, maps and more local resources there see as happening later on in the week. Thursday/Friday seem a key period. I think it was Nick S that mentioned some news from Maine last night.

[london-snow]

UKMO now following (as usual) GFS and ECMW:

UK Outlook for Saturday 1 Dec 2012 to Monday 10 Dec 2012:

Wintry showers are expected to affect some eastern areas on Saturday with snow on hills, possibly to low levels in the north. Elsewhere, largely dry and fine, but cold with the risk of frost, icy patches and overnight freezing fog. By Sunday, rain may spread into the far west, preceded by snow, mainly over higher ground. There is a lot of uncertainty for the remainder of the period but it is likely that northern and eastern areas remain cold with wintry showers for some. Elsewhere, it will probably become milder, at least for a time, but also more unsettled, with rain and hill snow accompanied by stronger winds. Later in the period, colder but drier conditions may become re-established across the UK, with frost and icy patches for many.

Updated: 1107 on Mon 26 Nov 2012

Hopefully the second shot will be more fortuitous.

The mods will be quite busy given the nature of that forecast imby will start kicking in from some...

After reading Ian's update no model or forecast can be taken as gospel at the moment as the difficulty of this pattern going forward. The one thing i will add is that it has a very classical old type pattern and feel about it. The only spin that could be put on this situation is that the further east you are the more chance that you will benefit.

Edited November 26, 2012 by london-snow

[phil nw.]

The 00z operationals again undecided wrt to the energy split off the Canadian vortex resulting in different medium term solutions.

It looks to me that a cold spell starting later this week is still very much supported if looking at the less volatile data.

The fledging block expected to build in the Atlantic will be enough to divert the flow towards the north bringing that colder air south by T72hrs.

Interesting that the latest fax for 12z Thurs hints that the block may be slightly stronger than last night`s issue if we compare here.

Again If we look at the NAEF`s pressure Anomls. and mean 850hPa temps we can see the cold set up established by the week end.

Beyond there is some suggestions from mean outputs and the later NAEF`s that some energy will cut through the block from the north west bringing in less cold air with rain crossing the UK as the signal for Greenland heights fade somewhat.Here are the 2 mean charts at day 7.

A quick look at the Warks Gefs graph and from both models the London 2m temps.

do suggest a slight warming again next week although still rather cold.

If we look at the 00z GFS mean hts pattern we can see that although we have a disrupted vortex with 2 centres it seems the Canadian sector is contolling the Atlantic pattern throwing too much energy towards Greenland and then south east preventing a stable Atlantic block.

From this information it looks more likely from the 00z outputs that our approaching cold spell may be modified both in length and severity.

Edited November 26, 2012 by phil n.warks.

[Matty M]

A last chart before we recess before the 12z. I have project work to do :'(

Comparing the NAM 6z and GFS 6z @T84hrs

I know which one i prefer!

GFS

NAM

[Glacier Point]

Seems like a parallel universe with the modelling I'm seeing and the reasoned assessment of teleconnectors compared to some other interpretations doing the rounds. The interesting period emerging where we establishing just where residual bits of polar vortex are positioned.

At the crucial t72-96 range, ensemble means and the balance of operationals appearing to suggest the troughing south of Greenland will disrupt allowing for an advection westwards of the cold air and lowering of pressure across Europe.

At t144-t168, we have some reasonable agreement for the second low to break through and head SE towards the European trough. Losts of complications there yet to be resolved but with blocking signature still in place to our north, probably not going to alter things too much - likelihood that the low will become adbsorbed with the main European trough with a temporary rise in temperatures towards average before falling away again, The mean temperature anomaly for the 15 day period looking between 2C and 4C below average (which is not far off ECM mean).

The day 6-10 and 11-15 GEFS H5 anomalies and T850 anomalies still very persistent in suggesting a sustained below average start to December.

Note the persistence of the upper low over Europe and the development of another ridge in the North Atlantic south of Greenland which re-inforces our cold block towards the end of the period. Note also, the ridge over the North Pacific which has not been well modelled in the last week. That teleconnects very strongly with the mid Atlantic ridge solution.

So a cyclonic cold block looks the order of the day for December, pressure higher to the south of Greenland.