Model Output Discussion

[johnholmes]

nice to see someone who has close links with UK Met giving a positive turn to what is often a fairly critical posting on the web.

As an ex senior forecaster with them, I too am critical when it seems to require it but like to see a balanced view showing up.

[Leigh]

Sorry to ask a stupid question. But if this cold spell does happen, what kind of daytime temps are we looking at getting middle of next week?

[snowlover2009]

Sorry to ask a stupid question. But if this cold spell does happen, what kind of daytime temps are we looking at getting middle of next week?

any fog, could make temps not get above 2c. If you look at gfs for next week it has a lot of snow and temps around 1 to 5c widely. so maybe even a few ice days. but not too sure yet.

[Leigh]

any fog, could make temps not get above 2c. If you look at gfs for next week it has a lot of snow and temps around 1 to 5c widely. so maybe even a few ice days. but not too sure yet.

Thank you

[Matty M]

The pesky shortwave.

Cause of and End of potential siberian blast.

Nevertheless the outlook is very positive and if the easterly doesnt pull through on the first,second... attempts

i think eventually it will, we must be patient and wise.

I would say however 9/1 for a white christmas in Paris may be bet of the month!

[Glacier Point]

Something of the GFS 12z being too progressive and the ECM 12z being too slow in its evolution I think.

The global wind oscillation is locked in phase 4, and will likely remain there for some time (short term fluctuations excepted) as increasing relative and total angular momentum plys with waxing and waning torques.

http://www.esrl.noaa...gcm/gwo_40d.gif

Under Nino conditions, the composite for phase 4 suggests low pressure to be the dominant Atlantic factor, which is important when a strongly negative Arctic Oscillation is in play as it suggests undercutting rather than the jet being forced north. It also suggests that the ECM 12z operation view with a sustained ridge to our west and close NW is unlikely to verify. Interestingly, the GFS is becoming quite bullish on a very negatively tilted longwave trough to attempt to slide in from the Atlantic. This is the battleground scenario. If the cold uppers make it to the UK before the trough works in, the chances of a sustained -NAO becoming locked increase significantly as the jet has no where to go but south, further advecting cold air from Scandinavia to the UK. The pattern becomes very progressive under such circumstances.

Phase 4 GWO

A cross section of ensemble mean guidance:

CPC blended 8-14 mean height anomaly

Core of the +ve height anomaly over Greenland and troughs centred either side of the western Atlantic and exending across much of central and eastern Europe.

GFS 00z mean height anomaly days 11-15

Similar to the CPC output but slightly weaker height anomaly over Greenland. 3-wave pattern still going strong.

GFS 12z mean height anomaly days 11-15

This is similar showing the core of the +ve anomaly over Greenland but much greater anomaly with the Atlantic trough extending across southern Europe to join with the trough dropping out of Russia. Probably the right evolution given the atmospheric variables but timing too soon ? The CPC outlook has this same level of blocking (+210 m) in this region. 3 wave even stronger.

CPC constructed historical analogue days 8-14

Broadly similar but everything slightly west centering the trough over the UK and Scandinavia.

The model guidance and synoptic analysis both pick out the +ve height anomaly over Greenland in the extended period. Therefore, as per yesterday, any models which do not show this should be a least treated with a degree of caution. That applied very much to the ECM 00z and still does a bit to the 12z. I am consistently drawn to the trend advertised by the GFS across its model suite to slide that trough in from the SW which is oozing snow potential.

Edited December 7, 2009 by Glacier Point

[Frosty.]

Ensembles are great HD...the ensembles are faster then the 12z ECM op by a good 12-24hrs in general, also the GFS is probably going to slow down over the next few outputs if it continues with this cold set-up, there is little chance things progress quite as fast as it suggests IMO.

The key thing is the ECM ensembles are trending colder and colder and that there are now around 80% going for a colder option...all good news guys at the moment.

What can possibly go wrong then kold, is it inevitable that we are heading towards a noteworthy cold spell or will it be scuppered by shortwaves as usually happens? Everything is pointing towards a potentially severe wintry spell but i'm sure some gremlin in the system will spoil it...hope not though

[swfc]

After seeing ecm earlier today was nice to see an upgrade.gfs as ever is showing arctic conditions later on but for all the talk of it being a know go its there for a reason.What will it bring later.???.Cant see an upgrade but you never know.In my humble opinion the ukmo is the one 2 follow in the reliable time frame.Paul hudson did say tonight that an exciting spell of weather could be on the cards for next week,thats blown that then.

[weather eater]

Just going to post a couple of charts here and would like to know what people think. The first chart is from the 1st December

predicting todays weather 7 days out. and the 2nd chart is todays +0 . How do you think GFS has done from these charts ?

1st Dec predicting the 7th

Not bad Chris is the easy answer; let’s hope it does as well in the coming week.

[snowmadchrisuk]

Not bad Chris is the easy answer; let’s hope it does as well in the coming week.

That is what I thought , overall GFS did great , I just wanted to know if anybody else would notice

[Thundery wintry showers]

...

(Incidentally - I will certainly defend the MO's guys on their February snow warnings this year; I sat through many briefings from them ahead of those events and with very few exceptions their areal and temporal accuracy down to sub-regional level was extremely good).

Good to see you on here!

Regarding the February warnings, I thought they were more accurate the further south we were- the warnings regarding the West Country and south-east England were generally spot on, but if I remember rightly the warnings for Scotland and NE England were not as accurate. (I must admit I can't recall specifics any more- those were just my feelings at the time).

Regarding their forecast output I think the FAX charts are the most accurate of anything out to T+120. I also think, personally, that the Met Office are right to be cautious on the detail re. snowfalls at this range. Where I do criticise them is in the vagueness of their seasonal forecast, especially as the maps show a lot more detail than is suggested by the forecast.

I still think the upcoming situation is on a knife edge. There is still a lot that could scupper this potential developing cold snowy setup but it's a good sign for cold/snow lovers that the teleconnections are in strong disagreement with the ECMWF and particularly its 00Z.

[kold weather]

It still could go wrong Frosty, there are always methods for it to go wrong but I think even the worst case is to get a sustained cold high nearby the UK, the best is anything you can spot from the very coldest of our winters in the past.

GP's post is once again great and shows what could happen, if we can get an easterly then and the upper high far enough north then its going to be very hard to see a short term way out of this, its the type of set-up that will beocming re-enforced by the jet...the type we've seen probably only a couple of times since 1997 but was quite present in many 60s winters...its the type of set-up where even if there is next to nothing over Greenland as long as pressure is relativly higher then it is to the south, we will keep in the cold set-up...its also the hallmark synoptic pattern of a long lasting pattern (usually anything from 2-3 weeks, and has a higher then normal chance of reloading again some point down the line) and is the type of set-up that when it did finally breakdown would see a big snow set-up.

For now the big pattern is getting into the pattern however, if we get there then odds have to favour a quite impressive spell of cold.

[Optimus Prime]

Depends whether we get low pressure undercutting the high to the north how long it'll last. If we don't get sandwiched between the two there goes the cold feed.

[The Eagle]

The GFS could well be wrong. The UKMO and ECM are almost identical at 96hrs. The GFS is the odd one out. This could mean the scenario of a blink and you miss it Easterly on the ECM. I hope this is not the case. Plenty of potential but like KW says there is still alot that can go wrong.

[Paul]

Just while the 18z starts to roll, just like to say thanks to all you model watchers, so far today we've been having just short of 30,000 charts viewed an hour on the free datacentre chart viewer, good work folks - lucky we upgraded the servers recently eh?!!

http://www.netweathe...tion=nwdc;sess=

Wonder if we can beat that tomorrow and beyond through the winter

[Frosty.]

It still could go wrong Frosty, there are always methods for it to go wrong but I think even the worst case is to get a sustained cold high nearby the UK, the best is anything you can spot from the very coldest of our winters in the past.

GP's post is once again great and shows what could happen, if we can get an easterly then and the upper high far enough north then its going to be very hard to see a short term way out of this, its the type of set-up that will beocming re-enforced by the jet...the type we've seen probably only a couple of times since 1997 but was quite present in many 60s winters...its the type of set-up where even if there is next to nothing over Greenland as long as pressure is relativly higher then it is to the south, we will keep in the cold set-up...its also the hallmark synoptic pattern of a long lasting pattern (usually anything from 2-3 weeks, and has a higher then normal chance of reloading again some point down the line) and is the type of set-up that when it did finally breakdown would see a big snow set-up.

For now the big pattern is getting into the pattern however, if we get there then odds have to favour a quite impressive spell of cold.

Thanks for the excellent reply kold

I can't see how today's GFS 12z could be improved on, it's the holy grail run in FI with a bitter E'ly followed by an equally bitter N'ly and a prolonged N'ly as well, one that could just reload and reload for many days. Like everyone else, I'm chewing my nails just hoping this really happens and it's not just setting us all up for bitter disappointment. All eyes down for the 18z

[kold weather]

Yep OP, whats interesting is both the UKMO and the GFS are more agreessive with developing cut-off lows even at 144hrs that head to our SW, of course the ECM does get there as well but I think GP's got the right idea, if this does come off then the GFS is too rapid, the ECM probably a little on the slow side.

By the way, the 12z ECM ensembles looking pretty cold in the long range:

http://www.knmi.nl/exp/pluim/vijftiendaagse/index.html

[Optimus Prime]

By the way, the 12z ECM ensembles looking pretty cold in the long range:

http://www.knmi.nl/exp/pluim/vijftiendaagse/index.html

Eventually I will force myself to learn those line graphs!

[cooling climate]

Excellent write up GP, cheers.

Reinforcing my own beliefs along with what JH wrote earlier we could well be on the

cusp of something not seen in December in this country since 81.

Another couple of days though to be certain.

Excellent extended ECM ensembles as well I see.

Edited December 7, 2009 by cooling climate

[johnholmes]

Eventually I will force myself to learn those line graphs!

you and me both-I struggle to understand why they are so popular!

[Ian Fergusson]

Good to see you on here!

Regarding the February warnings, I thought they were more accurate the further south we were- the warnings regarding the West Country and south-east England were generally spot on, but if I remember rightly the warnings for Scotland and NE England were not as accurate. (I must admit I can't recall specifics any more- those were just my feelings at the time).

Hi, thanks!

And you could well be correct: I should emphasise that we have two separate UKMO briefing conferences established throughout each day with BBC Weather Centre / BBC Regions: one covering southern & central regions / Wales, the other covering the north / Scotland and NI. Sitting solely through the former, I can only attest to their accuracy - perhaps the northern / Scottish warnings proved more problematic, as you suggest!

Best

Ian

[kold weather]

The thing is John shows us what the ECM super ensemble suite think will occur out to 360hrs, each line is a ensemble run, thus if they are mainly on the cold side, then like the GFS ensemble it just shows there is extra confidence to be had in the situation. Also useful to see if the eCM op run is on the warm/cold side of the suite.

Its all just another tool in the shed so to speak though really!

[Duane S.]

I'm not sure we could have hoped for a better 12 hours following on from the poor ECM 00z showing.

The ensembles have started to react as expected and I think there, a few blips aside, this is going to be fairly progressive now. If it starts trending back mild, key word trending, I am going to worry (not that I'm not already ).

The trouble is we have been so spoilt by some of the charts we become over-critical. Let's face it, we would have near killed for these sorts of chart runs only a couple of weeks ago.

I do worry about having to rely on a shortwave (if that is what it ends up coming down to) to aid this cold air down towards us. I mean, us and shortwaves haven't exactly had a very harmonious relationship of late. But one big difference here is that, at the moment, we could end up with very cold scenario via more than one route. i.e. with the help of a shortwave or my prefered route, without them!

WAA getting dragged polewards is the one thing that must occur, if this stops short we could be in trouble. Hopefully though this won't happen as signals point to it happening, for a change.

I have a question though. Quite a while before the PV split quite a few were saying it was nailed on and going to occur. How did you know this was going to happen? What makes the PV split? Is it just the stratospheric warming that did it? One of those things I suppose I ought to know but never quite fully understood.

Edited December 7, 2009 by s4lancia

[cyclonic happiness]

nice to see someone who has close links with UK Met giving a positive turn to what is often a fairly critical posting on the web.

As an ex senior forecaster with them, I too am critical when it seems to require it but like to see a balanced view showing up.

You are a nice chap john, let's hope this is gonna be one of the best christmases (omg is that the right spelling???, seems wrong?), you or i have ever seen Merry Christmas John!

(and I've been making holly wreathes by hand all day and my hands look like pin cushions, so that's where the spirit is coming from, I'm sure holly injects some sort of mania, anyway....)

Edited December 7, 2009 by cyclonic happiness

[Yeti]

Without doubt shortwaves have beenone of the curses of the christmas pudding, pulling the jet Northward, but here we may have the strong -AO signal overriding these features.

I'm not sure about the GFS synoptically, I don't think I've seen an easterly develop quite like that in the archives but good similarity between the UKMO and the ECM and that should rate the strongest benchmark as to where we are headed. No guarantees of a bitter easterly but it's a very interesting set-up.

Notice the sudden deepening of such a feature S of GL on the 18z:

http://www.wetterzentrale.de/pics/Rtavn781.png

That may throw a spanner in the works...