General Model Output Discussion

[Cloud 10]

You do have to laugh at the gfs ability to go from one extreme to another between its different runs, at 240 hrs the gfs 12hrs had a greenland high and this time the PV is up there on the gfs 18hrs. Anyway regardless of the gfs's behaviour it's been a very good evening in the model output.

Yes,the 18z certainly makes short work of the greenland block,which makes me slightly sceptical about this run!

predicted-- actual--

Rather poor modeling of todays low by the gfs,even though the actual chart i have posted is for 6 hours later its still a fair bit out.

[johnholmes]

Thank you John, as ever, to the point and balanced. a question, if i may. in my very amatuer way i am seeing the cold period to be mainly dry. do you feel that any PPN will be light, or sustained? i also (correctly or not) notice a possible setup in the models for a band of PPN to come in from the East and possibly create a Thames streamer. any ideas?

as for crystal balls. perhaps we should nickname you mystic John

regards, Mick

Mystic jh mmm might change my avatar!

tks-near enough with my lrf ideas at times although they give about 70% accuracy in my view over most of the time period.

anyway to your question

It 'appears' to be a largely dry and very cold spell inland BUT any marked flow off the sea, N Sea, Irish Sea, etc, will cause convection clouds to run inland, so some snow showers are probable. Nor, at more than 48 hours, can anyone tell if this may cause some kind of trough to develop and that would enhance shower activity.

UK Met and the other models all show some kind of system running south over the weekend. UK Met have already introduced a 'warm' sector too -just how warm, in relative terms, may govern how much precip and how mus of it is snow at all levels, so one to watch closely.

thanks also to the comment from Ian F.

night night folks-time fold oldies to go to bed.

Edited December 30, 2009 by johnholmes

[john mac]

Well the good point about that is that it reduces the chance of a western based negative NAO which is always frustrating and at least any angle of attack will be favourable, however i'm not convinced of any possible attack apart from the very uncertain possible event for early next week, basically we want the southern arm of the jet to remain as flat as possible, so we could make a case for the greenie high weakening but the jet still being favourable enough to keep the uk in a cold pattern.

I'm still thinking that perhaps we might see the ridge toppling ne' wards towards Scandi, and if this does happen and with the PV remaining disorganised this might be a case of waiting for another reload with the PV disrupting again and the high eventually backing nw with trough developing over Scandi and further WAA in the west Atlantic bringing back our friend the Greenland high. Anyway theres loads of weather to get through before then. At the moment it's hard to say where the pattern could eventually end up, often in recent winters you think when one door closes it's going to remain shut but this time perhaps we'll have a couple of keys up our sleeve!

Sorry Nick

could you please tell me what the WAA is ?

[phil nw.]

My only worry with that, is the increasing dew points due to the warm sector on the LP, probably nothing, as this 18z is messy, progressive and packed full of shortwaves.

Lewis

At that point T60hrs. we have cold uppers -5c or thereabouts Lewis.There is no more warm sector as such there`s cold air all around with even colder air behind,further north.

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=60&mode=1

The only concern is how much snow we can get and where.

[johnholmes]

I prefer this link JH

http://www.zamg.ac.a...Ms/PL/index.htm

nothing wrong with that M-more detail and a bit more complex for a newcomer to get a hold of which is why I wrote the one I quoted-its a highly complex subject just as all meteorology is.

To me the best idea is to try and keep it as simple as possible and then move on to the more complex ideas.

to the query above

WAA=Warm Air Advection

another hobby horse

PLEASE try every now and then to explain accronyms etc-it does so help any newcomer

there is a fairly extensive list by the way in the NW Guides

Edited December 30, 2009 by johnholmes

[Guest]

regarding Polar Lows-they are usually fairly obvious and are picked out by the models but there is a tendency amongst some posters to see them when they are not there!

I would be one of those posters John!! Although in my defence I have actually read up on polar lows since last year's failed "swirly cloud" chasing, and I do believe that conditions are ticking a lot of the boxes for formation of a PL off Norway in the next 24 hours.

500 temps of sub -40C with the approach of an upper trough.

A baroclinic zone of constant pressure with a large thermal gradient from cold land/ice surface onto sea (in this case from Norway at -20C onto the North Sea at 4C, is this the lee effect in your guide to PLs??)

This forming at the edge of a strong surface flow...

Basically I read this (amongst other info) and matched it to current conditions:

http://www.eumetcal.org.uk/polarlow/cometplows/polarlows/1.3_devofpolarlow.htm

Yes I am straw clutching, but this beats looking for swirly clouds hands down. We all have to learn somehow :) :)

[nick sussex]

Sorry Nick

could you please tell me what the WAA is ?

Yes it's Warm Air Advection, basically warm air heads north say for example on the eastern flank of a north/south aligned trough to the east of this this helps promote building pressure,so for example around Greenland the warm air heads north into the Arctic pressure builds over Greenland, the cold air from the Arctic then gets displaced and has to go somewhere so it heads south on the eastern flank of the high. I've got a link somewhere for a clearer explanation, i'll see if i can find it for you.

[grab my graupel]

Sorry Nick

could you please tell me what the WAA is ?

It's Warm Air Advection, John

[johnholmes]

CmD

glad you now feel more confident in what parameters are needed for them to form

perhaps the most obvious for anyone is a strong flow from the north usually over a mountain mass-like Iceland usually for the UK but now and then with a NE flow over or rather off the Norwegian coast/mountains.

[Lewis]

At that point T60hrs. we have cold uppers -5c or thereabouts Lewis.There is no more warm sector as such there`s cold air all around with even colder air behind,further north.

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=60&mode=1

The only concern is how much snow we can get and where.

Of course it's a warmer sector, it has -7 to -8 850's around it with -5 in and close to the center. And dews raise a lot, so it's a "warmer sector" not very warm at all, but still in snow terms, could make a huge difference for many, but... 850's, isotherm, thickness is fine. So worst case, is the evaperative cooling takes hold after a bit of sleet and turns readily to snow.

But as John say's and other members, this 18z is packed with shortwaves, and it's very progressive. But saying that the UKMO have already included the warmer sector..

Looking good any how.

Lewis

[MAF]

Thanks John

i am getting the impression the latest output is not to many likeings. but is it because this run is normally against the norm? (as its nickname suggests, "pub run" ) but if the trend has remained the same on previous outputs, should this run be ignored, or be taken seriously.

[Guest]

CmD

glad you now feel more confident in what parameters are needed for them to form

perhaps the most obvious for anyone is a strong flow from the north usually over a mountain mass-like Iceland usually for the UK but now and then with a NE flow over or rather off the Norwegian coast/mountains.

Thanks John, it does pay to take a step back and read up on these things. If I can go from chasing swirly clouds this time last year to a basic knowledge this year, then I reckon by the time I am 150 I may know half as much as you do.

[Lewis]

Thanks John

i am getting the impression the latest output is not to many likeings. but is it because this run is normally against the norm? (as its nickname suggests, "pub run" ) but if the trend has remained the same on previous outputs, should this run be ignored, or be taken seriously.

Hey MAF,

Generally i think it's best not to bin a RUN before looking at the ensembles, generally the GFS is very good with shortwaves etc and LP'S in a reliable time frame, we only see so much of the ECM, so really the GFS and UKMO is all we can run on in reliable time frame, because we can see the process of the models frame by frame.

I wouldn't write this run off in reliable time frame, but past 144 i would.

Lewis

Edited December 30, 2009 by Storm Force Lewis

[kold weather]

A very cold run on the 18z, doesn't look stunning BUT it would lead to a sub 0C 15 day period for the start of Jan I'd have thought...

Anyway ensembles coming out, the control run looks very similar.

Something to watch, IF the AO doesn't go as positive as is progged, we could see a east based -ve NAO eventually develop from the pattern we see from the 18z, though it probably wouldn't be as cold as the SE airflow shown on the 18z op given the type of evolution that would be needed.

[Cloud 10]

This is the ensemble mean from the 18z at +240 showing a cold high over us.

[kold weather]

Yep thats the general evolution I'd expect from this set-up Cloud 10, though I suspect its probably 2-3 days too quick in introducing it...though a few ensembles are insane in how progressive they are, indeed there are one or two in there that have it toppling as soon as 144hrs!

Which I can assure you all isn't very likely to happen!

A UK block could quite easily turn into another east based -ve NAO as well if the models have overdone the return of the PV...a few ensemnles also go for that set-up...

Edited December 30, 2009 by kold weather

[TEITS]

These charts are both for friday ???

I mean't thurs evening/friday on the first chart.

Latest +120 fax chart goes with the UKMO but if anything its slightly better!

http://www.meteociel.fr/ukmo/fax/fax120s.gif

A trough followed by a cold front moving S bringing even colder air.

[nick sussex]

An interesting trend developing in the ensembles which i've been keeping an eye on, a few days back many more southern european locations had a switch to milder conditions as the models were keeping the southerly tracking lows further north with of course these areas more often on the warmer side of these. For a few runs now more colder options appearing, for London some colder members down to around -12, now interestingly for Paris some down to -15 aswell as Toulouse. Now looking at Iceland a warm up going on up there later next week this seems now to suggest that possible high pressure closer to the north and possibly ne.

Also looking at the ecm ensembles the operational run is one of the warmer options for days 8-10 this probably because of the switch round to a more onshore flow for the Netherlands, I suspect the colder options are keeping the flow more from the east. So just a little uncertainty here as to where we go next with the pattern, a possible toppling of the ridge ne 'wards towards Scandi with another shot of colder upper air being pulled westwards on its southern flank this would go some way to explaining the increased colder ensemble members across central and southern europe.

The models may of course go off into a different pattern but just another possible option to think about.

Edited December 30, 2009 by nick sussex

[karyo]

The afternoon models have upgraded the little low coming from the northeast on Saturday! The gfs is even showing a stream of snow showers from the Irish sea as the wind temporarily switches to the northeast! Very interesting development for my region!

The 18z breaks down the northern blocking relatively fast but it settles a high over us which, as already mentioned, will give very cold weather on the surface. In fact this is my dream weather: snow and then a long lasting high for severe cold!

Karyo

[kold weather]

Yep Nick, if anything its IMO the more likely evolution, though I think most the severe cold uppers in such a pattern would be directed more at central Europe rather then the UK...I think we will instead keep a very cold surface flow.

Anyway for now reasonable agreement on the upper high decaying from Greenland between 168-240hrs and for an upper high to try and develop close to the UK, however the exacts of the upstream evolution will make a large difference on the actual weather of course...

[SP1986]

The afternoon models have upgraded the little low coming from the northeast on Saturday! The gfs is even showing a stream of snow showers from the Irish sea as the wind temporarily switches to the northeast! Very interesting development for my region!

The 18z breaks down the northern blocking relatively fast but it settles a high over us which, as already mentioned, will give very cold weather on the surface. In fact this is my dream weather: snow and then a long lasting high for severe cold!

Karyo

Indeed, whats more some rather interesting temperature gradients which if they come off, wouldnt be rather interesting for NW of England, which I suppose does some of the most varied weather in the country. It is likely as you say as the winds turn NW'ly for a while snow showers may well affect NW England/N Wales

[Lewis]

Cant wait to see what the 00z looks like.

Fingers crossed the warmer sector within the LP is some what less modification.

Lewis

[SMU]

Both the GFS & UKMO 00z are building a lot of surface cold out to the east of us around 120-168- together with possible surface high development over scandi-

watch subsequent model runs for a wedge ( triangle ish in shape move North Eastwards & ridge to the surface high over scandi-) this will indicate an old fashioned scandi easterly brewing (upper high moving into this region )- also lowering heights over France as the jet pulses back north

I think our best chance of sustained & heavy snow comes from these type of developments, combined with the fact that the low pressure over svalbard moves south at a slow pace-

GFS rebuilds heights over Scandi towards FI which is the evolution I hinted at last night, thats via a slight 'topple' if you like from greenland, but reinforced heights developing NE out of the azores- much how Jan 1947 started-

http://www.wetterzen...slp19470118.gif

Only our high would be more NW-

As for snow COVER I think this Thurs & friday will bring snow to the region 50/50 whether we get cover- Id image Kent & Suffolk ( & EA) with lucky cover elsewhere- turning dry & very frosty-

I think the GFS is overcooking temps- I see a surface cold pool developing-

The UKMO is also worth noting as the norwegian shortwave continues to move around bringing the threat of snow to anywhere......

http://www.meteociel...48-21.GIF?31-05

GFS ensemble mean at 120

http://91.121.94.83/...-21-1-120.png?0

The GH is a lot stronger & more NE than the 18z & also the low pressure over Spain is around 150 miles North- so excellent prospects........

RE my wedge of high pressure - the GFS ensemble mean now at 192 has now dropped down to -11C for London with the MAJORITY going for an easterly like this-

http://91.121.94.83/...-13-1-204.png?0

THis is the WEDGE you want to see-

S

[Eugene]

The 18z breaks down the northern blocking relatively fast but it settles a high over us which, as already mentioned, will give very cold weather on the surface. In fact this is my dream weather: snow and then a long lasting high for severe cold!

I haven't looked at the models for two days but steve murrs post just now is all i need to look at really

Yes people forget strong winds like now keeps temps up in the UK and the coldest weather comes from stagnant conditions it may be boring but it brings the coldest weather to the UK after an initial cold flow.

If he's posting enthusiastically you know future prospects for cold weather must be good

ps steve your links aren't working.

[snowmadchrisuk]

Very Cold NE'ly from the ECM with plenty of Snow about . The high starts to topple around +216 but then it looks like we will need to look towards Scandi. Very impressive yet again .