Model output discussion - mid-winter

[Tamara]

This excerpt was posted last weekend:

... suffice to say the GEFS plot forecast not to be taken at face value with its extreme forecast, but still suggests good evidence for an amplitude low angular momentum Phase 2 response

This set to trigger a significant Pacific amplification with deceleration of the jet upstream. The downstream effect of this evolution is to draw the Azores high westwards and northwards, and with the jet looping south in the Atlantic with breakaway vorticity and energy spilling SE from the Canadian vortex. 

The amplification of the pattern upstream, in tandem with eastwards moving tropical convection is then set to help balance the AAM budget with -MT leading to +ve frictional torque which will serve to weaken the sub tropical highs and leave a poleward height anomaly in the North Pacific. The alignment of the hemispheric H500 pattern becoming more sharply amplified and aligned to external wave 2 SSW precursor combustion of the polar vortex (late month and into Feb)

Meanwhile in the tropics and extra tropics its a case of seeing how eastward moving tropical convection supplies a bounce back in AAM tendency and sends the GWO onwards towards Phase 3/4 and helps set up that H500 precursor pattern, reduces influence of cyclonic overspill from the Canadian vortex, and also supplies the downstream amplification to orientate the Scandinavian High more and favourably for chances of cold air advection south westwards across the North Sea

--------------------------------

The Indian Ocean tropical forcing sector analysis in that post has duly come into NWP focus with upstream amplification sharpening the pattern over the Pacific and setting in motion the axial slant of the jet in the coming days and breakaway vorticity (low heights) engulfing the Atlantic/NW European sector resulting in a fairly cold unsettled and unstable pattern. I will as usual leave the micro dissection of the modelling to the very many others on this thread in terms of 'will it and where will it snow' .

The next part of that analysis, based on evolved evidence of recent days is that tropical/extra tropical interaction is not necessarily going to be as straightforward as many might prefer, at least as I see it, in terms of bringing the anticipated amplification signal downstream as fleshed out in that last post based on 'upside potential'.

This post, especially in light of recent developments, emphasises downside - and in the absence of any other analysis on here that appears to do so :

A significant swathe of easterly trades have scrubbed all 'a-typical' La Nina signals of early winter from the atmospheric circulation. As a consequence total global angular momentum has plunged to levels last seen before the autumn rally which began with the amplitude MJO wave in October - and which imprinted the extra tropical pattern with an increasingly amplified La Nina based mid latitude pattern.

However, Seasonal wavelength cycle changes make the atmosphere more resistant to MJO induced wave train amplification the later into winter one heads.

Already though, the December MJO progression failed the amplification test set timed for the approach of mid winter seasonal cycle wavelength to naturally transfer the long-wave pattern from the ridgey Atlantic profile of December into the one that could supply meaningful cold air advection from stable Scandinavian heights as January arrived.

In short, the base Nina state in the Pacific proved more resilient to the a-typical MJO convection pattern further west. Net result : more La Nina easterly trade wind inertia remaining in the atmospheric circulation at the expense of tropical convection induced westerly wind bursts to sustain the Pacific jet extension into the Pacific (as alluded to by Michael Ventrice in his well worth watching video on @Bring Back1962-63 equally well presented thread).

So, the latest MJO wave headed from the Indian Ocean faces an added surplus of La Nina forcing in the system compared to December. The implications of La Nina -ve torques have been discussed by me in at least the last two summaries in terms of the strength produced of the sub tropical highs (which naturally includes the Azores high in this sector)

Its not just about the MJO of course. A reversal of low angular momentum -ve torque to a sufficiently+ve frictional torque response within the tropics c/o the MJO making amplitude progress to the Western Pacific (without then encountering easterly trade wind shear and cause weakening and loss of amplitude to the 'COD'), will  in turn shape the net amplification response within the extra tropics (the synoptic pattern) and associated +MT wave breaking to impact the stratosphere.

Translated into synoptic evolution parlance: If the natural re-bound response of angular momentum from its present low cycle point does not sustain strongly enough into the extra tropics (and we have to take the logical perspective that the atmosphere is potentially less conducive to processing amplifying +AAM anomalies than December) it means that once next weeks cold trough heads slowly off into mainland Europe, we  may not see sufficient and/or sustained enough poleward amplification of the Azores/Atlantic ridge in its wake, to take full advantage of the developments towards - AO within the polar field through the month 

If the analysis of preceding posts is taken into account explaining how +ve frictional torques in tandem with rising AAM tendency weaken the influence of the sub tropical ridges at mid latitude (inc Azores High) and -ve torques c/o of low AAM sustain them, then based on  those GWO composites for Phase 3 and Phase 4:

 

-as usual, best not to take literal interpretation of those composites at face value.  But suffice to say that  GWO Phase 3 (the lower AAM phase) infers greater suggestion of less amplification downstream of mid latitude ridging with the greater forcing for this kept upstream across the Pacific to also retain strength in the Canadian vortex- albeit slightly retracted to allow at least a temporary amplification of the Atlantic ridge.

With this in mind, frictional torque tendency remains depressed:

We have to take into account lag time responses for any upturn also, of which there is (currently) no indication.

So my niggling concern , which was omnipresent when I made a pre-seasonal post back in November is that the sub tropical ridging (the Azores High) as part of the traditional La Nina pattern, could well continue to do its best to be a potential spoiler to help evolve a colder and colder progressing pattern into what it might otherwise be towards the latter part of the month.

The last part of that previous sentence needs careful interpretation. Its taken within the context of any wave 2 external forcing on the stratospheric vortex later this month. Clearly though, a stubborn +NAO pattern, would not be helpful to maximise the cold air advection potential of any vortex submission that might occur

As ever, not a predictive forecast summary from me. I leave it to others to make competitive predictions. Its simply my usual take on what might go wrong as well as might evolve according to what the majority would prefer to hear.

At some stage, when time permits, I intend to contribute to the learning thread using this kind of neutral approach to these tools that are usefully intended to help pre-empt NWP - as it is the biased use of them trying to fit signals to a desired pattern (cold or otherwise) that lends itself to the parochial dismissal echoed by some on these fast pace, very reactionary threads. Many others of course are more open-minded.

 

 

 

 

 

[Methuselah]

2 minutes ago, jgbgt said:

Is that a 6 hour possible Easterly/North easterly at 13 days away.....all predicated on a second system going SE

Don't knock it...it might even happen during the daytime!

[Singularity]

Well you can certainly see from the day ten charts of GFS and GEM how much we do need the 4-5 MJO signal that they're reading to prove insignificant relative to the GLAAM behaviour with respect to the GWO etc.

Not keen on having an extra trough leaving Canada around Friday though compared to the GFS 06z; that means an extra feature that has to slot into place correctly for us to benefit.

[BlackburnChris]

11 minutes ago, Mucka said:

I berated this mornings extended MetO forecast, with its talk of milder spells for the South and lack of the "s" word, but it has changed significantly this afternoon.

Now talk of accumulations in the North and no mention of mild. not sure what the the morning forecaster was on.

Im pretty sure there warning will extend South into NW england over the weekend . The Snow signal has been consistant for days now . 

[ACCOUNT DELETED]

2 minutes ago, stewfox said:

Yes west next week , east week after ??

And the south coast of Wales & England not at all

[MR EXTREMES]

12 minutes ago, Mucka said:

I berated this mornings extended MetO forecast, with its talk of milder spells for the South and lack of the "s" word, but it has changed significantly this afternoon.

Now talk of accumulations in the North and no mention of mild. not sure what the the morning forecaster was on.

Most likely on yesterday's data but fresh set of data and a more wintry flavour for next week.

[Frosty.]

There's further snow opportunities right through low res on the Gfs 12z as lows / fronts bump into cold air, especially scotland, blizzards for some..

Edited January 12, 2018 by Frosty.

[tight isobar]

Ok im going to put to bed the m4 corridoor mania..and explain why dynamical input data -via computer modeling takes into account minor diverse, and factors in high population/heat surge/bubble dynamics.

Its an overview as desecting details would be tedious...

However the overall basis is for science and data to factor in these virtues, and analysis on a simple basis of indeed mathmatical-science structure..

The nutshell of all in- is that micro dynamics are of complete and utter diabolics..

So in a word being a polar maritime island shoots further into a black hole where this is leaned on....

So as -again the very slightest tweaks/gradients are of massive ramifications on our part...

And thats why both uk weather...and moreso snowfall is a complete and utter pain in the a##e to decipher and predict..

And thats why we are all so interested in the very first place..

I have had copious amounts of ground lying snow...when it was 48/72 hrs out modeled as cold rain...

Await closer exactions before highlight of expectations...

The surprise element upcoming will be stark...

Beleive me....

Edit;..

Anyway some spectacular synoptics playing out atm...for notable winter interlude/interludes..

Edited January 12, 2018 by tight isobar

[MR EXTREMES]

Interesting that next week's weather reminds me of a beefed up version of winter 2014 although not sure if the atmosphere was similar or not.

[carinthian]

Evening all, wonder if some of you lot will be waking up by this time next week to deep snow like this morning outside my apartment ? Anyway, here is the latest thoughts of our forecaster from a foreign field with specifics to the UK. They think the British Isles are in the cusp of some real winter wonderland charts towards the end of the coming week. What could go wrong I ask ?  Convective snow in the NW at first and possibly later in the SE. The wave disturbance around mid week really being monitored over here. Its development and track looks to be right could produce some snowfall , possibly in the Midland. The same disturbance tracking into Europe. Our snow model has 30 cm fresh snow on Thursday. The outer perimeter results show NW England with 10cm in places at the same time. The Northern Russian Block is really dictating the weather over Europe. Their medium range charts continue to show a squeeze on the Atlantic trough and for it to transfer SE into mainland Europe by the end of next week. It is also suggested increasing heights over Greenland by next weekend . These look like some of the best Mid -Winter charts for quite sometime with possible widespread snow events to evolve as the Polar Maritime and Polar Continent air flow mix. Longer term charts still indicate strongly negative NAO for the last 10 days of January.   Hope this update cheers some up on here ( snow lovers ). Taking a break over the weekend as have some old boys coming out frpm blighty, so few beers no doubt. Report back on Monday.

C

Edited January 12, 2018 by carinthian

[Mucka]

15 minutes ago, Steve Murr said:

pretty Decent 12Z our to about day 10-

If I lived in any of these locations I would be salivating at the models ( +150 M )

-Lake District

-Yorkshire dales

- western Pennines

- peak district

- Shropshire hills

- Exmoor

- Beacons & Snowdonia

- Sperrin in Ireland-

possibly even the Cotswolds - 

we could see DEEP snow accumulating from when the cold spell starts lasting 7-10 days... 

somewhere is going to get North of 30cm of snow from convective showers followed by frontal Atlantic sliders -

The most snowiest spell for the UK since 2010 ... let’s hope it’s backed up by the North Easterly Post 192....

S

That is the only fly in the GFS12z ointment, development of that Atlantic low and downgraded Atlantic ridge, supported by control run and some GFS ensemble members.

It would great if that were just a blip and the Op went back to making nothing of that low and building an Atlantic ridge in future runs.

 

[Tim Bland]

So...is the Easterly still on then? ?

I think the models are already firming up on a more widespread snow event on Weds night / Thurs.  if you look at the UKMO - Where red dot is - it is also producing the slider low in a similar position. Definitely one to watch !

 

[Methuselah]

8 minutes ago, Mucka said:

That is the only fly in the GFS12z ointment, development of that Atlantic low and downgraded Atlantic ridge, supported by control run and some GFS ensemble members.

It would great if that were just a blip and the Op went back to making nothing of that low and building an Atlantic ridge in future runs.

 

The 'fly in the ointment' is, unfortunately, the size of a Carboniferan dragonfly - it has a 3ft wingspan!

[nick sussex]

No clearance of the trough on the GFS 12hrs run with a hangback of low heights nw wards.

Its flatter upstream and only saves itself because the Russian high comes to its aid. Whilst it still delivers because it keeps those lows weak as they approach the UK and they manage to slide it looks like a more complicated and messier outcome with more that can go wrong.

The UKMO is cleaner upstream and has that high ahead of the Canadian low.

I'd much rather this limpet trough could clear as that really increases the chances to lock the cold in and the GFS just looks like an accident waiting to happen.

 

[Snowmadsam]

5 minutes ago, nick sussex said:

No clearance of the trough on the GFS 12hrs run with a hangback of low heights nw wards.

Its flatter upstream and only saves itself because the Russian high comes to its aid. Whilst it still delivers because it keeps those lows weak as they approach the UK and they manage to slide it looks like a more complicated and messier outcome with more that can go wrong.

The UKMO is cleaner upstream and has that high ahead of the Canadian low.

I'd much rather this limpet trough could clear as that really increases the chances to lock the cold in and the GFS just looks like an accident waiting to happen.

 

Agreed. As cliche as this sounds, I'm going to hold fire and see what the ECM does say. I may also take a break from model watching 

[Snowman.]

Blocking over the pole in the GEFS has increased quite a bit compared to the 6z lot. Especially around Greenland. And mark the 22nd of Jan as around this time a low crosses somewhere through the UK on plenty of the members bringing snow.

Edited January 12, 2018 by SN0WM4N

[MattTarrant]

6 minutes ago, nick sussex said:

No clearance of the trough on the GFS 12hrs run with a hangback of low heights nw wards.

Its flatter upstream and only saves itself because the Russian high comes to its aid. Whilst it still delivers because it keeps those lows weak as they approach the UK and they manage to slide it looks like a more complicated and messier outcome with more that can go wrong.

The UKMO is cleaner upstream and has that high ahead of the Canadian low.

I'd much rather this limpet trough could clear as that really increases the chances to lock the cold in and the GFS just looks like an accident waiting to happen.

 

Accidents can be good...

[nick sussex]

Just now, Snowmadsam said:

Agreed. As cliche as this sounds, I'm going to hold fire and see what the ECM does say. I may also take a break from model watching 

Originally the models seemed inclined to clear the Euro trough se, now we’re seeing these different and messier solutions .

The GFS is walking a tightrope with its output and could easily fall off.

You can see from the GEM what could easily happen. Hardly any margin for error and you need that in the UK.

 

[MR EXTREMES]

ECM I reckon will be good tonight.

As for GFS it's fine it's cold and we all know how the GFS loves to complicate things.

Although things are pretty complicated to be fair 

Still believe there's more upgrades on the way over the next few days.

[Snowmadsam]

4 minutes ago, MattTarrant said:

Accidents can be good...

Or bad, as my parents found out the hard way....

I will still say, surprises can happen

[Iceaxecrampon]

12z

Third potential Midlands slider at +201

[karlos1983]

57 minutes ago, mulzy said:

AZH could start spoiling the fun after day 10 but that's way out in FI...

Isn’t this what GP alluded to earlier? I.e. dont read to much into it?

 

Edited January 12, 2018 by karlos1983

[nick sussex]

2 minutes ago, MattTarrant said:

Accidents can be good...

Not with UK cold synoptics.

Don’t cheer the GFS evolution on it will end in tears!

Keep the NW helpline on speed dial! 

 

[Dancerwithwings]

GFS 12Z....In the medium term...NEXTWEEK.....Temperatures are low, 850's low enough and pressure low...... SNOW 

[feb1991blizzard]

1 minute ago, nick sussex said:

Not with UK cold synoptics.

Don’t cheer the GFS evolution on it will end in tears!

Keep the NW helpline on speed dial! 

 

To be fair the GFS evolution wouldn't end in tears it would end in dumpings for huge swathes of the country including the South because if it went wrong then it wouldn't be the exact GFS evolution - pedantic I know