Model output discussion 14/04/18

[Tamara]

On ‎10‎/‎06‎/‎2018 at 11:38, Tamara said:

The important thing for summer prospects beyond this period is that the AAM floor limit continues to be higher than that of April, and then higher still than that of May. According to Spring seasonal wavelength patterns, this type of lower angular momentum atmospheric phasing is much more forgiving than summer seasonal wavelengths would be.

.. (assuming for one moment and not predicting!) the atmosphere were to now get "stuck" and unable to progress further and break away properly from the vestiges and lag of 2 years of La Nina influence - this would imply ever increasing emphasis on the ridge/trough arrangement backing west (with the Jetstream suppressed from its better summer position) the more we progress through June, but more especially arrive at July..

*If* there is anything that nags away wanting very much to be proved wrong, it is this which remains the test to be overcome and represents the lingering remaining suspicion of what "could go wrong".

In this scenario, much like last nights ECM 12z operational (NB: purely for illustration purposes and clearly not to be taken at face value) this means the ridge retracted in the Atlantic, and the trough over UK, much of the rest of Western Europe and Scandinavia.

This extract from a post last weekend, with particular emphasis on the bolded section, continues to be key.

Just a little conflict emerging between the shorter-medium term in term of persisting hints of an evolving mid Atlantic ridge and how this then evolves in the further outlook in light of what continue to be positive signs from a swathe of data discussed in all previous summary updates (and supported by seasonal modelling) that La Nina atmospheric feedbacks, which are not ideal for the UK summer, look set to finally fade away.

At the outset, the explanation for suggestions of a somewhat amplified Atlantic ridge and some downstream troughing is quite easily illustrated and then explained:

The tropical cycle, which supplied +AAM anomaly propagation from tropics to extra tropics has ended its cycle and is set to return to the Indian Ocean as per last discussion. This process is complicated (and potentially delayed) by the eastward moving convectively coupled kelvin wave (or CCKW as discussed in last post) leaving the East Pacific and now heading onwards through the tropical Atlantic to Africa and then Western Hemisphere. Though separate to the MJO itself, the CCKW mirrors the movement of the MJO towards the Western Hemisphere to re-start the next tropical cycle and will support a substantive loss of atmospheric angular momentum.

This process involves the scrubbing of the westerly wind bursts triggered by the recent tropical convection convection cycle and translates to a fall in atmospheric angular momentum tendency. This fall has been well anticipated in previous posts, so in itself is not a cause for concern.

The working (and evidential) assumption has been that the active periodicity of the MJO would lead to a limited time for AAM to fall back before being re-booted by the next eastward moving tropical cycle. There is no evidence to suggest this won't happen - but the time factor involved is very important to the degree that angular momentum falls. This is because the amount that AAM falls dictates the ability, strength and duration of any amplifying Atlantic ridge - critically with July on the horizon which *would* further augment this pattern in lieu of change of month and change in seasonal wavelengths.

Its now possible, to have sight of some updated GSDM data to get an idea of the likely size of this fall (linked to where the atmosphere/ocean relationship is right now and also to the time factor between tropical/extra tropical time changeover)

Frictional torque, which is a gauge to changes in wind-flow patterns, is registering quite a large fall - this means that westerly wind additions to the atmosphere c/o tropical activity are being lost from the global system

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The fall in frictional torque is mirrored by a parallel fall in AAM tendency

 

 

Frictional torque tendency is the lead for the direction of travel in mountain torques - and global mountain torque tendency has now also started to register a fall. Notice though the lag timeline of +ve Asian MT (in red) which is indicative of the westerlies remaining in the Pacific (c/o MJO and East Pacific CCKW)

For the moment, this is holding up GLAAM and preventing a corresponding fall as identified by the rest of the AAM atmospheric budget. This also underpins the ridging profile within the period into next week to allow some return of warmth.

The real difficulties and great uncertainties appear thereafter and depend on how the tropical and extra tropical cycle evolves to arrest the decline and keep any programming by the models of an emerging Atlantic ridge in check.

Why does a fall in AAM precipitate a retraction of the Azores High into the guise of an Atlantic ridge?

Synoptically, the effect of falling angular momentum from upstream is to re-amplify the Pacific pattern (remember the effect of tropical convection is a symbol of a jet extension in the Pacific from Asia and cyclonicity,  The downstream effect of pressure rising and amplifying in the Pacific is to retract the Azores ridge westwards and also amplify it in response to the upstream wavelength (as the jet extension c/o the +ve Asian mountain torque is lost, and reversed). The amplification is created by the deceleration of the jet upstream and the vacuum that is created. Hence a somewhat discontinuous retrogression of the pattern downstream.

There is nothing, yet, not to suppose that the upstream pattern won't reboot when the next tropical cycle resumes. But based on the potential extent of the fall-back in AAM that is being suggested it is very important that this verifies both as soon as possible and also at as high an amplitude and eastward progression as possible

Reflective of all of the above the Global Wind Oscillation, which is a plot depiction of global wind-flows, has moved into Phase 8

This is representing the wiping out of westerly wind anomalies gained from tropics>extra tropics in the atmospheric circulation. If the favourable progressive MJO orbit verifies (continuing the theme of the fast recurrence envelope of recent months) then a fairly fast and shallow orbit of the GWO is likely back to Phase 4 - the signal to push (or retain) heights back east and dissolve any downstream trough *should it evolve* (the much desired pathway to continuance of a warm pattern into Juy).

Alternatively, should the process be convoluted and the time and extent of fall of angular momentum not be checked sooner, than a deeper low angular momentum GWO orbit is likely which may become snagged around GWO Phase 2 (signal for entrenched amplified Atlantic ridge and UK trough)

NB: I am certainly not a seasonal forecaster and would never pretend to be - and would leave such analysis for those prepared to decipher the variables involved. But for what little it is worth, I would repeat the previously stated opinion that there is nothing in any shorter-medium term hurdles and uncertainties to automatically imply that an Atlantic ridging phase *should it evolve* has any long term repercussions this summer - beyond a frustration maybe to delay the suggested promising evolution to a better summer than some seen of recent years, that still has good background support. Not just my words, but various professional sources.

At this stage its worth watching how the next tropical cycle shapes up. If it follows Scenario A then the models will drop any spectre of Atlantic ridging setting up camp to overstay a visit and backtrack to an eastward ridging profile and an Atlantic trough (in contrast to a ridge) setting up to draw up southerlies and heat from the Spanish plateau.

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Edited June 15, 2018 by Tamara

[Man With Beard]

EC clusters for D10-D15 - clearly troughing across the north, ridging across the south (as one would expect) - but still great divergence between how far ridging will extend north, and also whether an area of lower heights might split the ridge in two halves (Atlantic ridge + continental ridge = trough in the middle close to the UK). Here's the last 3 cluster sets for 27th June (now D12)

    

Not enough unanimity for even tentative predictions yet (based on these charts). Most favoured scenario seems the N/S split (a little like next week). With slightly less favoured possibilities of a more extensive ridge through the UK, or a trough towards the UK.

 

[Frosty.]

The Gfs 6z operational looks summery next week across southern uk with heights from the azores and the near continent extending across most of the uk with temps across southern britain into the low to mid 20's celsius and next thursday as high as 30c 86f across the SE..28/29c next friday too..with the temperatures shown there would be plenty of sunshine but also the risk of some thundery showers developing in response to the very warm temperatures and increasing humidity...it should be noted that northern uk is much cooler in comparison but still with some fine conditions at times.

[Frosty.]

The Ukmo 12z shows plenty of high pressure ridging next week, especially across southern uk and early next week it's set to turn warmer and sunnier across the s / se, progressively cooler and more changeable further n / nw..that same n / s - nw / se split continues through to T+144 hours with the south doing well for fine and warm weather, the risk of a few showers but predominantly settled the further south you are.

[Cheshire Freeze]

One thing that has been a clear trend recently is that troughing is shown to push further and further S. It's a gradual deterioration with the NW of the UK seeing the most unsettled of the conditions first of all.

In the mid range we see the unsettled weather push across most of the country 

Not dire by any means but a far cry from the glorious May we experienced.

On another note, the CFS (which was so bullish about an extensive mid lat high across the UK) is backing away from the glorious July it once promised

The UKMO now shows at day 6 the beginning of the break up of the +NAO signature which has promoted the fine weather we have enjoyed, you can see the deeper Greenland troughing begin to dissipate

Edited June 15, 2018 by CreweCold

[Frosty.]

The Gfs 12z operational also shows plenty of high pressure / ridging next week with the best of the fine weather further south but the most noticeable thing about next week is the temperature contrast from north to south with scotland / n.ireland varying anywhere from low / mid / upper teens celsius, coolest across the far n / nw compared to low to mid 20's celsius across most of england and wales, very warm upper 20's c for a time across southern / southeast england between mid / late next week. 

Edited June 15, 2018 by Frosty.

[northwestsnow]

Much to like about GFS12Z but again its another run showing extensive blocking  longer termwhich is not supported by the background signals or Exeters update.

Hoping GFS is totally overdoing this love affair with a Greenland high!

 

[I remember Atlantic 252]

Making less of Wednesday's cold front as well! wednesday has looked crappy day, on and off on GFS

[Frosty.]

The Gem 12z shows increasingly warm and predominantly fine weather across southern uk next week, especially for s / se england..much cooler and less settled across northern uk ( scotland / n.ireland)..there's a brief cooler blip further south much later next week before it again warms up considerably for the s / se..more humid too with a risk of thundery showers.

Edited June 15, 2018 by Frosty.

[northwestsnow]

Very nice ECM this evening out to 144, looks better than GFS/UKMO to my eye with more emphasis on the azores high..

[northwestsnow]

In fact you have to say EC looks lovely from monday through to next saturday away from the far NW.

Inc next sunday as well! A really beautiful EC this evening for many!!

 

Edited June 15, 2018 by northwestsnow

[Frosty.]

26 minutes ago, northwestsnow said:

Very nice ECM this evening out to 144, looks better than GFS/UKMO to my eye with more emphasis on the azores high..

Yes indeed, what a run this Ecm 12z is turning into for southern uk in particular, plenty of azores / continental heights and some very warm weather too with 564 dam thicknesses...stunning charts and finish..going to post them shortly.

Edited June 15, 2018 by Frosty.

[Frosty.]

And here it is, the Ecm 12z which is head and shoulders above other output so far today..really summery charts here for southern uk with a lot of azores high / ridge influence..and northern uk eventually warms up...what a finish too!

Edited June 15, 2018 by Frosty.

[mb018538]

Barnstorming ecm tonight....but when it’s so much better than any other run, you’ve got to feel it’s prpnably a warm outlier. The others aren’t terrible by any stretch, but the ecm is a corker.

[Mike Poole]

Catching up on models after a long day, I think the 12s as a whole are an improvement on the last 24 hours, I think the ECM ensemble mean at T240 sums this view up:

[Fourty Point Three Degrees]

I can see our first 30C of the year next week if the ECM is right.

[mb018538]

Could be possible, bbc long range tonight has 27c in east anglia by Tuesday, ecm evolution is great for heat!

[northwestsnow]

After viewing ECM and its 10 day mean this evening i think its fair to say the balance has swung back towards the Azores high making a real fist of it - esp after this mornings apparent swing towards a trough scenario, esp after viewing the clusters.

Lets hope the former scenario in Tamaras anaylysis is proved to be the winner,its great to be able to go outside or to the pub for footy lovers like myself, with the world cup being on.

Hoping GFS now follows suit and send the jet packing north..

[damianslaw]

Mmm too much variability between intra-model runs at the moment , a north-south split is the form horse, warm and sunny conditions in the south into next week, cooler wetter weather for the north. Longer term - the balance appears to be in favour that the azores high will retract back west,rather than ridge NE and to our east. All quite normal stuff for this stage in the year, a fine balance - preety decent for the south, more standard for the north.. but compared to recent weeks, rather underwhelming overall - perhaps expectations this year are much higher though - given how things have recently been.

[northwestsnow]

At 96 we can see why bbc long ranger going for low 80s on tues in the SE, even as far north as Manchester has jumped up to 24 on Tues on BBC raw data.

[Fourty Point Three Degrees]

Not sure if its Manchester or Birmingham where the BBC have 25C forecast for, it shows 25c on the map where Manchester is but is it actually the temp being forecast for Birmingham?

[northwestsnow]

GFS following ECM tonight-

Certainly a couple of very warm days and most likely very little rainfall generally.

 

[northwestsnow]

Just now, 38.5*C said:

Not sure if its Manchester or Birmingham where the BBC have 25C forecast for, it shows 25c on the map where Manchester is but is it actually the temp being forecast for Birmingham?

Yes, very warm temps across the country- 24/25 for manchester it lovely!

Probably dropping to 20 by thur / fri but still very nice, further SE still mid 20s on thursday.

Acc to GFS18Z.

[Mike Poole]

GFS 18z to T192:

Great run for much of the country, tonight's output definitely looking like an improvement.   

Then we get to this T276, where did this come from?

Edited June 15, 2018 by Mike Poole

[northwestsnow]

Well looking at ECM and GFS tonight i think we can safely say it looks very dry over the next 7 to 10 days away from NW Britain- no heatwave but temps into the low to mid 20s for many next week, esp mon/tue/wed.

After that yes the high gets dragged a little west on GFS but high teens low 20s across much of the country.

Really positive about next week and a little beyond now.