Model Output Discussions 12z 01/06/2017

[Frosty.]

Nice end to July under high pressure and an increasingly hot early August on the Gfs 00z..Bank!

Edited July 17, 2017 by Frosty.

[Man With Beard]

The ECM is struggling in particular at the moment. Its op run has reverted back to the standard UK low scenario (albeit slightly more north-based) for T96/T120. However, one wouldn't expect the ensemble mean to be so volatile at just T96 and T120. Here is its last three runs for the coming Friday:

    

Last night's is clearly out of sync, being much flatter. But even comparing the first and last charts, big differences for D4/D5 charts with respect to Atlantic ridging and the size/shape of the trough.

The one idea from last night's ECM that seems to have "stuck" this morning, however, is a link-up between the UK trough and the upper trough towards Finland/Russia. This prevents the "lock-in" of heights sweeping round from Scandinavia, and allows a swift exit for the UK low after Saturday:

    

and by Monday it has gone

  

compared with GFS, which does not have the UK/Scandi trough link up, and keeps the trough slow moving - the GFS has also seen the small ridge, and has pushed the trough further north now, though:

  

The UKMO and GEM return to a more stable UK low

  

So still major modelling issues for next weekend.

Beyond that, another climbdown is starting to appear on some models. I would guess that, in the last 5 years, the failure rate of ridging to Greenland modelled at T168 must be about 90%. So I can't help thinking that this

 

rather than this

 

will be the way forward for next week.

[Frosty.]

In a nutshell, the first half of this week looks very summery with long spells of sunshine, unbroken sun for some during today and tomorrow and becoming warmer, hotter and more humid. Tues night and weds bring an increasing chance of thunderstorms across southern uk which will gradually spread north through wednesday..enjoy the warmth. 

[geoffw]

what are your opinions of august for weather ?

[Man With Beard]

48 minutes ago, geoffw said:

what are your opinions of august for weather ?

Bit of a guess, but maybe Scandi trough / Azores High displaced N, with a reasonably balance between settled/unsettled spells and best conditions in the south and west.

But hey, look at the trouble we're having with Thursday this week, let alone next month!!

The GFS 06Z is keen on getting more of a ridge underneath the main trough and as a result both Saturday and Sunday are back into play as warmer days in the SE. Also note the continuing trend to diminish the N Atlantic ridge:

 

[northwestsnow]

6z better for the SE as the trough is ever so slightly shifted north, still dreadful IMBY and further north though..

Is the azores high going to make it???

Edit probably not its getting squashed at228h by ANOTHER Atlantic low...

Edited July 17, 2017 by northwestsnow

[Glacier Point]

Are we starting to see some stabilising in the models ?

I suspect modelling has had one of its 'once every six months' type failures, reacting to false signals (or perhaps more accurately over-reacting). Funny how we tend to get these at the peak of winter and summer seasons.

GFS angular momentum plots highlight the recent upsurge in tendency in relative angular momentum, followed by a rapid loss of inertia (phase 4 to phase 8 transition).

NWP almost certainly saw the phase 4 evolution and projected more of the same resulting in an inflated Azores ridge, and then overcompensated when this started looking like a phase 8 transition, with too much of a loss of angular momentum being forecast.

So perhaps a period of us moving back to the pattern observed through much of June, but with more influence of ridging over Scandinavia and a broadly +AO / +NAO regime.

[Singularity]

  

In a bid to try and figure out what's messing with the models - it seems there's two adjustments taking effect;

The first is a reduction in the strength of the jet stream crossing the Atlantic, with the deceleration zone now (i.e. on the 06z) occurring before it reaches Europe, instead of over France as was the case on  yesterday's 12z run (shown left).

The second is an extra small trough being picked up by some model runs over or near the S. tip of Greenland. It's a very shallow feature - indeed still only just evident on the 00z GFS shown middle - but it's enough to reduce the ability of a ridge to nose in between the Canadian trough and the one trying to become cut-off over the UK.

  

So it is that during the weekend, the jet is a bit flatter between the mid-Atlantic and Scandinavia, which as a result means that instead of the progression being determined largely by a fairly strong NW-SE section of jet stream that would push the low SE, there is more of a balance against the poleward-returning jet on the eastern flank of the trough, with the combined force keeping the low near-stationary while positioned N-NW of the UK. There's even the chance of a weak ridge building over the SE as there is a net deceleration of upper winds over there.

 

This, then, is an explanation for GFS' behaviour - but clearly UKMO has not applied any of these adjustments, and ECM has swung from applying them in the extreme to, it appears, retaining the weakened jet stream but also backtracking to produce greater ridging in between the two main troughs.

So it's not the best of times for making any weekend plans .

 

Next week we have been seeing another round of mid-Atlantic amplification being suggested but, as MWB has well documented, there has been a trend away from this toward a flatter pattern.

  

Hints are that envy of southerners nabbing the best of the weather may remain a major theme of this summer. 

For those up north to stand a better chance of seeing fine summer conditions, the upward AAM swing that has been increasingly advertised in recent GEFS output may offer some hope but it does depend on how those C-Pac trades behave and also the MJO. GFS keeps the latter quiet while the trades slacken off again and this (combined with the AAM trend) is probably why it's recently started to take on a more positive mood in the 8-16 day range. Really though, those trades could do with returning to at least neutral anomalies.

[Dancerwithwings]

GFS 06Z It's only one run but definitely in the right direction.

06 Z 00 Z

[Frosty.]

Again the Gfs 6z shows an improving low res, especially further south as high pressure builds in with plenty of dry, sunny and increasingly warm weather.

Edited July 17, 2017 by Frosty.

[Man With Beard]

Ensembles now trending this low even further N and W - about 50% have the continental air even further into England/Wales by Sunday, when compared with the op. A couple of members:

  

17 out of 22 now do not have the centre of the trough over any part of the UK by T144. A massive turnaround from this time yesterday.

Not a single ensemble member follows the ECM op idea of merging the trough with a Swedish/Finnish trough.

[Frosty.]

It increasingly looks like a north / south split in the weather will take place with a broadly westerly pattern becoming established, for a time more north of west and then trending south of west. The most changeable / unsettled and coolest atlantic conditions across northern uk with the south rather better with longer fine and pleasantly warm spells and short-lived changeable blips but rather breezy at times everywhere. Looking at the GEFS 6z mean shows a gradual improvement further south as time goes on with Azores ridge influence increasing. In the meantime, enjoy the summery next few days.

Edited July 17, 2017 by Frosty.

[Singularity]

6 minutes ago, Man With Beard said:

Ensembles now trending this low even further N and W - about 50% have the continental air even further into England/Wales by Sunday, when compared with the op. A couple of members:

  

17 out of 22 now do not have the centre of the trough over any part of the UK by T144. A massive turnaround from this time yesterday.

Not a single ensemble member follows the ECM op idea of merging the trough with a Swedish/Finnish trough.

Quite something watching these ideas bounce between the models. GEM was keen on the merger with S/F trough yesterday leading to a fast exit but a cool and showery NW-N flow behind (this model does tend to over-amplify in a big way though), but handed that over to each of UKMO, ECM and the EC Ens. this morning. Meanwhile GFS/GEFS have been trending toward yesterday's 12z ECM and in doing so has now gone beyond the shift that the ECM Ens. underwent yesterday evening. 

I am starting to suspect that a factor behind this extreme variability in LP position and movement is a struggle to resolve finer detail regarding a fairly dramatic N. Atlantic SST anomaly pattern and how that affects the strength of thermal winds (i.e. the jet stream) on a relatively small scale; we have a vast area of anomalously cold waters to the west interacting with a swathe of notably above average SSTs extending from the eastern subtropical Atlantic to the North Sea (and increasingly Barents/Kara so far this month).

This being superimposed onto the issues handling AAM changes. Orbits in the GSDM plot are not that often as fast - with large dM/dt (difference over unit time) threatening to rapidly amplify any errors in modelling - as we're currently seeing.

A word of caution is however due regarding the emerging Euro-Scandi idea from GFS/GEFS, as this model is the most enthusiastic by far with propagating tropical convective signals east - something that's hit the wall a number of times lately. While the convective signal looks weak, such can still tip the balance in such delicate situations. I wish I could see what the EC ens. are doing with the MJO for example but the plots from this model have gone AWOL as they are known to do from time to time.

[Man With Beard]

Still very embarrassing differences between models at T96. UKMO still going for a low centred over central England - GFS never lets the low make landfall on the UK mainland, and by Saturday summer is pushing back in from the south. Why can't they just get on??

[mb018538]

5 minutes ago, Man With Beard said:

Still very embarrassing differences between models at T96. UKMO still going for a low centred over central England - GFS never lets the low make landfall on the UK mainland, and by Saturday summer is pushing back in from the south. Why can't they just get on??

Indeed. UKMO 12z run tonight - horrendous, low pressure, trough over the UK. GFS keeps the muck at arms length with a westerly regime. Stark differences.

[Glacier Point]

Such a crucial next week for so many being the start of the school holidays. If you're looking at the GFS and parallel GFS ops, looking good for the south and SW, probably a slower start with the UKMO. GEM, hmm, that might take a little longer but the GFS banging the drum for a quicker return to summer.

[Frosty.]

Spot the difference today's Gfs 12z looks a lot better than yesterday's regarding the weekend and next week.

Edited July 17, 2017 by Frosty.

[mb018538]

Sadly, the UKMO run matches the anomaly charts more closely than the GFS....add to the fact that it shows diabolical weather, and you know all too well that it will end up being right!

[Frosty.]

Much much better looking Gfs 12z compared to the last few days with high pressure returning from early next week and becoming warmer and warmer..looking great..wow

Edited July 17, 2017 by Frosty.

[Guest]

Bugger, typical GFS raising my hopes, dreams and expectations with a good 12z run. 

Will we see a positive ECM later?

[mushymanrob]

indeed the gfs is another run supporting yesterdays ecm that had the nasty little low exit northwards quite quickly and not getting trapped in. ok this gfs run like yesterdays ecm is still unlikely to happen... but....

yes please mr god!

Edited July 17, 2017 by mushymanrob

[Guest]

Just now, mushymanrob said:

indeed the gfs is another run supporting yesterdays ecm that had the nasty little low exit northwards quite quickly and not getting trapped in. ok this gfs run like yesterdays ecm is still unlikely to happen... but....

Well quite! Given the summer so far we can't discount another shot of heat later next week. After all we've had a few plumes already  Rinse and repeat?....

[Frosty.]

Awesome..what a transformation, much nicer to look at anyway!

[Man With Beard]

37 minutes ago, Man With Beard said:

Still very embarrassing differences between models at T96. UKMO still going for a low centred over central England - GFS never lets the low make landfall on the UK mainland, and by Saturday summer is pushing back in from the south. Why can't they just get on??

Actually, make that T48. I've just looked at 5 models for that timeframe (someone's got to do it eh) and there is absolutely no agreement over the shape, size and position of the low(s) beneath Iceland in just 2 days time. The highest Shannon Entropy event ever??

[mushymanrob]

1 minute ago, Steel City Skies said:

Well quite! Given the summer so far we can't discount another shot of heat later next week. After all we've had a few plumes already  Rinse and repeat?....

i dunno... tbh i still think we have a good chance of a superb hot august. ive been looking at some charts from july prior to a august heatwave... just look at this from 1995, who would have thought what would follow?..