Model output discussion - August hot spell - how hot, how long?

[Mike Poole]

GEM 12z T192, not quite as pronounced with the plume scenario as ICON, but still gets +20 uppers into the SE:

South only story on this one though.

[Mike Poole]

ECM and JMA not interested in the plume scenario at T192, but a more organised move of high pressure towards our islands, take that too, its better than the current dross, overall good output tonight...

Edit, ECM should settle things down!

Edited August 28, 2020 by Mike Poole

[Mike Poole]

ECM mean T144, T168, T192:

I think the plume scenario is a dud, but the UK high is looking increasingly more likely as per ECM op run.  I’ll take that, it will be nice to see some September sun!

[Mark wheeler]

12 minutes ago, Mike Poole said:

ECM mean T144, T168, T192:

I think the plume scenario is a dud, but the UK high is looking increasingly more likely as per ECM op run.  I’ll take that, it will be nice to see some September sun!

No plume in the mighty Navgem either . Looks ok though .This is the final chart .

[Man With Beard]

Yep, at D9, 70% of ecm ensembles spill over an Atlantic ridge before a plume gets a foothold. Usually the threshold value for a forecast. Fine weather = yes, hot weather = not likely.

Edited August 28, 2020 by Man With Beard

[Frosty.]

The message from the GEFS 0z is don’t give up on a return of very summery weather in early september / autumn!!!!!   

Edited August 29, 2020 by JON SNOW

[Sky Full]

ECM does appear to show a change from low pressure influence to dominant high pressure over six days from Wednesday......

  

This would allow a period of settled weather so let’s cross our fingers that it happens!

 

[mb018538]

Here comes high pressure (so says ecm and gem):

 

GFS not keen - though the op is an outlier:

[mushymanrob]

57 minutes ago, mb018538 said:

Here comes high pressure (so says ecm and gem):

 

GFS not keen - though the op is an outlier:

Id have thought the GFS is closer to the mark if these charts are right, but i do like the hints of the positive pressure anomaly in the Western Atlantic progressing towards us. IF that continues then a decent settled spell in time for mid month?

 

[mb018538]

1 hour ago, mushymanrob said:

Id have thought the GFS is closer to the mark if these charts are right, but i do like the hints of the positive pressure anomaly in the Western Atlantic progressing towards us. IF that continues then a decent settled spell in time for mid month?

 

 

Quite possibly - the ecm op was a big outlier too this morning....just the opposite way to the gfs!

[Man With Beard]

Clusters 00Z run for D10 - looking good for a UK high - should feel pleasantly warm - outside chance of southerly draw.

[northwestsnow]

46 minutes ago, Man With Beard said:

Clusters 00Z run for D10 - looking good for a UK high - should feel pleasantly warm - outside chance of southerly draw.

Yes,looks dry mid term,infact,hardly any rain forecast for Manchester for the next 10 days!!

 

[kumquat]

2 hours ago, mb018538 said:

 

Quite possibly - the ecm op was a big outlier too this morning....just the opposite way to the gfs!

Take the mean of them all and I'd be happy. (Self interest at play here as I have a south coast holiday coming up)

[Scorcher]

2 hours ago, northwestsnow said:

Yes,looks dry mid term,infact,hardly any rain forecast for Manchester for the next 10 days!!

 

We certainly need some dry days. Today is just atrocious for August. Just started raining here and feels genuinely cold. These conditions can wait until late October at the very earliest.

[East Lancs Rain]

1 minute ago, Scorcher said:

We certainly need some dry days. Today is just atrocious for August. Just started raining here and feels genuinely cold. These conditions can wait until late October at the very earliest.

I know. What happened to the forecast for a dry sunny weekend?

[Scorcher]

1 minute ago, East Lancs Rain said:

I know. What happened to the forecast for a dry sunny weekend?

Just horrible. Again, another example of the Netweather myth about northerly airflows being dry and clear. Most of the time polar air brings with it cloudy skies and a genuinely unpleasant feel.

[mb018538]

2 hours ago, Scorcher said:

Just horrible. Again, another example of the Netweather myth about northerly airflows being dry and clear. Most of the time polar air brings with it cloudy skies and a genuinely unpleasant feel.

Echo that - been 14c and miserable all day here. Rubbish.

12z op runs aren’t up to much - none showing high pressure moving in. I think if we’re presented with 2 scenarios at this juncture, we all know what will win.

[I remember Atlantic 252]

2 hours ago, Scorcher said:

Just horrible. Again, another example of the Netweather myth about northerly airflows being dry and clear. Most of the time polar air brings with it cloudy skies and a genuinely unpleasant feel.

not myth from late Nov to late Jan here, northerlies are dry and clear (wishbone effect) sun too weak for inland convection, rain/sleet/snow showers down west and east coasts, this area poor for snow in direct northerly, setups that create Pembrokeshire Danglers,

[Tamara]

So, deposit paid, and "escritura" scheduled to be signed (by a POA representative near Lisbon) during September to be the owner of new property in destination Portugal. Summer blazes on there, regardless of technical change of season.

Back here, as ever not so straightforward - with airmasses from polar and sub tropical origins doing their usual act of vying for supremacy at this crossroads and higher latitude to where I ultimately will reside at 39N. 

Current momentum budgets underpin why this neck of the woods is, at present, on the "wrong side" of an Atlantic ridge, unlike southern europe, - and instead fully on the side of another  departing trough and below average temperatures coming into the end of this week and leading to slow recovery (but still these highly disappointing temperatures for many of us) this Bank holiday weekend. 

Globally averaged angular momentum has returned below average c/o -ve mountain torques in the extra tropics - these torques and their associated easterly inertia to hemispheric wind-flow, have fully scrubbed out the angular momentum rally that inspired the seemingly distant memory of an anomalous downstream heat ridge and some blazing NW European summer weather earlier this month.

 

This change equates to re-amplification of the upstream pattern within the Pacific sector as a result of the stabilising effects of tropical convective suppression (shaded orange), at the same time as the convectively active low frequency signal (shaded blue) re-asserts itself across Africa and the Indian ocean as illustrated by the velocity potential precipitation anomalies. N.B Note the forecast trend and eastward progression with time - more on that later.

 

 

This re-sets the downstream wavelength of retracted Atlantic/Azores ridge and overriding NW European trough that was seen extensively in the mid summer period - encompassing almost the whole of July (bar that final day heatwave!)

What of the change of month and the previous suggestions of improved warmer weather once again ?    Still on cue whatever numerical operational model vicissitudes appear to indicate - albeit as per the theme of the beginning of this post, never a straightforward process at this junction of the Northern Hemisphere.

As was the case in late July and early this month, the high frequency MJO signal has growing confidence to embark on another eastward progression across the topics, much as discussed in detail in previous posts. The cue to promote attempts at further downstream ridging.  Deterministic RMM modelling between the ECM and GFS is coming together with this trend, but this less reliable MJO modelling in terms of suggested amplitude path is very open to further forecast adjustment (i.e on the upside)

 

6 hours ago, Man With Beard said:

Clusters 00Z run for D10 - looking good for a UK high - should feel pleasantly warm - outside chance of southerly draw.

Following on from the above, this snapshot in time cluster analysis represents in synoptic terms the same interesting juncture.

At present NWP is very keen to retain an element of a more dominant Atlantic ridge and  the tendency to allow fronts to slip south as pressure falls ahead of it  sometime around next weekend.  Some caution is required in the coming days as to whether or not any further retrogressive Atlantic ridge signal is overdone or not beyond this time - not so much as to how it evolves at face value - but more in terms of how the longer term pans out. 

The lessons from late July and the obfuscated lead up to when the intense heatwave suddenly started to appear in numerical modelling were that these same numerical models were, as anticipated back then,  too blindsided by the low frequency upstream signal and made late adjustments to over progressive inroads on cold fronts and return of cool North Atlantic air, due to over amplifying the incoming Atlantic ridge to the west of the UK, and therefore digging the polar front too far south.  The reality was that downstream ridging across Europe and Scandinavia proved more resistant than advertised and the subsequent heat dome persisted for much longer than initially suggested

This time around, there is clearly not going to be anything close to any extent of advected heat source later this week. However, irrespective of this, based on the suggestion of an amplitude MJO wave progressing right into the Pacific through September, this ultimately supports the sort of de-amplification upstream (and subsequent downstream amplification in this neck of the woods) that happened in early August. On this basis there is support for the idea of any renewed ridging coming in from the west, adjusting itself eastwards to become positioned to the east and drawing a plume of very warm air from southern europe and up through France and to knock on the door of the UK.

It is more a question of timing of this, rather than if it happens at all. I am, as ever, suspicious and questioning of NWP trends and do not take each and every output at face value.

 Latest updated 12z GFS ensembles indicate a shift away from what the operational continues to insist and with the Atlantic ridging element overdone on the latter as suggested might be the case within this post. 

The ideas behind any further plume potential lie beyond  next weekends proceedings and based on the movements of the renewed ridge  advancing eastwards into the following week. This rather than trying to retract the high westwards and lower pressure again across the UK and other parts of NW Europe.

It seems counter intuitive to expect further retrogressive flow after next weekend when the tropical signal supports increased momentum upstream in the longer term. On that basis the 12z GFS ensemble mean is closer to that of the ECM mean this morning and makes more sense.

So, and for what it is worth as I do not profess at all to be a "forecaster" as such and rather simply look at diagnostic probabilities -,  it may take some time to benefit areas further north and west, but the evolution of ridging may ultimately manifest itself as stretched between the European mainland, the UK and with some slight and very subtle discontinuous retrogression to the Azores/Atlantic high pressure zone. But not too much discontinuous retrogression to allow pressure to fall too much ahead of it across NW Europe.

Bias susceptibility to too much Pacific amplification possibly an element with this within some numerical products and it is on this basis to not be overly distracted by operational output but monitor the trends on numerical models c/o the ensemble means and clusters. This over a day or two at a time, rather than draw instant conclusions from each and every suite.

A very conducive outcome remains possible for some calm, warm and sunny September weather and evenings still warm enough for sitting outside with the lanterns and tea lights glowing in the now darker evenings than were enjoyed during the previous summer type spell of weather 

 

 

 

 

 

Edited August 29, 2020 by Tamara

[Zak M]

12z GEM shows severe thunderstorms breaking out across S England on days 8 and 9

[Mike Poole]

ECM going against the grain of the other 12z suite as far as I can see, and builds in the UK high as per some of the 0z and yesterday’s 12z suite.  Not done and dusted this, by any means:

 

[wellington boot]

1 hour ago, Mike Poole said:

ECM going against the grain of the other 12z suite as far as I can see, and builds in the UK high as per some of the 0z and yesterday’s 12z suite.  Not done and dusted this, by any means:

 

Well definitely not, no. One of the most reliable models is consistently showing a build of pressure and a potentially sustained period of warm, settled weather. Another shows an improvement but we can't see beyond 6 days. The NOAA anomaly charts are promising and show a significant positive anomaly close to us or over us during the next 14 days. Meanwhile, Tamara has outlined a series of factors that support not only a much improved spell of weather, but which also undermine some models' attempts at a rapid breakdown. 

I'm surprised at the lack of activity in here and the lukewarm chatter of a possible warm up towards mid month - from tomorrow it starts to look much better for the South and within a week there is a real prospect of widespread warmth and sunshine...

[davehsug]

Thanks for another informative post Tamara and good luck with the move. Getting out just before smelly stuff really collides with air-moving machine, I envy you the great weather too!

[Guest]

It's been a difficult two weeks but happy to hear of some calmer warmer weather on the horizon. Thanks all.

[Mike Poole]

ECM mean 12z here, T168 and T240:

This would strongly suggest that the ridge in the Atlantic would build over the UK over the next 10 days, with 1020 over most of UK at day 10.  Clusters will tell us more I would think...