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

[Mr Frost]

9 minutes ago, Djdazzle said:

Doesn’t equate that today’s max will be 3C short though.

Indeed - UKV still going for a max of 36C today.

 

@Alderc some nice cells approaching the SW - lightning kicking off.

Edited August 7, 2020 by Mr Frost

[Bradley in Kent]

Yes temperature are definitely several degrees down compared with 7 days ago; the temperature dropped lower than forecasted in the early hours this morning.

However last week the wind picked up and cloud arrived around 15:30 onwards putting the brakes on any further rise despite being quick out the blocks in the morning.

Today is off to a slower start but the lighter winds and all day sun should still result in a very hot one with a more sustained climb peaking later in the afternoon in comparison. It will be interesting to see how much of an impact having a cooler night beforehand has! Stay cool everyone! 

[Guest]

2 minutes ago, Mr Frost said:

Indeed - UKV going for a max of 36C today.

 

@Alderc some nice cells approaching the SW - lightning kicking off.

The cells near plymouth are good and developing in a clean environment, the rest behind is just convective leftovers and a mess. All it will do is hinder sunshine and provide light to moderate rain over a much wider area than currently forecast. This isn't a grumble but an almost certainty.

[Captain Shortwave]

22 minutes ago, kold weather said:

Broadly 1-2c lower.

Today will give a good indication of the models grasp of the actual heat and will show whether the models backed off too much.

Still very hot to be fair as well.

Steve, given we are still accepting warm air in its entirely possible we carry on at near 2c an hour through to 3.30-4pm before things level off.

The dynamics compared to last week are different, in fact there was a fairly sustained southerly breeze last week, which quickly dried the air at the surface out. This allows temperatures to rise quicker. Obviously today we have very light winds so a slower rise during the morning isn’t a surprise.

GFS op on the 06z has 34c for central London, actually given the set up I think London will be the top spot (Kew Gardens or St James park) and I will still go for 36c today. Stronger winds tend to push the hottest temperatures towards the north/west of London and areas north of that (Oxford/Cambridge corridor for instance).

More model related - Euros still fairly consistent in trying to break the heat down mid-week with low pressure trying to lift northwards. However following it does seem that the Azores high could build back in from the west again to bring more settled conditions after the potential fireworks next week.

Edited August 7, 2020 by Captain Shortwave

[clark3r]

Latest gfs has 35 tomorrow, 34 Sunday and Monday 31 Tuesday then it’s all over, maybe gfs is being over progressive again we will see in future runs, looks like for now   Records will not be broken maybe night time, how ever can change in future runs 

[sunnijim]

41 minutes ago, Djdazzle said:

Doesn’t equate that today’s max will be 3C short though.

It doesn't, but the maximums at 11am are still 2 or 3 degrees lower than last Friday.

Although we could have longer heating today when maximums are achieved at 4pm or 5pm instead of 3pm as last week.We shall see.

My view is that we will fall some way short.

35c, with 34c more generally.

[Guest]

6 minutes ago, sunnijim said:

It doesn't, but the maximums at 11am are still 2 or 3 degrees lower than last Friday.

Although we could have longer heating today when maximums are achieved at 4pm or 5pm instead of 3pm as last week.We shall see.

My view is that we will fall some way short.

35c, with 34c more generally.

Just to add on this, when we are shooting for those really high maxes heating the ground from say 34-37c after 3pm is very very difficult. I think on most of the hottest days maxes are usually 1-3pm not the usual 4-5pm we normally expect. Let’s see what happens in the next few hours. 

[kold weather]

3 minutes ago, Alderc said:

Just to add on this, when we are shooting for those really high maxes heating the ground from say 34-37c after 3pm is very very difficult. I think on most of the hottest days maxes are usually 1-3pm not the usual 4-5pm we normally expect. Let’s see what happens in the next few hours. 

that's only the case recently because usually a cold front started spilling cloud into the areas that are likely to get the highest temperature. Between 3 and 4.30 normally is  peak temperature of the day typically and unless we get a surprise amount of convective cloud come over the usual hotspots I would guess that will also be the case today. Cou

IMO we are gunning for something in the 36s, maybe low 37. 

Edited August 7, 2020 by kold weather

[SMU]

11 minutes ago, sunnijim said:

It doesn't, but the maximums at 11am are still 2 or 3 degrees lower than last Friday.

Although we could have longer heating today when maximums are achieved at 4pm or 5pm instead of 3pm as last week.We shall see.

My view is that we will fall some way short.

35c, with 34c more generally.

You will be surprised when you see the 12-2 maximas we will surpassing 35c by then.

Max temp today around 5-530 pm

[Djdazzle]

The wind is also meant to veer more SE from midday, so this may facilitate a sharp rise.

Edited August 7, 2020 by Djdazzle

[Guest]

6 minutes ago, Steve Murr said:

You will be surprised when you see the 12-2 maximas we will surpassing 35c by then.

Max temp today around 5-530 pm

Not so sure on that one today, a large mass of cloud is going to be over London before 5pm today and that is a cloud shield that is expanding, certainly not breaking up. 

Also surprise Arpege still going for 33C just inland from here today, to be honest its not got a good handle on the precip in the channel, there's a lot more in reality, its further and moving more NE opposed to NNE, that to me suggests there is less of a push of heights from the south east. 

Edited August 7, 2020 by Guest

[wellington boot]

For what it's worth, temperature here appears to be 89f or just shy of 32C at 1130. Should note the thermometer is on the east side of the house but in shade on a North facing wall.

Currently Arome cloud cover charts looking accurate incidentally.

[danm]

Edit. 

Edited August 7, 2020 by danm
wrong thread.

[Frosty.]

For those who say I only post nice charts, stick this in your pipe and smoke it!!!..joking aside, there’s plenty of heat, and sunshine and thunderstorms to come before that possibility!?

Edited August 7, 2020 by JON SNOW

[Guest]

Right lets be honest, not a single model has got a truly great handle of the situation this afternoon. Not a single has any showery activity at all through the spine of the country around midday yet we have thundery showers breaking out from York down to the midlands. All also under-estimating the precip in the south west significantly as well. 

[Djdazzle]

Heathrow recording 33C.

[Weather-history]

Hmm that's interesting 

 

That set-up reminds me of a couple of famous past thunderstorm events, July 1923

July 1925

[Tamara]

@Tamara I wonder if you might answer a couple of questions I have about this.

- You note the inevitability of the low frequency tropical signal returning (which you have previously described as "default") and promoting upstream amplification etc.

- You also later refer to the default pattern and seasonal wavelength changes in August.

I wonder if you could flesh this out a little more. Firstly I'm interested in what you mean by the default pattern. Do you mean on a global basis, or are you specifically referring to default patterns for a UK summer, because there seems to be an implication regarding the effects of this change on UK conditions? 

I understand that there are more and less typical broader patterns, so an Azores high is typical while a Scandinavian high is less so, but even if you imagine a default Azores high in place, there are many ways for the UK to retain a warm/continental flow, for instance, just as there are ways to be cool and wet with warm southerly or easterly draws. 

So what is the implication when you say the default pattern will return? Do you just mean, default broader patterns will return, accompanied by relatively moderate conditions and all the usual uncertainty, or are you implying specific conditions in the UK? 

I suppose I'm asking here because if default broader conditions are to be significant (as you suggest they are) in this context they must imply specific effects for local weather. And yet looking through recent summers we have had prolonged spells of both warm, settled weather (probably majority overall) as well as wetter, cooler weather, such that it is not clear to me what our default summer weather is. So it's not clear to me either what default conditions locally are, or that globally default conditions have predictable effects on weather locally...

Secondly, can you add any further detail about seasonal wavelength changes? Is this to do with the jet firing up? Does this mean August should actually be expected to be a less summery month overall than June/July? My understanding was that our recent run of our Augusts was anomalous set against long term averages.

 

A few questions in there I know. I hope you can pick that apart...

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

An interruption to the heatwave nowcasting being monitored on this thread, but still an important one in my opinion nonetheless. This valuable post by @wellington bootrisked being lost as the previous model thread was closed yesterday morning very soon after it was posted.  So I have re-pasted it because it covers the essential points that have been made throughout this summer, so far, and which will continue to be primary influence for the rest of summer and into the following season as well.

When I have spoken of the "default" pattern, so often as it has been referred to, that means the primary driver in the tropical> extra tropical circulation that has created the negative inertia wind patterns that translate to an upstream amplification across the Pacific - and which has led to the synoptic downstream configuration of an Atlantic ridge and trough across NW Europe and Scandinavia. So the default pattern hasn't been referring to traditional UK summer patterns - but specific signals that have been primary influence over the summer (at least up until very recently) across the UK and other parts of NW Europe and Scandinavia

The low frequency tropical convection signal (the default) controlling the pattern has been rooted, since the beginning of June, across Africa and the Indian Ocean. The persistence of this signal is denoted by the velocity potential convection anomalies (shaded blue). Note how the twin low frequency signal faded in the last part of May across the Pacific and left the African and I/O standing wave to dominate.

 

This positioning of the dominant singular low frequency signal creates uptick in low level easterly trade winds (the African Easterly Waves or AEW) that steer the tropical cyclone developments westwards in the Equatorial Atlantic along the southern perimeter of amplified Bermuda and Azores ridges to the west of the UK. On the northern perimeter of these ridges, has been compensatory increased polar jet energy - hence the low pressure sequences that have kept passing close to the NW of the UK with persistent relatively cool air flowing across the UK from the Northern Atlantic.

Recent posts, at least since the second half of July, anticipated a break in the influence of the low frequency signal as the high frequency intra seasonal MJO has overridden this c/o an eastward progression across the tropics which has scrubbed out quite a significant amount of this easterly inertia which has re-configured the upstream pattern (de-amplification). This added momentum transport between the tropics and extra tropics, c/o positive frictional and mountain torque mechanisms, creating rossby wave-breaking downstream, carving our our downstream anticyclone,  reversing the pattern configuration of the summer to date and promoting heat advection from southern europe, .

The extent of the uptick in momentum (and the reflected extent of the pattern change for the better downstream) is evident from the Global Synoptic Dynamical modelling (GSDM). This is a diagnostic (non numerical) model that calculates wind-flow inertia across the globe. In other words in simple terms it is a useful guide to deciphering jet stream patterns.

With so much of the excess easterly inertia scrubbed from the atmospheric circulation at this time, AAM has recovered with a sharp uptick to average levels for the first time since the end of Spring. It is no coincidence that the fall in AAM began just as summer appeared and at the same time as the low frequency tropical signal switched away from the Pacific. It is equally no coincidence that AAM has recovered just as that low frequency (default) signal has been interrupted for the first time since the start of summer.

 

The longevity of the current heatwave centres around the wavelength (timespan) of the influence of the MJO phase cycle that created the distinct upturn in angular momentum tendency. When this intra seasonal signal fades, this will be the cue for the upstream pattern too re-amplify and what I refer to as the underlying default pattern to return. 

NWP is playing with this evolution in the extended period with hints of a return of an Atlantic ridge with the strngest anomaly to the west of the UK, rather than downstream to the east.. However, considerable intense heat build-up across huge expanses of the mid latitudes, very much including all of Europe means that this process is nowhere near as clear cut in terms of any progressive cool-down as during the earlier summer. Additionally late summer seasonal wavelength changes make it likely that pressure fall across Scandinavia will not be as easily achieved as earlier in the summer. Therefore the pattern could well evolve into an an anomalous Atlantic ridge bridging with enough strength to keep deflecting the polar front far enough north to retain a warm airmass in place for some time. Whilst these will be almost certainly less intense then currently, any dramatic modelling over the coming 10 days that tries to show a sharply retracted ridge, falling pressure to any great extent over and to the NE of the UK - and substantial inroads of very much cooler air very far southwards should be, at present at least, treated as suspicious and an overreaction to falling momentum upstream.

That said, angular momentum will definitely fall back when the high frequency signal completes its eastward progression across the tropics, and the trade wind influence returns across the Equatorial Pacific and Equatorial Atlantic, but it is far from certain that the downstream pattern will revert to precisely as it was before the heatwave.

The coming several days, at least, are set for some quite remarkable summer weather - irrespective of whether 38C is breached or not in the immediate future

 

 

 

 

 

 

Edited August 7, 2020 by Tamara

[Frosty.]

WOW Tamara, you are a legend!..as for the GEFS 6z mean, sure there’s a stutter mid range but generally speaking this is light years better than July..apart from July 31st that is!..enjoy all the heat, sunshine and thunderstorms blah blah in the meantime!

 

[Gordon Webb]

54 minutes ago, Alderc said:

Right lets be honest, not a single model has got a truly great handle of the situation this afternoon. Not a single has any showery activity at all through the spine of the country around midday yet we have thundery showers breaking out from York down to the midlands. All also under-estimating the precip in the south west significantly as well. 

got that right , anyone have any idea why

Edited August 7, 2020 by Gordon Webb

[Arch Stanton]

At 1400 it was 29.5 and rising with Humidity at 47%.

Heathrow seems to have it's own micro-climate or dodgy instruments, I can understand Kent being hotter with the SE wind flow.

This time last week it was 33c here.

 

 

Edited August 7, 2020 by Arch Stanton

[johnholmes]

58 minutes ago, Gordon Webb said:

got that right , anyone have any idea why

Look at the Met O Fax 0600 Z chart, the reason is clear, a trough from the low in Biscay!

[Portsmouth Sun]

With winds swinging N/NE the south coast looks primed for some nice temperatures this weekend, especially if tomorrow is a dry day. 

[stodge]

Afternoon all

33c here in lowland East London and looking at Arpege, has it backed off some of the really high values forecast a day or so back?

First two are from the 06Z run, last one is from the 00Z.

The curiousity is the heat being "confiend" to the southern coastal counties suggesting an onshore flow for eastern areas and that keeping London fractionally cooler. 

The WRF 06Z also suggests a change in the heat profile from the more traditional broad push north of the classic plume to a more sharply defined event for the far SE.

 

[ribster]

2 minutes ago, stodge said:

Afternoon all

33c here in lowland East London and looking at Arpege, has it backed off some of the really high values forecast a day or so back?

First two are from the 06Z run, last one is from the 00Z.

The curiousity is the heat being "confiend" to the southern coastal counties suggesting an onshore flow for eastern areas and that keeping London fractionally cooler. 

The WRF 06Z also suggests a change in the heat profile from the more traditional broad push north of the classic plume to a more sharply defined event for the far SE.

 

Yes, one day wonder here, temp for today slightly revised down and more so for next four days, before dropping away further.