21st Aug 2011 - Storm/convective Forecast Discussion And Reports

[Nick F]

New thread for today, haven't been around much the last few days so only just aware of the storms this morning/overnight across SW England, Wales, Midlands and across to Cambs/Lincs/Norfolk. Looks like warm and moist plume has advected north later yesterday and overnight with a trough in the warm sector moving NE and the cold front itself triggering storms overnight/this morning. 0600hrs fax this morning:

Another batch of heavy thundery rain and storms currently south of Dorset atm, looks like pushing NE across SE England by lunch time, sferics currently in the Channel:

Looks like another batch of heavy thundery rain will move up from the south later tomorrow and through Tuesday as an area of low pressure over Biscay off SW France moves north towards southern England Tuesday morning.Not quite clear yet whether there will be much in the way of storms this side of the channel, but the models aren't handling these situations too well atm, so sit tight!

[vortex_liam]

Well, good morning!

I think that as that storm moves over land it'll grow. Also as it moves NE it will grow as it hits the lower pressure!

That cell is growing!!!!

This looks good as it comes over land

[Coast]

Why does all the real action have to happen 200 miles away?

Storm Forecast

Valid: Sun 21 Aug 2011 06:00 to Mon 22 Aug 2011 06:00 UTC

Issued: Sat 20 Aug 2011 20:05

Forecaster: GROENEMEIJER

A level 2 was issued for extreme E Benelux countries and parts of Nrn and central Germany mainly for severe wind gusts and large hail, and perhaps a tornado.

UKASF have two areas covered, one of which is over me, but I think that was or the early hours, which didn't happen for me:

Forecaster: Dan

Last Updated: 18:14 Saturday, 20th August 2011

Valid: 00:00 Sunday, 21st August 2011 - 23:59 Sunday, 21st August 2011

Areas Affected: SLGT: West and East Sussex, Kent, Cornwall

Synopsis:

An upper trough slowly moves eastwards across the United Kingdom during the forecast period. Ahead, warm air is advected northwards to the east (and south) of a cold front. Focus for convection will be within this warm (sector) airmass.

Discussion:

A plume of high ThetaW, accompanied by high ThetaE (typically 18C and 55C at 850mb respectively) will cross the eastern English Channel and far SE through the early hours of Sunday morning. Destablisation of this airmass will occur over northern France during the early hours of the morning as frontogenesis takes place. It is possible, given previous similar setups, for mid-level convection to take place over the eastern English Channel during the early hours, especially given a couple hundred J/kg CAPE, ELT's as low as -30C and warm SST's (17-19C).

As a result, there is a slight risk of a few thunderstorms developing, which may graze the coasts of Sussex and Kent in particular, primarily between 3z-8z. Such storms that do develop could benefit from 50kts DLS, although given their high-based nature, severe weather seems unlikely.

Any storms that do manage to develop will clear eastwards by mid-morning and the remainder of the forecast period is expected to be significant-convection-free as the cold front clears eastwards across these aforementioned areas.

A SLGT level was issued for W Cornwall for any residual thunderstorms that may continue through the first few hours of the forecast period along the first cold front. There is also a possibility of isolated lightning along the cold front over Yorkshire and Lincolnshire during the first few hours of the forecast period, but coverage is considered to be too low to warrant a SLGT.

EXTENDED OUTLOOK: There is a renewed possibility of thunderstorm potential early next week, from Monday night through to Tuesday, but this is highly dependent on the track of a frontal boundary and how far west it may move.

[vortex_liam]

My house in portugal is in the level 2 :o wish i was there, to see if it gets damaged :S

[Coast]

Met Office chart from 21st OWS also shows something in The Channel later:

As does the WAFC aviation chart for today:

GFS CAPE and LI has Belgium and France written all over it:

KO index also shows Belgium, but not the intense stuff in France:

Hmmmmmm

Rain later, but the sky over Sussex doesn't currently look like it

Funnel threat on the other side of the water?

A really slight risk today, if at all, I'm going for a wander down along the beach and will take the camera anyway!

[Nick F]

Think the models, especially GFS has not really showed the convective energy too well overnight and this morning across southern UK, as the SEward advancing cold front interacts with the warm moist air which has been advecting in across southern parts. Certainly didn't expect as much thundery activity as there has been across SW England, Wales, Midlands and part of E England as there has been over the last 12 hours. So be interesting to see what develops towards the SE as this cold front clears by early afternoon and what evolves from this heavy thundery rain which moves up from the south late tomorrow and through Tuesday.

Anyway, brief forecast for today, not really much going on but slight potential before the cold front clears the SE after lunch to fresher more stable conditions everywhere:

Convective / Storm forecast - Issued 21/08/2011 10:00

Forecast Summary Map

Click for full size

Valid: 21/08/2011 10:00 - 22/08/2011 00:00

Headline: ...THERE IS A RISK OF THUNDERSTORMS FORECAST...

Synopsis

NE-SW aligned upper trough advances from the west this morning, with trailing cold front ahead of this trough front lying from Cornwall-Lincs at 06z this morning, E of cold front is an unstable warm moist Tm airmass. Cold front should clear SE England early this afternoon, with a fresher more stable WSW flow behind

... S and SE ENGLAND ...

Overnight/early morning storms that developed along and ahead of cold front across Wales, Midlands and E England have now cleared away eastwards. Another area of heavy thundery rain, moving NE along SEward advancing cold front, with some sferics currently noted off Dorset coast, will affect parts of central southern and SE England later this morning before clearing early afternoon. Torrential rain with possible localised flooding, gusty winds and isolated lightning is possible for a time before the cold front clears away eastwards early afternoon to more stable and fresher conditions.

Issued by: Nick F - Forecaster for Netweather

[Harry]

MetOffice warnings have been updated....entirely southern slice from Cornwall to Kent (anywhere south of M4 roughly) for Monday evening and overnight. Then much larger chunk of England and Wales for Tuesday, incorporating SE Wales, whole of the Midlands, NE England up towards the Humber, EA, SE and S England.

Could be interesting....I hope it'll be much more electrified than frontal masses normally are...

[Chris K]

Clear blue sky out there today and very warm. Lovely day! Also interested in the fact the met office have updated the warnings to cover my area, as well as the whole of SW, saying that thundery rain is likely to spread on monday night into tuesday. Also thundery showers for weds and thurs.

With a bit of luck I may finally see something electrical this year. Only downside obviously is the risk of flooding :/

Edited August 21, 2011 by Bugganuts

[Harry]

Estofex have updated their forecast, extending the level 2 over Benelux further West....its mouthwatering and so disappointing it's the other side of the channel.

Also quite interesting is that the FAX chart are bring the BoB low slightly further West, probably reflecting the updated warnings.

Lovely and warm and humid today - shame the CF will be moving through soon

[Adi F]

If we get anything today I think it will just clip me. But time will tell!

[Jane Louise]

Beautiful day here too, with sunshine and a nice light breeze. Looking forward to the next few days with the weather warning s for heavy thundery rain. Like bugganuts says, let's hope this dosent produce any flooding. Friday, although it's still early has a 51% storm risk for here, the first in a long time.

[Harry]

Just realised that UKASF have changed the wording of their extended outlook, replacing what I think was reference to heavy/thundery rain, to now thunderstorm potential....

EXTENDED OUTLOOK: There is a renewed possibility of thunderstorm potential early next week, from Monday night through to Tuesday, but this is highly dependent on the track of a frontal boundary and how far west it may move

For me, as I've maintained all along, am rather pessimistic and really cant call it....naturally, I'm praying for fireworks!! Wonder whether the frontal boundary will be made up of clusters of MCS thunderstorms rather than frontal nimbostratic (oooh new word lol) mess....whether that is even remotely possible! Given the very high progged 'juice' over France I dare say it's not outside the realms of possibility

[Harry]

That CF seems to be slower in clearing than the MetO FAX charts were predicting...also, the wave is starting to form (judging by the bends in the front and moisture trying to push northwards) much earlier than the FAX charts were thinking...unless I'm totally reading it wrong

[A.J]

Blimey I must have been knackered last night as I've just read about storms & heavy rain in my vicinity last night...I didn't hear a thing!

[Neilsouth]

I was in north essex at 6am, drove down to central essex woke up at 11 then drove back to kent.. NO storms, NO showers nothing

[chrisbell-nottheweatherman]

Were the storms last night anywhere near me?

[staplehurst]

Yet another upper frontal feature (in this case a cold front) on today's ASXX's, reiterating my point from a couple of days ago how this year (summer) has seen a lot more occurrences of dry continental air underneath moist tropical air.

[gottolovethisweather]

Yet another upper frontal feature (in this case a cold front) on today's ASXX's, reiterating my point from a couple of days ago how this year (summer) has seen a lot more occurrences of dry continental air underneath moist tropical air.

Hi staplehurst,

Without my "thinking head" being attached, can you tell me what effect the above has on the outcome of the weather at the time.

Also, well done Harry for yesterday. You called it right in that the MCS (a large storm indeed) took a sharp right after exiting the northern section of the BoB and headed due east across northern France into Belgium, just for a change.

Looking forward to tomorrow night and beyond.

Kind Regards

gottolovethisweather

Edited August 21, 2011 by gottolovethisweather

[Harry]

Hi staplehurst,

Without my "thinking head" being attached, can you tell me what effect the above has on the potentially thundery rain/showers etc.

Also, well done Harry for yesterday. You called it right in that the MCS (a large storm indeed) took a sharp right after exiting the northern section of the BoB and headed due east across northern France into Belgium, just for a change.

Looking forward to tomorrow night and beyond.

Kind Regards

gottolovethisweather

Haha, yay to me

I was watching it veer sharply towards the East and questioned whether you would ever see such ludicrous changes in direction elsewhere in the world - only a storm on course for our shores could be knocked away so easily...sometimes MCS' due to their scale will move more independently of veering/steering winds and plough on through...one example of that was a couple of years ago where the general flow was from a SSW to the NNE...an MCS was forecast to run up from France, possibly clipping Kent en route...woke the following day to see it had veered towards the NW and clattered into Cornwall/Devon causing immense frustration as it scuppered convective potential for that day.

Alas...here we are still waiting for a proper import to reach the south coast this year...almost all have stayed in the channel or veered east towards Benelux.

On the plus side, and it's limited enthusiasm I have to say, the thundery low forecast to move up Monday night is not showing much sign of shunting eastwards...in fact, subtle changes are pushing it ever so slightly westwards...if we want to see proper thunderstorm weather we need to be on the eastern side of that low as much as possible...western or northern edge and it'll be frontal mess.

Still can't call it

[gottolovethisweather]

Haha, yay to me

I was watching it veer sharply towards the East and questioned whether you would ever see such ludicrous changes in direction elsewhere in the world - only a storm on course for our shores could be knocked away so easily...sometimes MCS' due to their scale will move more independently of veering/steering winds and plough on through...one example of that was a couple of years ago where the general flow was from a SSW to the NNE...an MCS was forecast to run up from France, possibly clipping Kent en route...woke the following day to see it had veered towards the NW and clattered into Cornwall/Devon causing immense frustration as it scuppered convective potential for that day.

Alas...here we are still waiting for a proper import to reach the south coast this year...almost all have stayed in the channel or veered east towards Benelux.

Thanks Harry for a great explanation.

This is a brilliant place to the learn about our weather and I like the way these beasts from down south tend have a mind of their own.

Concerning your highlighted comment, can you remember where this baby was born? as it is my understanding that we need the developments to start down as far south-west in europe as at all possible!

Re: your last comment, it really has been years for my neck of the woods too.

Here's hoping for a MCS to come to our shores before the descent into autumn proper.

Cheers

gottolovethisweather

Edited August 21, 2011 by gottolovethisweather

[staplehurst]

Haha, yay to me

I was watching it veer sharply towards the East and questioned whether you would ever see such ludicrous changes in direction elsewhere in the world - only a storm on course for our shores could be knocked away so easily...sometimes MCS' due to their scale will move more independently of veering/steering winds and plough on through...one example of that was a couple of years ago where the general flow was from a SSW to the NNE...an MCS was forecast to run up from France, possibly clipping Kent en route...woke the following day to see it had veered towards the NW and clattered into Cornwall/Devon causing immense frustration as it scuppered convective potential for that day.

Alas...here we are still waiting for a proper import to reach the south coast this year...almost all have stayed in the channel or veered east towards Benelux.

On the plus side, and it's limited enthusiasm I have to say, the thundery low forecast to move up Monday night is not showing much sign of shunting eastwards...in fact, subtle changes are pushing it ever so slightly westwards...if we want to see proper thunderstorm weather we need to be on the eastern side of that low as much as possible...western or northern edge and it'll be frontal mess.

Still can't call it

25th May 2009 - ask Dogs32/Pat/(whatever name he has now haha).

Significance of upper frontal feature really just brings a change of airmass in the upper levels of the atmosphere rather than at the surface. As a result, you may have noticed many of the fronts recently have had rain not only falling from mid-high level cloud (rather than low cloud), but also a significant distance away from the actual surface front (take Thursday for example, huge mass of rain over CS Eng and S Midlands, yet the actual front was over the Channel closest to France).

EDIT: Notice on the current T+48 a warm front ahead of the main frontal system, this will no doubt be an upper level feature, and by T+60 it's drawn as an upper occlusion. Something's certainly out of the ordinary with our summer weather this year!

Edited August 21, 2011 by staplehurst

[Harry]

Just seen the new FAX chart which does make the risk of storm cells far less likely...the centre of the low though is slightly further West which is far better than being further East...whether we can hope/expect embedded thunderstorms within the frontal zones I don't know...but what we do have is a pretty vigorous triple point which do have a reputation of bringing more intense prolonged rainfall.

I would like to ask a question though to the more knowledgeable and experienced chaps...can MCS' type features exist amongst frontal zones?

Traditionally in the UK MCS features are not associated with fronts but with more subtle atmospheric disturbances...given the presence of the trough over France with very high CAPE/instability, thunderstorms are almost certainly going to develop here and drift north/north east with the low pressure system...could an MCS develop along the fronts or is that neither possible or likely? Anything convective that does develop I would suspect is liable to bear supercellular characteristics and turn possibly tornadic at times given the shear likely to be present.

Noticed that the first few storms are starting to fire over Benelux, bit later than I was expecting.

Edited August 21, 2011 by Harry

[Nick F]

Certainly wouldn't rule out some thundery activity Monday night and through Tuesday across parts of England and Wales, as the thundery low currently across Iberia tracks N across the Bay of Biscay and reaches south coast of England on Tuesday morning. Rather warm moist air aloft drifts north across England and Wales by Tuesday morning, as shown by the theta-w (wet bulb potential charts):

When you've got this amount of warm moist air in the atmosphere, a low with a triple point moving over - you can't rule out thunderstorms - even if GFS shows no CAPE. I don't think the models (i.e. GFS) handle storms too well in these plume type set-ups.

Lots of storms across iberia this evening looking at the sferic plots and sat pics (below), some cold cloud tops looking at spectral too, showing these storms are pretty strong in places. Be interesting see how the models handle this thundery low moving north over the next 36-48 hours.

Been plenty of let downs from similar set-ups in recent years, but there is always the unexpected when thinking that nothing is likely looking at GFS CAPE charts, last night was a good example when storms broke out across parts of England and Wales, despite GFS seeming rather uninterested in the idea.

[Neilsouth]

After all the mention of a storm I think i'll give up, what a terrible few years. However by that we've had some fantastic winters!

[Harry]

Certainly wouldn't rule out some thundery activity Monday night and through Tuesday across parts of England and Wales, as the thundery low currently across Iberia tracks N across the Bay of Biscay and reaches south coast of England on Tuesday morning. Rather warm moist air aloft drifts north across England and Wales by Tuesday morning, as shown by the theta-w (wet bulb potential charts):

When you've got this amount of warm moist air in the atmosphere, a low with a triple point moving over - you can't rule out thunderstorms - even if GFS shows no CAPE. I don't think the models (i.e. GFS) handle storms too well in these plume type set-ups.

Lots of storms across iberia this evening looking at the sferic plots and sat pics (below), some cold cloud tops looking at spectral too, showing these storms are pretty strong in places. Be interesting see how the models handle this thundery low moving north over the next 36-48 hours.

Been plenty of let downs from similar set-ups in recent years, but there is always the unexpected when thinking that nothing is likely looking at GFS CAPE charts, last night was a good example when storms broke out across parts of England and Wales, despite GFS seeming rather uninterested in the idea.

Wow cheers Nick....stunned by some of the cloud tops over Spain - circa -65C!!!

Fingers crossed!! I notice as ever estofex seemingly taking their time...it appears they like GFS struggle a wee bit with the dynamics of these setups!!