20th Aug 2011 - Storm/Convective Forecast Discussion and Reports

[Jane Louise]

Ok, l thought I'd start a new thread. Now, UKASF seems to think that some of us have a slight chance Saturday and according to them the areas affected will be W + SW Scotland, Northern Ireland, SW England, SE Wales, West Country, S + E Midlands I expect Stu will be along to post up a more detailed forecast from UKASF later on .

Nothing from Estofex yet, but to be honest I think there will be more potential Sunday night until Tuesday most especially for our SE lot.

Anyhow, it's good luck to all for the next few days and remember to always expect the unexpected .

[ZONE 51]

keeping a watch for the potential electric activity for saturday night in the southeast, some humid moist air moving in, i would say the risk being for IOW along the Sussex coast into Kent and possiblly some isolated cells northwest of this area saturday night, its the type of storms that could spark off somewhere unexpected, do we take a trip to France? well i dont think we need to this time!

[Guest Quantumsnow]

Good luck Jane! I'm still waiting for a decent storm here, nothing this year except a distant rumble and a small one near the beginning of the year, doubt this will be much different but it's something to watch out for I suppose!

Will be F5-ing Estofex on a regular basis

[bobbydog]

Good luck Jane! I'm still waiting for a decent storm here, nothing this year except a distant rumble and a small one near the beginning of the year, doubt this will be much different but it's something to watch out for I suppose!

Will be F5-ing Estofex on a regular basis

you must have had something during the 'spanish plume'! we had some reasonable storms here that day and you're only a few miles away from me

[Jane Louise]

Good luck Jane! I'm still waiting for a decent storm here, nothing this year except a distant rumble and a small one near the beginning of the year, doubt this will be much different but it's something to watch out for I suppose!

Will be F5-ing Estofex on a regular basis

Good luck to you too QS. I know you haven't had much luck either,like me it's always been so near yet so far away grrrrr

I'm continuously refreshing Estofex too, hopefully they won't be too long lol.

[Guest Quantumsnow]

you must have had something during the 'spanish plume'! we had some reasonable storms here that day and you're only a few miles away from me

Nope, sadly not bobbydog, I was stuck at work in Henfield that day. I remember sneaking a look at the radar on my mobile and seeing the cells in an arc over Horsham area though. Did get a rumble or two at work but had to have the windows closed because one of the patients was affraid of thunder.

Jane, you are well overdue a supercell or two

[bobbydog]

Nope, sadly not bobbydog, I was stuck at work in Henfield that day. I remember sneaking a look at the radar on my mobile and seeing the cells in an arc over Horsham area though. Did get a rumble or two at work but had to have the windows closed because one of the patients was affraid of thunder.

Jane, you are well overdue a supercell or two

ha ha! i remember i was due to be stuck in a loft that day, plumbing in a shower and you told me to call in dead, or anything to get out of it! as it happens, the job was postponed and i was off when we got the storms. with your line of work, i'm surprised you couln't think of an 'acute illness' that day lol

[MR CB]

keeping a watch for the potential electric activity for saturday night in the southeast, some humid moist air moving in, i would say the risk being for IOW along the Sussex coast into Kent and possiblly some isolated cells northwest of this area saturday night, its the type of storms that could spark off somewhere unexpected, do we take a trip to France? well i dont think we need to this time!

Indeed there was a very good sign this evening out over the Channel - some Cumulus Castellanus in a long row with well defined heaps and virga, I'm sure this is a sign of the humid air. As soon as I saw it I though thundery rain possibly tommorow or Sunday

[ZONE 51]

There was a line of altocumulus cas here across Surrey friday evening, thin lines of bubbles, not virga on this row but a sure sign of increase of mid level humid air. this is what il be watching out for saturday today!

Edited August 20, 2011 by ElectricSnowStorm

[Nick F]

Can't really see much in the way of storm potential today, risk of some elevated convection bringing thundery showers and perhaps an isolated storm to the SE of England tomorrow morning - before it clears away by early afternoon.

Then on Monday we see a new low develop over Biscay which then tracks NE over France, unforuntately it looks like most of the storms will stay over the near continent and we get the heavy rain, always seems the models overplay the NW extent of a plume days out then as usual it end up being shunted SE with any decent storm potnential staying the other side of the channel.

[Coast]

ESTOFEX going with the French storms, but nothing reaching our shores:

Extended Forecast

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

Issued: Fri 19 Aug 2011 23:35

Forecaster: TUSCHY

A level 1 was issued for parts of France and Germany mainly for large to very large hail and severe wind gusts.

A level 1 was issued for parts of Spain and Portugal mainly for large to very large hail and severe wind gusts. An isolated tornado threat may evolve along the coastal areas.

SYNOPSIS

A cut-off low west of the Iberian Peninsula becomes trapped in the westerlies and starts to move northeastwards, eventually approaching the Bay of Biscay during the night hours. A sharp frontal boundary over parts of W-Europe will be the focus for isolated to scattered thunderstorm initiation. Stable and hot conditions prevail over most of the Mediterranean.

DISCUSSION

... France to Germany ...

Stalling stationary front from the 18th/19th cold front event continues to retreat northwards during the day and seems to be placed somewhere over central France and central Germany as an active warm front boundary. Air mass over far S-France/W-Mediterranean features a rich BL moisture content with surface dewpoint readings in the upper tens/lower twenties. As an upper low moves towards the Bay of Biscay, WAA strengthens over France and helps to sharpen the boundary during the day with EZMWF/GFS indicating a quite intense moisture gradient along this boundary....with moisture pooling bringing surface dewpoints well above 20°C on a local scale. Steep mid-level lapse rates atop this air mass feature a confined area with high CAPE build-up....probably in the order of 1-2 kJ/kg. However major uncertainties exist in the exact placement of this front due to numerous weak short waves crossing that boundary during the severe weather outlook day-1 period, each of them being able to generate strong to severe thunderstorms, which may modulate final boundary position.

Despite that, overall conditions favor rapid to explosive thunderstorm development on an isolated to scattered scale regarding thunderstorm coverage. Shear in order of 20-25 m/s (6 km bulk shear) will be more than adequate for intense multicell/supercell evolution with the main risk being large to very large hail and severe downbursts. Near parallel alignement of the deep layer shear vector to the frontal boundary also favors the development of numerous smaller scale and progressive clusters with a more widespread severe to damaging wind gust risk. However, major uncertainties in the meso-alpha/beta scale preclude any higher probabilities for now. Also, latest QPF outputs hint on more isolated thunderstorm coverage especially over central France, which also neglects a level 2 for now.

UKASF:

Storm Forecast - Slight

Forecaster: Dan

Last Updated: 20:02 Friday, 19th August 2011

Valid: 00:00 Saturday, 20th August 2011 - 23:59 Saturday, 20th August 2011

Areas Affected:

SLGT: W + SW Scotland, Northern Ireland, SW England, SE Wales, West Country, S + E Midlands

Synopsis:

Upper ridging slowly weakens and moves eastwards as an upper low approaches from the Atlantic. The associated upper trough weakens progressively during the forecast period. Limited convection is expected.

Discussion:

In the post frontal airmass, numerous showers are forecast over SW + W Scotland and Northern Ireland during the morning. Slight instability is indicated with a few hundred J/kg CAPE, and ELT's marginally conducive for lightning. Chances for isolated thundery activity increase here during the evening hours across Northern Ireland and SW Scotland in particular.

Further south, in the redeveloping warm sector as a wave develops along the cold front, slight instability is also signalled to run NEwards along/ahead of the front between 12z-23z from SW Eng towards Lincolnshire. Isolated lightning is possible from embedded convection, but not expected to be widespread given weak support.

During the evening hours it is likely an area, possibly linear, of showers and thunderstorms may develop in the W English Channel, drifting NEwards just offshore of Cornwall and Devon, and possibly making landfall somewhere in S/E Dorset (Swanage, Poole etc for example). However, this is expected to be beyond this forecast period and thus we will monitor this situation and will provide an update in the Forecast issued for Sunday.

EXTENDED OUTLOOK: There exists a chance of a few thunderstorms over the C + E English Channel during Sunday morning, which may move onshore at times, although this remains uncertain. Further activity is expected next week.

21st OWS have nothing even close!

Their Met O interpretation shows it way too far South for our interest:

WAFC Aviation chart looks better:

KO index looks in about the right place:

CAPE doesn't really get going until the early hours of tomorrow, but is encouraging:

Gone by lunchtime tomorrow:

I'm not convinced for daylight hours at all during today, but later overnight and into the morning, still has the merest smidgen of a chance for the South coast:

Maybe even a funnel in the Channel if you can see out there?

Yet again down to radar and lightning detector watching - presupposing you want to be up in the wee small hours in Hants, Sussex and Kent coastal areas!!!!!

[Coast]

Just had a squint at the Netweather model and the CAPE and LI are not in the right place I'm afraid, even at 03Z tomorrow morning:

Might just see it out to sea......

[chionomaniac]

Coast, the estofex chart that you have displayed is the extended forecast for tomorrow that was issued yesterday. That may still change though (probably for the worst!).

Todays estofex forecast doesn't set the heart racing either!

[Coast]

Coast, the estofex chart that you have displayed is the extended forecast for tomorrow that was issued yesterday. That may still change though (probably for the worst!).

I just couldn't see people go through the pain of today's chart!!

I'm not sure that today up until midnight will give us anything, but from 0330 hrs onwards is where the slight risk for Kent and Sussex coasts seems to be, dying out after breakfast.

When I say risk, I mean

[Jane Louise]

Hmmm, I must admit I was a little doubtful whether anything would occur. I guess it's wishful thinking on my behalf. UKASF has a small chance which gave me hope lol. Oh well, roll on the next storm potential.

[Harry]

Horrifically disappointing

I would say in terms of storms, the best chance for anyone in the SE is in fact later tonight (based on GFS). After that it just stays almost entirely over the channel.

Still, that's the 00z...lets see what the 06z has to offer

[NUT]

Hasn't it really been the case for most of the summer? Things so near yet so far?

[gottolovethisweather]

Hello all.

On the face of it today, so far, looks disappointing pretty much as forecast, despite some spurious blitzortung sferics showing, however..........my friends

I will draw your attention to a development, currently showing up off the western coast of Portugal (shown in red).

On it's current track I would say the SW and maybe areas around Dorset/Somerset/Bristol and Avon could be in the firing line during the wee hours of tonight.

Once we're clear of the frontal mass (in blue) then maybe the real moisture and subsequent storms can come our way.

If tonight goes down the pan, I believe there's still hope for Monday/Tuesday, is there not? so we should all keep our chins up.

All of my assumptions need backup from the more knowledgeable in here and effectively go against current forecasts but I have a gut feeling that THIS IS NOT CLEAR CUT.

Thundery Regards

gottolovethisweather

[sunnijim]

I would agree there. GFS 06z has improved CAPE/Lift values compared to recent runs with the extreme SE showing precipitation and decent values between midnight and 6am.

This far from all over with regards potential for some tonight imo.

[staplehurst]

I hold on to my thoughts for the early hours offshore of Kent/Sussex...

[Harry]

I hold on to my thoughts for the early hours offshore of Kent/Sussex...

No no no that's a cop our Staplehurst....please divulge as you tend to be right

[staplehurst]

No no no that's a cop our Staplehurst....please divulge as you tend to be right

haha

well in this sort of setup, you can get storms randomly developing in the eastern Chanel, primarily due to the high SSTs. I still think this is a possibility tonight, it happened a couple of years ago I remember so I am still optimistic for some overseas stuff tonight.

[Chris K]

On the face of it today, so far, looks disappointing pretty much as forecast, despite some spurious blitzortung sferics showing, however..........my friends

I will draw your attention to a development, currently showing up off the western coast of Portugal (shown in red).

On it's current track I would say the SW and maybe areas around Dorset/Somerset/Bristol and Avon could be in the firing line during the wee hours of tonight.

Once we're clear of the frontal mass (in blue) then maybe the real moisture and subsequent storms can come our way.

Im not an expert in these things, but with the development you are talking about west of portugal... wont the fronts currently over the UK just push it away from us when they approach each other? Ive seen it do this many times - it heads our way then eventually changes course eastwards over the continent.

As for monday night into tuesday...looks more promising but I fear for my area it wont amount to anything bar a bit of rain. Looking better for the SE with the current forecast of the fronts positioning. Obviously it can and most likely will change. But for me the general trend has been it shifts further east and misses me completely.

[staplehurst]

I can't remember the exact date (some of you guys on here might though) but it was definitely a Saturday night/early hours of Sunday type thing when storms popped up in the eastern Channel unexpectedly and gave a good coastal show in the early hours ahead of an advancing cold front. I think I've found the date (below) but it might've been some other time - nevertheless, the chart does look similar to today:

This is what I'm hoping will happen tonight

[gottolovethisweather]

Im not an expert in these things, but with the development you are talking about west of portugal... wont the fronts currently over the UK just push it away from us when they approach each other? Ive seen it do this many times - it heads our way then eventually changes course eastwards over the continent.

Hi Bugganuts,

The surface low, being located where it is, is exactly where a MCS, should it develop would need to be in order for a widespread hit of the UK, eg to avoid being just a kent clipper or a belgium beast etc. With regards to your first part as highlighted above, yes is the most likely answer. However, this much depends on the speed of the mess clearing eastwards, i.e. if it gets out of the way sharpish (which it seems to be oing) then IMHO, it may well be game on. This is actuially what is also hinted at by the NW homepage headline summary. Keeping tabs on satellite animations, radars and atmospheric conditions is much advised even at this early stage, I would say.

Monday/Tuesday is still too far away to be sure in my opinion and the thundery(hopefully) mass could go anywhere.

I believe there is a lot of "clutching at straws" going on, even by the official folk (MetO etc.) as to what develops from this point forward.

An interesting few days ahead I feel.

Cheers

gottolovethisweather

Edited August 20, 2011 by gottolovethisweather