Storm risk - Monday 9th May onwards

[William Grimsley]

@Harry,

Maybe my post came over phrased incorrectly, I was saying that there wasn't any electrical activity in the showers yesterday because the PWAT values were higher than normal meaning the rainfall intensity outwheighed the actual instability in the atmosphere.

[Ben Sainsbury]

This is the area I see, having the greatest potential for showers to form within the next hour or two as already mentioned. This is where cloud levels seem to be thinnest too according to Sat24. (Possibly along South Coast too!)

 

Edited May 11, 2016 by Ben Sainsbury

[alexisj9]

Just now, Ben Sainsbury said:

This is the area I see, having the greatest potential for showers to form within the next hour or two as already mentioned. This is where cloud levels seem to be thinnest too according to Sat24. 

We have fog here.

[Gordon Webb]

met office has updated warnings but still a lot of uncertainty their focus for slow moving intense showers should they occur is southwest midlands se wales and the west country , of course i've got some fairly heavy rain heading my way anyway and I'm not convinced how much sunshine I'll be seeing

Edited May 11, 2016 by Gordon Webb

[Ben Sainsbury]

1 minute ago, alexisj9 said:

We have fog here.

But that's what I mean, if cloud is going to break anywhere, it is mostly likely to be where you are rather than the system moving west across England.

[Supacell]

18 minutes ago, William Grimsley said:

@Harry,

Maybe my post came over phrased incorrectly, I was saying that there wasn't any electrical activity in the showers yesterday because the PWAT values were higher than normal meaning the rainfall intensity outwheighed the actual instability in the atmosphere.

I may be talking rubbish here but I agree that the amount of moisture in the air could be to blame. It meant almost constant cloud cover. Any breaks were quickly filled in by more cloud. The heavy rainfall also cools the land and cold downdrafts can shut down the updrafts if there is no shear in place to split the two apart. Essentially what we had yesterday was a weather front and moisture saturated air producing a rain making machine that just keeps on going, producing large rainfall totals but at the same time scuppers thunderstorm chances. In these situations you can sometimes still get isolated lightning but it tends to be a rain mass punctuated by the odd sferic.

I am hopeful that some brightness today may allow something a bit more thundery too form. GFS seem to agree with this hence the larger CAPE values being shown in comparison to yesterday. William, your area looks good for tomorrow too.

 

Edited May 11, 2016 by Supacell

[William Grimsley]

11 minutes ago, Supacell said:

I am hopeful that some brightness today may allow something a bit more thundery too form. GFS seem to agree with this hence the larger CAPE values being shown in comparison to yesterday. William, your area looks good for tomorrow too.

Sounds good to me! Thanks, Supacell!

Just have a look at the rainfall radar again. I would imagine if the rain doesn't move off soon, there could some hefty rain totals under these showers:

[Nick F]

Netweather storm forecast for today, storms dependent on cloud breaks, which is always difficult to forecast where in the UK!

Storm & Convective Forecast

Issued 2016-05-11 09:36:00Valid: 11/05/2016 0600z to 12/05/2016 0600z

DAY 1 CONVECTIVE OUTLOOK: WEDS 11TH MAY 2016

Synopsis

Upper low will be centred close to the west of Portugal on Wednesday, with ridging to the north stretching from Iceland/N Atlantic to Baltic Sea. At the surface, a shallow low will drift west across southern counties of England with a wrap-around frontal boundary, bringing cloud and rain, gradually drifting west and NW across England, Wales and eventually Ireland. To the south of the boundary, a slack warm and moist E to SEly flow will become unstable with insolation/surface heating - bringing a risk of thunderstorms in places this afternoon.

... S ENGLAND, MIDLANDS, WALES, NW ENGLAND ...

Wednesday morning sees extensive cloud and outbreaks of rain drifting west across England and Wales associated with wrap around frontal boundary. Brighter skies with increasing cloud breaks are thought to spread in from the E and SE later this morning and through the afternoon, as the frontal rain retreats W and NW. Surface heating of moist airmass (PWAT values of 1 - 1.4 inches) is indicated to generate 300-800 j/kg CAPE - as per 00z GFS and ECMWF guidance. So scattered heavy showers and a few thunderstorms will likely develop given sufficient surface heating from cloud breaks though the afternoon across the above areas. Storms may focus along a W-E convergence zone between SEly and NEly flow, which is indicated by ECMWF to develop from northern Home counties west across central southern England/south Midlands to south Wales. Also, see breeze convegence appears likely across Devon and Cornwall. Along these convergence zones will likely see the strongest storms, with a risk of hail, gusty winds and torrential rain leading to localised flooding. There is a risk of isolated marginally large hail (1-2cm) given convective energy forecast, however, given uncertainty over how many storms will develop and lack of vertical shear for storm organisation - will refrain issuing severe probabilities for now.

Later this evening and overnight, there is a risk of thunderstorms developing over Belgium and northern France drifting west across SE England and East Anglia, though some uncertainty whether storms will survive the sea crossing, dependent on strong enough mid-level convection. If they do, they may pose of risk of hail, frequent cloud-to-ground lightning, gusty winds and localised flooding. 

... S IRELAND ...

Some uncertainty over cloud cover and where cloud breaks may occur, but where insolation /surface heating occurs this afternoon, models indicate a few 100 j/kg CAPE developing - which will allow heavy showers and a few thunderstorms to develop - bringing hail, gusty winds and risk of localised flooding.

Forecast can bee seen on the website here: http://www.netweather.tv/index.cgi?action=convective;sess=

 

Issued by: Nick Finnis

[William Grimsley]

Those downpours still continuing to dominate to our NE, in fact it looks like they're starting to move this way, let's just hope that doesn't scupper our chances. However, to me most of the frontal rain is fizzling out and moving west quite sharply now.

[weather09]

Yes, was quite a saturated air mass throughout the profile yesterday which isn't ideal, as can hinder updrafts with water loading for example. Ideally we want dry air at mid-level, which increases convective instability. Lapse rates were pretty meagre as well. Though unlikely the moist profile alone would have been the issue. Indeed, we see much higher moisture advect across UK during the year.  June 28th 2012, for example, saw highly moist environment.  Issue was cloud debris and lack of surface heating to build CAPE and trigger storms.  

No shear and jet aloft to support with lift and storm organisation as well makes storm development that more difficult

[MP-R]

Big blob of rain is taking a hike westwards now which is good. More intense downpours starting to form and move westwards. Could just do with some cloud thinning to really get things going.

[Harry]

Pouring down in London in the past 30 minutes or so and recently gone very dark.

No sferics to speak of however.

Lets hope once this clears through we may be able to develop some breaks for some good surface heating.

To clarify my earlier post, I was referring to the idea that the amount of rainfall/rainfall rate/PWAT was linked to the probability of lightning. I think recent posts by Supacell and Weather09 deal with the issue beautifully - it is a lack of instability and wind shear (and therefore surface heating I would suggest) that has scuppered our lightning chances, not by virtue of relatively PWAT values. Take 3/4 July last year - if I recall correctly, we experienced exceedingly high PWAT levels, but also strong surface heating and very high (mid level in particular) instability. Added to this was a trough which helped destabilise the build up in instability.This resulted in very high lightning rates by UK standards. It is therefore more accurate to say, in my view, that the lack of lightning was not because of high PWAT levels/rainfall, but a lack of surface heating/wind shear and therefore lack of instability....broadly speaking.

It is because of this I supported Surrey's appraisal of lightning risk today, but highlighting (rightly in my view) that because of an apparent repeat performance of yesterday (bucket loads of cloud cover) that we may see disappointing lightning occurrences today.

 

Edited May 11, 2016 by Harry

[William Grimsley]

1 minute ago, Harry said:

Pouring down in London in the past 30 minutes or so and recently gone very dark.

No sferics to speak of however.

Lets hope once this clears through we may be able to develop some breaks for some good surface heating.

To clarify my earlier post, I was referring to the idea that the amount of rainfall/rainfall rate/PWAT was linked to the probability of lightning. I think recent posts by Supacell and Weather09 deal with the issue beautifully - it is a lack of instability and wind shear (and therefore surface heating I would suggest) that has scuppered our lightning chances, not by virtue of relatively PWAT values. Take 3/4 July last year - if I recall correctly, we experienced exceedingly high PWAT levels, but also strong surface heating and very high (mid level in particular) instability. Added to this was a trough which helped destabilise the build up in instability.This resulted in very high lightning rates by UK standards. It is therefore more accurate to say, in my view, that the lack of lightning was not because of high PWAT levels/rainfall, but a lack of surface heating/wind shear and therefore lack of instability....broadly speaking.

 

True, but I never said it was specifically to do just with PWAT values, I said they contributed to it...

[Flash bang flash bang etc]

For the South East would you say it's better chances for the overnight period?

 

The BBC forecast seems to put most homegrown activity over central areas, which is fine for me if it means we have the chance of imports (which sound like they might be more convective given favourable conditions)

[William Grimsley]

There we go! We have ignition over SE England!

[Gordon Webb]

34 minutes ago, Flash bang flash bang etc said:

For the South East would you say it's better chances for the overnight period?

 

The BBC forecast seems to put most homegrown activity over central areas, which is fine for me if it means we have the chance of imports (which sound like they might be more convective given favourable conditions)

depends what forecast you put your faith in

Nick F has his focus for the worst in a line from E - W from northern home counties across central southern / south midlands to south wales but is incertain how many will develop

metcheck has there focus over the higher ground notably the Mendips, North Downs, Chilterns, Exmoor, Dartmoor and Bodmin

bbc has warnings out for a few areas but with focus for sw midlands se wales and west country but with the proviso that most will probably miss them and also ones overnight travelling East - West along southern england which is what i think your referring to

currently a line of showers over m4 corridor or maybe just north or south of it not sure where they going to go or do bbc i think has them going W/NW heading into midlands then eventually wales according to latest News 24 forecast , probably change again by 1.30pm since there only mention of sunshine this afternoon over southern parts was for the SE

currently overcast where i am but showers se of me may encroach

so take a pick and see what comes off is my view

Edited May 11, 2016 by Gordon Webb

[Spikecollie]

Indeed! And here too in Limousin it's looking good. The sun has just come out, although it is still raining, so the ingredients just might be coming together for this afternoon.

[Gordon Webb]

32 minutes ago, William Grimsley said:

There we go! We have ignition over SE England!

doesn't look like it lasted

[Flash bang flash bang etc]

8 minutes ago, Gordon Webb said:

doesn't look like it lasted

That's the central southern bit of the South east of England for you ;-)

[NO LONGER POSTING]

There are showers.. Non thundery though again South east will import the left overs later today from france 

[Evening thunder]

Just a couple pics from yesterday, went on a little 'chase' with lots of heavy/torrential showers (no thunder of course). Saw this 'dangly' scud rising rapidly into the cloud base for a minute or two. Sort of looked like a funnel cloud trying to form even if it wasn't, there was hardly any (visible) rotation but perhaps a bit of variable direction noticeable in the rising scud. Though it was probably rising at least 20-30mph maybe more, given the speed relative to the tree size on that hill, it really was quite rapid despite being dead calm at my location.
 

As for today and tomorrow, I'm kind of disappointed how they have both downgraded, were looking much better on the GFS charts and it will feel a pretty poor show here if we come through this without a hint of thunder here given several days of what had looked like ok potential.

Edited May 11, 2016 by Evening thunder

[Harry]

Sky is trying to brighten up here. If it is able too in the next hour or so we may be in some luck.

Judging by the radar, rashes of heavy showers developing south of London. Given the motion of the precipitation, i.e comparing precipitation travel along the M4 corridor (due W) with the showers south of London (NNW) I'd be surprised if there's not, locally at least, more shear than perhaps anticipated.

Edited May 11, 2016 by Harry

[Bolt from the Blue]

Looking good for the SW tomorrow:

 

[Harry]

10 minutes ago, Evening thunder said:

Just a couple pics from yesterday, went on a little 'chase' with lots of heavy/torrential showers (no thunder of course). Saw this 'dangly' scud rising rapidly into the cloud base for a minute or two. Sort of looked like a funnel cloud trying to form even if it wasn't, there was hardly any (visible) rotation but perhaps a bit of variable direction noticeable in the rising scud. Though it was probably rising at least 20-30mph maybe more, given the speed relative to the tree size on that hill, it really was quite rapid despite being dead calm at my location.
 

As for today and tomorrow, I'm kind of disappointed how they have both downgraded, were looking much better on the GFS charts and it will feel a pretty poor show here if we come through this without a hint of thunder here given several days of what had looked like ok potential.

Awesome...everything about that picture strongly suggests to me that this is in fact a funnel cloud, not scud. Furthermore, from the middle picture it suggests to me that this 'scud' is forming in area of lowered cloud base, looking like a subtle wall cloud.

If I was a betting man (which I'm not), I'd wager what you witnessed was a funnel all day long.

[William Grimsley]

11 minutes ago, Harry said:

Awesome...everything about that picture strongly suggests to me that this is in fact a funnel cloud, not scud. Furthermore, from the middle picture it suggests to me that this 'scud' is forming in area of lowered cloud base, looking like a subtle wall cloud.

If I was a betting man (which I'm not), I'd wager what you witnessed was a funnel all day long.

WOW! As I stated in a previous post, I was with Evening thunder yesterday, so that's the first funnel cloud I've ever seen! Fantastic!

Edited May 11, 2016 by William Grimsley