Uk Convective General Discussion & Forecasts, 16th August 2012>

[Coast]

So are we still on for tomorrow then in the SE?

GFS simplified says not.....

Until after dark!!!!!!!!!

Kent particularly favoured?

Higher level lapse rates:

and shear:

Anywhere from Brighton to Dover I guess, fingers crossed!

Surface CIN is high:

Mid levels less so:

Could be some good elevated storms after dark and into the early hours out over the Channel and maybe along the coast??

Come on you storms!!!!!!

[fergieweather]

Yes, the key area, at least in UKMO analysis, remains northern/northwestern boundary of developing heat low through Sunday. The last 2 NAE runs have been consistent in this respect, with 06z analysis & high-res (UKV/UK4) output due in next 40mins. In broad terms, AcCas showers Sunday AM expected to spread N from S Coast early AM, primarily into W Country environs, with subsequent deeper destabilization into midday on periphery of frontal boundary as a marked area of convergence (signalled in last 3 runs) threatens to fire things up in a broad zone extending roughly Bristol / Wilts N'wards & NE'wards across the flank of the heat low. However, confidence on distributional bounds for threat of torrential downpours has been low based on recent runs, so UKMO Chief Forecaster awaiting better continuity before warning is given consideration (might be issued this morning). None of the last 3 runs keen to generate storms in SE quadrant, but detail not wholly trustworthy. Will update on sight of next analysis. Cheers, Ian.

[Bobby]

Some torrential downpours here yesterday, 45mm in all from that front - 20mm came in an hour or so, some minor flooding, no thunder or lightning though. Looks like Sunday could be a repeat looking at the charts. Sceptical on any thundery potential though.

[Coast]

None of the last 3 runs keen to generate storms in SE quadrant, but detail not wholly trustworthy. Will update on sight of next analysis. Cheers, Ian.

Good morning Ian,

What is limiting or not initiating thunderstorms into the darker hours of tomorrow here in the SE? I must admit your colleague locally here, Tomasz Schafernaker, made no mention of anything in his weekend forecast last night, but then I know you guys hang on until more detail becomes apparent.

Just checking a couple of the Hi-Res charts available to us mere mortals, they do point to things occurring out over the Channel, but is the cap too much for us?

[fergieweather]

Yes, Sunday's concern is potential for heaviest rain in similar areas to those affected yesterday, as the final wave (with shortwave accompanying) runs up NE from SW peninsula. Latest EC Op rainfall accumulation for 06-18z Sunday shows strong signal for this very similar distribution to yesterday, with a very muted signal in SE, much akin to UKMO-NAE, GM & extended UKV.

[fergieweather]

Hi Coast, re the SE, UKMO suggests 'low risk' isolated storms in that area overnight into Sun & again later Sun, but notes that low risk is tempered by potential high impacts. However, new analysis (just in) carries similar story to before: early AcCas showers frm S, some possibly thundery into W Country, with then the frontal/wave showery rain aligned roughly Cornwall-Pembrokeshire-Cumbria into day (some heavy), whilst convergence feeds showers/storms (potential severe) in area by mid afternoon from approx Wilts/Glos N'wards into Midlands. This latter concern will be focus of warning which I understand will be issued at some point later this morning. Potential for isolated storms further SE later Sunday remains, but is, as noted above, considered "...low risk, but high impact". It's a tricky one, frankly, much as we dealt with yesterday as the chaotic elements of upper forcing kept us second-guessing what would happen each hour!!

[Bobby]

It's a tricky one, frankly, much as we dealt with yesterday as the chaotic elements of upper forcing kept us second-guessing what would happen each hour!!

Yes the intensity and location is varying by quite a bit by each run for this next event too. The GFS 06z has the main rain area further E than the NAE, while the 06z NAE seems to have ramped up the intensity again compared to 00z. Another one of those radar watching and looking out the window days probably.

Some torrential rain for some anyway with flooding a concern considering the same areas might get hit, already had 45mm here from the last batch so the ground is soaked.

[fergieweather]

Hi Barb, yup, latest NAE ramps-up totals again; 30mm or so in places. Re positioning, given the broadly good agreement between EC & UKMO-GM & NAE, that's the story they're running with at Exeter for now. Chief makes no comment re GFS prog, but they'll of course considered it (albeit EC tends to be favoured anyway in many respects... we'll often see comparative analyses sent to us comprising UKMO output made alongside those from ECMWF, GFS, ARPEGE & sometimes JMA... they rate the latter v highly indeed). I can't post the EC output here graphically, so you'll just have to take my word on it, descriptively!

[Coast]

Potential for isolated storms further SE later Sunday remains, but is, as noted above, considered "...low risk, but high impact". It's a tricky one, frankly, much as we dealt with yesterday as the chaotic elements of upper forcing kept us second-guessing what would happen each hour!!

Ahh yes!!!! that will be the 'roller coaster' effect that we get in here every so often!!!

Thanks for clarifying how knife edge it is.

As usual then, some potential that might or might not appear and down to some ATD radar watching + keeping an eye on the detectors tomorrow!

[VillagePlank]

An interesting day for the SE, Sunday.

A curious set of vertical velocity charts, this morning,

This shows sinking air at the lower levels, and rapidly rising air at 500hPa and above; I can see why the talk is off elevated storms.

The forecast skew-t shows why this is the case,

Essentially, CIN is present. Now will surface heating be enough for the breakthrough? At this stage I don't think so, we need another trigger - but *should* CIN melt away in the hot sunshine, the stage is set for some explosive action. Best time at the minute is around 18z, I reckon.

The midday Herstmonceaux ascent will be absolutely essential viewing on the day.

Edited August 18, 2012 by Boar Wrinklestorm

[johnholmes]

An interesting day for the SE, Sunday.

A curious set of vertical velocity charts, this morning,

This shows sinking air at the lower levels, and rapidly rising air at 500hPa and above; I can see why the talk is off elevated storms.

The forecast skew-t shows why this is the case,

Essentially, CIN is present. Now will surface heating be enough for the breakthrough? At this stage I don't think so, we need another trigger - but *should* CIN melt away in the hot sunshine, the stage is set for some explosive action. Best time at the minute is around 18z, I reckon.

a nicely presented post, if only some in the model thread would use this as a model, viewpoint expressed,charts shown and explained.

thank you

[Bobby]

Throwing it down again here, the BBC said a dry day with the sun coming through.

[andymusic]

Throwing it down again here, the BBC said a dry day with the sun coming through.

Raining once again in the land of song - this green and soggy land - was passing through just north of swansea last night on the way up to newquay - and wow what a torrential amount of rain - motorways flooded - quite scary and bad conditions! - amazingly though not one flash of lightning!!!!!!!!!!

[weatherobsuk]

Its the kind of weather you daren't attempt hiking in the moorlands because it can go from being sultry and humid one hour to rain and low cloud base the next. Definitely a evening to head to the local where the only flashes guarantee'd are that of the whiskey shots.

Even the forecast's of it being a dry afternoon here in the Peak District are looking diminishing with every hour as that rainpulse from the SW begins to creep nearer.

[Bobby]

Raining once again in the land of song - this green and soggy land - was passing through just north of swansea last night on the way up to newquay - and wow what a torrential amount of rain - motorways flooded - quite scary and bad conditions! - amazingly though not one flash of lightning!!!!!!!!!!

I read that as the Land of Sog which suits nicely too eh? Some flooding here too from the torrential downpours, some of the smaller roads were knee deep in water. ~20mm in little over an hour at the worst of it.

The Met Office Wales forecast looks interesting for tonight

Today:

A wet and breezy day with some heavy rain at first, easing for a time then further rain this afternoon and evening with torrential and thundery downpours in places. Feeling warm and humid despite the breeze. Maximum Temperature 21 °C.

Tonight:

The chance of some heavy spells of rain at first. The rain will become more patchy later but still warm and humid with low cloud and hill and coastal fog. Minimum Temperature 14 °C.

Edited August 18, 2012 by Barb-

[legritter]

30mins ago absolute huge spots of rain lasted on and off for 3/4 mins ,but no wherenear enough to wet the ground ,nothing on radar . next 24/36 hrs very hard to predict at the moment ,so many elements to come together to get any storms or torrential down pours .but it is becoming increasingly likely that several areas of UK will get some good action as the weather breaks down .at the moment i will be keeping an eye down south and s/west as iv a gut feeling the front will pep up later today and bring something for our area later today and overnight ,with places further east getting their action later tomorrow .so will it be a BIG BANG or abig let down .some good posts today and thanks for those posters who have been posting charts data etc ,things will be much clearer come about 8pm this evening especially with a good view on SATTELITE .strange yesterday between 5PM AND 9PM we have free view telly ,and usually we cannot get BBC WEST AS SIGNAL IS always BREAKING UP ,we watch bbc WALES instead but last night brilliant signal for four hours ,i wonder if weather action to my west influenced this ,any thoughts ,will be back later ,must get lawns cut ,cheers legritter

[paul m]

where is everyone ?????

[Cheshire Freeze]

where is everyone ?????

Not much going on atm.

[AWD]

Is this the Meto starting to come onboard?

Sunday:

Showery rain will spread northeast, with some intense thunderstorms possible at times. Largely dry for far northwestern areas, and also for the southeast where it will be very hot.

[Supacell]

I'm lurking but as CC says, not much going on. I have lost interest in today and did do by about 9am this morning as I just cannot see anything happening anywhere away from N Scotland, and even I am not willing to travel that far to see a storm.

I am hoping for better tonight into tomorrow. I was wondering when the plume gets here if there is likely to be elevated lightning tonight?

BBC going for thunderstorms across the Midlands and up into Lincolnshire and East Yorkshire tomorrow.... can't see Lincolnshire getting anything though - they never do

Edited August 18, 2012 by Supacell

[AWD]

I'm lurking but as CC says, not much going on. I have lost interest in today and did do by about 9am this morning as I just cannot see anything happening anywhere away from N Scotland, and even I am not willing to travel that far to see a storm.

I am hoping for better tonight into tomorrow. I was wondering when the plume gets here if there is likely to be elevated lightning tonight?

BBC going for thunderstorms across the Midlands and up into Lincolnshire and East Yorkshire tomorrow.... can't see Lincolnshire getting anything though - they never do

Where have I heard that before? Hah yeah, June 28th springs to mind..................

[Chris K]

Ian Fergusson â€@fergieweather

We'll see 22-26C in W Country Sunday, prob higher in Wilts. Showers/storms likely for some. Settled much of nxt week... until Friday onwards

Maybe...just maybe... although at the moment forecast rainfall is kept further West from here into Wales and any showers look likely to head NE across the Midlands.

Edited August 18, 2012 by Chris K

[Cheshire Freeze]

Ian Fergusson â€@fergieweather

We'll see 22-26C in W Country Sunday, prob higher in Wilts. Showers/storms likely for some. Settled much of nxt week... until Friday onwards

Maybe...just maybe... although at the moment forecast rainfall is kept further West from here into Wales and any showers look likely to head NE across the Midlands.

Yeah I can envisage it now....torrential rain heading up through Wales then into Lancashire, a batch of storms moving NE through the Midlands into Lincolnshire etc and a great big dry slice inbetween the two areas, with Crewe at the centre of this dry slice. I bet this is how it plays out.

[Supacell]

Hirlam charts looking very promising for areas on the SE flank of the wriggling cold to warm front tomorrow for some really good storms to develop if the cap can be broken. This is also the area on the Northern flank of the low coming up from France.

I will try and forecast using this, but it is obviously subject to change at this range.

Parcel layer depth very impressive:

This chart would indicate to me that most activity would be fairly elevated to the SE of the waving front.

But with Parcel layer depth being so high then all this MLCAPE will be available through the layers (around 1250-1500j/kg)

So, with all this CAPE what else can be used to help develop some intense storms. How about 60knts of Deep layer shear (see above chart)

And a decent dose of helicity just to get those clouds spinning and increase the risk of a tornado or two

There is some rain in there too, although worth noting that the precip in the storm risk zone does not appear until 6pm and so looks like hirlam believe the cap to hold until later on in the afternoon. The front can be seen across Wales and Northern England too and there could definitely be some embedded cells in here.

If these charts verify, and the cap is broken, then I can foresee a line of storms developing just south east of the front and they will have the ability to turn severe as they move slowly NE.

Edited August 18, 2012 by Supacell

[VillagePlank]

Herstmonceaux sounding, 12z

Observation time: 120818/1200

Station latitude: 50.90

Station longitude: 0.32

Station elevation: 52.0

Showalter index: 3.35

Lifted index: 4.04

LIFT computed using virtual temperature: 3.89

SWEAT index: 166.12

K index: 29.30

Cross totals index: 14.70

Vertical totals index: 29.70

Totals totals index: 44.40

Convective Available Potential Energy: 0.00

CAPE using virtual temperature: 0.00

Convective Inhibition: 0.00

CINS using virtual temperature: 0.00

Bulk Richardson Number: 0.00

Bulk Richardson Number using CAPV: 0.00

Temp [K] of the Lifted Condensation Level: 281.32

Pres [hPa] of the Lifted Condensation Level: 820.02

Mean mixed layer potential temperature: 297.74

Mean mixed layer mixing ratio: 8.40

1000 hPa to 500 hPa thickness: 5716.00

Precipitable water [mm] for entire sounding: 23.98

The Showalter index indicates severe weather is unlikely, the SWEAT index indicate that there is a (ever so) slight chance of severe storms, k-index puts that chance between 40-60%, and cross totals, and vertical totals indicate that any showers will not be thundery. No CIN, no CAPE, and positive LI means a hot stable evening ahead for the SE ...

Looking to tomorrow, then

Edited August 18, 2012 by Boar Wrinklestorm