Uk Convective General Discussion & Forecasts, 27th June 2012>

[Supacell]

UKASF forecast is out and its a beauty!!!! If someone could post the map that would be great as i cannot see it for some reason with my internet connection

Areas Affected:

SVR: SE Scotland/Borders, NE Eng, Yorkshire, E Mids, East Anglia

MDT: NE Eng, Yorkshire, E Mids, N Cambs/W Norfolk

SLGT: Majority of United Kingdom, excluding NW Highlands and N Isles

Synopsis:

Upper trough gradually approaches from the west, allowing a plume of high WBPT and very high ThetaE to advect northwards across Britain. Destabilisation will occur on the plume axis during the passage of a pronounced shortwave trough, with potential for severe convection over Lincolnshire, Cambridgeshire and Norfolk during the afternoon.

Discussion:Several areas of interest exist during this forecast period, with varying levels of severity.

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

From the start of the forecast period through to daylight, indications are that elevated thunderstorms will develop over the English Channel/move northwestwards from northern France as the moisture plume destabilises with the approach of a shortwave. These mid-level showers/storms will tend to merge as they cross SW England and across the Irish Sea to Ireland. >40kts DLS coupled with >25kts would suggest some organisation of such storms into multicells or elevated supercells, sustaining such storms for quite a long period of time.

>800J/kg MLCAPE also highlights reasonably frequent lightning with these storms at first, although the exact interaction of such storms with long sea tracks leaves us with a cautious SLGT level for sferic coverage for the time being - this may be upgraded to MDT if necessary. Other than very heavy rain, with PWAT values near 36mm locally, generally due to the elevated nature of storms severe weather is unlikely.

This general area of thundery rain will then move north across the Irish Sea, grazing both Ireland/Northern Ireland and western parts of Britain during the day on Thursday, with ever-decreasing values of MLCAPE. Nevertheless, it is quite probable that lightning activity will continue throughout the day as cells remain sustained with decent shear available.

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

Across the rest of England and Wales, a few mid-level showers will migrate northwards later in the night and first thing Thursday morning, although significant WAA in the mid-levels will provide strong capping, thus most showers will be light with a low probability of thunder/lightning. Capping is strongest towards SE England, and weaker towards the NorthWest.

Behind these showers, conditions should improve with some patchy insolation expected as cloud cover begins to break. Dewpoints of ~17C and surface temps of 22-27C are simulated by most models, and will be substantial enough to generate some surface-based convection.

Initially this will take place over eastern England, east of the Pennines with the plume axis and approach of a shortwave, combined with topographical forcing. Initially numerous thunderstorms are forecast to develop from ~12z onwards, with forecast soundings suggested ample shear (typically 40kts DLS, 30kts LLS) and some helicity (>300m^2/s^2). This would generate an environment, coupled with 1000-1300J/kg CAPE, capable of producing multicells or supercells, with the potential for a tornado.

However, due to the weakly-capped environment it is likely that mass forced ascent will quickly merge such storms into a linear formation along the trough, eventually forming an MCS producing frequent lightning and possibly some large hail (locally >3cm). PWAT of up to 39mm also increases the risk of flooding from torrential downpours.

This line will continue to move quickly northeastwards, wrapping back into eastern Scotland during the evening so increasing sferic activity is expected here during the evening.

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

It is not clear where the southeastern edge of such line will be, with some inter-model disagreements. Current thinking is south Lincolnshire southwards will be in a more-capped environment, and here such storms that develop will be more scattered and less likely to merge into any linear structures. Indeed, by adjusting forecast soundings to realistic T and Td values, 1800-2000J/kg CAPE is potentially available, with a slight cap being broken by combination of rising LSTs and the approach of a shortwave.

Thus, in a zone from northwest Essex to southern Lincolnshire, a few scattered surface-based thunderstorms are expected to form in the early-mid afternoon hours, rapidly moving northeastwards. Given such CAPE values, combined with >40kts DLS, >25kts LLS and >250m^2s^-2 helicity, then a few discrete supercells are possible, especially in southern Lincolnshire across The Fens to West Norfolk.

Veering wind profiles with height, and a substantial increase in speed shear with height suggest the potential for one or two severe thunderstorms, with rotating mesocyclones. With forecast dry air aloft, such storms will be capable of producing hail 3-4cm in diameter (up to golf ball), frequent lightning, with local flooding possible given PWAT values approaching ~40mm, although fast storm-motion should ease this risk somewhat. If the cap can be broken to generate such storms, then this is borderline our threshold for EXTRMLY SVR. Low LCLs as a result of relatively low dewpoint depressions (generally ~7C) with such shear values poses a significant tornado risk. Convective gusts may be as high as 50-60mph, and outflow boundaries may pose a focus for breaking the cap downstream and generating new cells.

Storms will rapidly move northeastwards, generally clearing by 18z.

In SE England, capping appears to be too strong, and the surface flow not backed enough, to generate any noteworthy convection, although some uncertainty remains.

[Purplepixii]

For those who may not know they're on here yet:

Free Radar (subscribers will be taken to the extra radar from this link too)

http://www.netweathe...ion=radar;sess=

And free ATD lightning detection

http://www.netweathe...lightning;sess=

True Paul however it should be added that the NetWeather subscription service is fabulous and invaluable in times like this and costs peanuts, really!

[Deluxe]

UKASF storm forecast

Wow that is a beauty! Must remember not to get carried away now! Easier said than done

Edited June 27, 2012 by Polar Thole

[Phelpsy]

Very dark to th south now can see it edging in!

[Aaron]

As ever, green = slight, orange = moderate, red = severe

Edited June 27, 2012 by Aaron

[Kain]

As ever, green = slight, orange = moderate, red = severe

Too far east :o

Well I hope it is anyway

[Aaron]

Forecasters have conflicting opinions, so it can go either way.

[Love Snow]

Just talking to lad on twiiter from Belfast said the powers gone off and the streets flooded..

Edited June 27, 2012 by Love Snow

[I Cumbria Marra I]

UKASF forecast is out and its a beauty!!!! If someone could post the map that would be great as i cannot see it for some reason with my internet connection

Areas Affected:

SVR: SE Scotland/Borders, NE Eng, Yorkshire, E Mids, East Anglia

MDT: NE Eng, Yorkshire, E Mids, N Cambs/W Norfolk

SLGT: Majority of United Kingdom, excluding NW Highlands and N Isles

Always the *BLEEP* EAST!! Edited June 27, 2012 by I Cumbria Marra I

[Supacell]

As ever, green = slight, orange = moderate, red = severe

Thanks Aaron. Needless to say, if they do end up putting an Extremely Severe up then it will be the first time i have ever seen it on the ukasf forecast. I am wondering now if estofex could give us a level 2 for tomorrow?

[TheToastPeople]

Yet another forecast to the east. Thank "£$% i was getting worried!!

[A.J]

Alex Deakin used this term in his forecast not too long ago. Couldn't believe it!

He'll be hearing from my lawyers

lol

[camiscool]

i personally think something could spark anywhere.

[Darren Bown]

Any ideas when estofex will update their forecast? im sure they used to do it the night before.

Normally around this time!

[toggerob]

As ever, green = slight, orange = moderate, red = severe

From a personal forecasting point I would have kept the Severe limit where it is, but shifted the moderate risk westwards to cover the Pennines under CAPE values/forcing alone. Seems the slight risk is to cover the law of averages and forecasting errors.

[liambailey09]

If only I could drive and chase tomorrow, it'd be so easy to get on the A1 and drive north towards Lincolnshire or even towards Cambridge.

[aggy]

cam i agree look at central scotland the other day and that was not forcast

[A.J]

Dan's a good forecaster, I just hope he's wrong, as it will be a case of 'close, but no cigar' for the west midlands if his forecast verifies....ie an overnight MCS affecting areas less than 100 miles to the west, and possible severe convection initiating less than 50 miles to the east tomorrow, with us left high & dry....yes, it reminds me of a certain song....'MCS to the left of me, Supercells to the right, here I am, stuck in the middle near Crewe'....

[Hodgheg]

Think i will take a trip to the humber bridge tomorrow, loving the ukasf forecast, Shame the west pathway is closed would love to get some pics of the storm moving in from the bridge. I have a picture in my mind how id like it to look, Nice gust front with a dark rain core behind and constant lightning, too much to ask for hmm.

[lfcdude]

Ian Fergusson â€@fergieweather

Full modified UKMO run now in. T-storm threat looks v small for W Country Thurs AM; indeed, many districts missing heavy showers entirely.

Ian Fergusson â€@fergieweather

UKMO highlights threat of severe storms with large hail tomoro afternoon E Midlands & E/NE England. Could turn v lively in those areas....

[Harry]

Alex Deakin used this term in his forecast not too long ago. Couldn't believe it!

Are you serious????

[NUT]

Too far east for me too (stuck right in the middle of scotland) wouldn't even see any decent clouds to the east with the above forecast. You never know though

[AWD]

Ian Fergusson â€@fergieweather

Full modified UKMO run now in. T-storm threat looks v small for W Country Thurs AM; indeed, many districts missing heavy showers entirely.

Ian Fergusson â€@fergieweather

UKMO highlights threat of severe storms with large hail tomoro afternoon E Midlands & E/NE England. Could turn v lively in those areas....

Yeah, thats what I was expecting! You could see it dwindling away throughout the day for this area unfortunetly. Oh well, best of luck to the guys up North!

[Supacell]

Dan's a good forecaster, I just hope he's wrong, as it will be a case of 'close, but no cigar' for the west midlands if his forecast verifies....ie an overnight MCS affecting areas less than 100 miles to the west, and possible severe convection initiating less than 50 miles to the east tomorrow, with us left high & dry....yes, it reminds me of a certain song....'MCS to the left of me, Supercells to the right, here I am, stuck in the middle near Crewe'....

..... why don't you take a trip west AJ? (I meant east - think i need to go to bed lol)

Edited June 27, 2012 by Supacell

[Kain]

Looks like the all nighter for me might be off then

Damn it!