[staffsstormspotter]

SKYWARN UK SEVERE THUNDERSTORM WATCH #031
ISSUED: 1415UTC WEDNESDAY 20TH JUNE 2012

SKYWARN UK HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR THE FOLLOWING REGIONS:
WALES
SOUTHWEST ENGLAND
SOUTHERN ENGLAND
SOUTHEAST ENGLAND
MIDLANDS
EAST ANGLIA
NORTHERN ENGLAND

IN EFFECT FROM 1700UTC WEDNESDAY 20TH UNTIL 2100UTC THURSDAY 21ST JUNE 2012

SHALLOW LOW DEVELOPMENT TO THE WEST AND SOUTHWEST ENCOURAGING A WARM AND SATURATED PLUME FROM FRANCE, WITH EMBEDDED AND POST-FRONTAL INSTABILITY.

THERE IS A POSSIBILITY OF SEVERE WEATHER AFFECTING THE INDICATED REGIONS IN THE TIME PERIOD SPECIFIED. THREATS WITHIN THIS WATCH INCLUDE, BUT ARE NOT LIMITED TO:
HEAVY RAIN...FLASH FLOODING...HAIL...LIGHTNING...FUNNELS/WEAK TORNADOES

DISCUSSION:
THERE IS GOOD MODEL AGREEMENT ON THE POTENTIAL FOR A SEVERE WEATHER RISK OVERNIGHT WEDNESDAY AND THROUGH THURSDAY. A CYCLONIC JET LIMB ROUNDING THE BASE OF THE PERSISTENT UPPER TROUGH WITH STRONG VORTICITY FOLLOWING ON WILL DEVELOP AN INITIALLY COMPLEX SURFACE LOW AREA OVER THE WEST AND SOUTHWEST OF THE UK. A PLUME OF HIGH THETA-E VALUES FROM NEARBY FRANCE WILL RESULT IN A DEVELOPING FRONT WITH EMBEDDED ELEVATED INSTABILITY PUSHING INTO THE SOUTH FROM THIS EVENING. PRECIPITABLE WATER VALUES OF AROUND 3CM ALONG WITH WELL-SHEARED CONVECTIVE ELEMENTS MAY RESULT IN HIGH RAIN RATES ACROSS THE SOUTHWEST OF ENGLAND AND WALES, AND ALSO THE SOUTHEAST OF ENGLAND INTO LATE THURSDAY MORNING THOUGH THE ENTIRE FRONTAL ZONE WILL HAVE THE POTENTIAL TO PRODUCE STRONG CONVECTION. ACCUMULATIONS OF OVER 20MM IN SHORT PERIODS MAY OCCUR, PROVIDING A RISK OF FLASH FLOODING AND EXCESS SURFACE WATER.

INTO THURSDAY AFTERNOON, THE FOLLOWING UPPER COLD POOL AND VORTICITY WILL ALSO GENERATE AN AREA OF STRONG CONVECTIVE DEVELOPMENT IN THE POST-FRONTAL ENVIRONMENT ACROSS THE MIDLANDS, WALES AND THE SOUTHWEST OF ENGLAND. THE COMPLEX SURFACE LOW PATTERN WILL CREATE ZONES OF SUPPORTIVE SHEAR AS WELL AS WITHIN THE STALLING FRONTAL ENVIRONMENT OVER NORTHERN ENGLAND. AS THE LOW CONSOLIDATES AND BEGINS TO DEEPEN, A SHARP DRY AIR INCURSION ALOFT WILL PUSH INTO THE SOUTHWEST POSSIBLY ENHANCING THE CONVECTIVE RISK AND GENERATING MODERATE HAIL SIZES AND FREQUENT LIGHTNING. ANY STRONG UPDRAFTS ATOP EASTERLY SURFACE FLOWS, OR IN LIEU OF TOPOGRAPHY, MAY HAVE THE POTENTIAL TO PRODUCE FUNNEL CLOUDS AND/OR WEAK TORNADOES. THIS WATCH MAY BE UPDATED OR UPGRADED. PLEASE MONITOR WEATHER AND TRAVEL INFO AS NECESSARY.

SPOTTER ACTIVATION IS REQUESTED AND SPOTTERS ARE REQUESTED TO REPORT ALL FACTORS EXCEEDING ACTIVATION CRITERIA.

Chris

[PJB]

I think any threats for Severe Thunderstorms are very small.

[Tony Gilbert]

Posted Weds 9pm

The post below is for Thurs 21st specifically as I can't really see any convective risk during today Weds!

Slight Risk of Isolated Thunderstorms Central, NE and SE UK regions 15Z-19Z Thurs 21st June 2012
Prime Risk Hail & CG's

Confidence level; Medium

Low pressure deepens and remains quasi stable, centred over Ireland. Short wave upper trough moves eastward through period increasing upper lapse primarily across southern regions only, later in the day. Moderate divergent upper jet stream generates further lift within the front left exit region effecting the prime convective zone (white box).
Initial overnight occluded front looks primarily non convective and likely to bring heavy rain to many parts early Thurs morning. This occlusion moves northward allowing for surface temps to rapidly recover. The post frontal airmass looks to become unstable as temps peak by early afternoon where we should see a surface trough develop triggering convective showers and isolated thunderstorms through the afternoon. Deep layer shear looks to overlay instability best within the zone (yellow box). Where we could see some stronger storm cells develop with a risk of moderate hail.

All in all whilst convection may be fairly well distributed, not all activity will become thundery due to lack of positioning in relation to upper PVA and the mid level dry incursion. Winds look fairly straight lined right across the board with relatively weak low level shear. So it would seem unlikely for tornadoes to occur. Though there may be some scope later in the day for the SE where DLS increases and the CAPE and strong potential instability moves further south. This will need to be addressed closer to the time. NMM and GFS fairly confident for Tstorms though WRF pushes any risk much further east across E. Anglia and NE coastal regions.

Concerns at this point will be regarding the potential for heavy cloud cover in the wake of the earlier occlusion restricting surface heating and ultimately the predicted CAPE values. We need to remember that there will be very little cooling aloft generated over the predicted CAPE. Hence the low level lapse rates will need to essentially peak at around 19 deg C. Not sure how much influence the upper trough will assert over the first round of convection central east regions?

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This post has been edited by Tony Gilbert: 20 June 2012 - 20:38

[Dave W]

There does seem to be meduim level convection firing off though atm just off Devon & Dorset coast ?

[Tony Gilbert]

Be interesting to see if they can hold it together over the mainland?

This post has been edited by Tony Gilbert: 20 June 2012 - 21:05

[Dave W]

As PJB mentioned the storms are developing from mid level destablisation so whether they are land/sea based in this case shouldnt make a great deal of difference

[Matt D]

Yep storms developing out of high theta-w plume at approx 850mb as lead short wave trough overspreads the plume from the SW. Highest theta-w values (>17C) remain over N France tonight but axis of the plume does extend northwards into S England. Certainly would expect a few sferics along S Coastal counties overnight (Dorset eastwards) a perhaps up into London area towards 0500-0700BST.

[Tony Gilbert]

Very little sferic activity actually made it to the mainland overnight but some very heavy rainfall returns noted.

PS. I was aware that any instability would be associated with the theta plume. I was in fact refering to how far the theta plume was likely to venture northward across the UK

This post has been edited by Tony Gilbert: 21 June 2012 - 07:29

[Tony Gilbert]

UPDATE 9am Thurs

As per earlier forecast with no changes.

CONFIDENCE LEVEL; Medium

Very little change though whilst the Tstorm box remains broad, in reality we may only see sferics develop Camb, Norfolk, Lincs today. Later in the day a lobe of CAPE is predicted to build around central south but with all the earlier precip the whole region looks to be effected by subsidence. Though this will need some monitoring today for a possible update!

This post has been edited by Tony Gilbert: 21 June 2012 - 08:06

[Matt D]

Yes, highest theta-w values remained on near continent overnight (associated with greatest instability). Despite the strong synoptic scale forcing, instability remained fairly weak over the UK, profiles fairly saturated as well which limited the scope for embedded TS.

[Sonia]

Well, this isn't technical I'm afraid, but I can tell you that I was woken at 3am by torrential rain. It was so heavy that I was surprised at the lack of thunder or lightning (that I'm aware of).

[Matt D]

Tony Gilbert, on 21 June 2012 - 07:57, said:

UPDATE 9am Thurs

As per earlier forecast with no changes.

CONFIDENCE LEVEL; Medium

Very little change though whilst the Tstorm box remains broad, in reality we may only see sferics develop Camb, Norfolk, Lincs today. Later in the day a lobe of CAPE is predicted to build around central south but with all the earlier precip the whole region looks to be effected by subsidence. Though this will need some monitoring today for a possible update!

Anything but subsidence moving into southern England this afternoon though, strong deep lifting ahead of a sharp and negatively tilted shortwave trough. Forcing very strong so widespread cloud development - but prob too much saturation of mid/upper levels over southern England later this afternoon for anything more than a few well scattered/embedded storms here.

As you mention, greatest TS risk will be N and E of London from scattered cells, given some surface heating

[Tony Gilbert]

Matt D, on 21 June 2012 - 10:46, said:

Anything but subsidence moving into southern England this afternoon though, strong deep lifting ahead of a sharp and negatively tilted shortwave trough. Forcing very strong so widespread cloud development - but prob too much saturation of mid/upper levels over southern England later this afternoon for anything more than a few well scattered/embedded storms here.

As you mention, greatest TS risk will be N and E of London from scattered cells, given some surface heating

Matt what model are you using? This is not the case regarding GFS at all. Fairly moderate mid level subsidence looks to cover the whole of the 'central south' near the secondary lobe of CAPE by 18Z. Realistically these CAPE values are probably erroneously bolstered higher by the almost saturated boundary layer. Re lift and subsidence try looking to vertical Velocity values when determining such information. Look for positive values at around 700 mb as is the case here. This will help you to see rising momentum and falling momentum at any given level of any lifted parcel. With the moisture lifted into the mid level and cooling temps dragged down to surface earlier today the atmosphere will do a sort of vertical role over. In short you will see mild air lifted above cooler air creating an almost stable climate. Not sure if the PVA and increasing upper lapse rates building from the west will have any quantifiable effect on this situation with the lowered temps in place at surface. Though wouldn't rule out the odd 'shallow' convective shower around Dorset if the sun can make it through later on.

Still some reasonable potential today initiating maybe around Cambridge today as you say

This post has been edited by Tony Gilbert: 21 June 2012 - 12:05

[PJB]

GFS 700mb Vertical Velocity Fields are highly negative this afternoon, This implies strong dynamic uplift is present ahead of the approaching shortwave. there is no NVA present and strong dyamic ascent from 700mb up to 300mb is present.

http://www.wetterzen...cs/Rtavn066.png

I am not sure which charts you derive your assumptions from , but there is certainly no NVA forecast. However profiles are largely saturated and the dynamic forcing overwhelming - therefore its not a terribly convective situation.

However despite its not overly convective - that does not imply large scale dynamic ascent is not happening.

paul

[Tony Gilbert]

Have I mentioned NVA anywhere? Not sure my point is an assumption? Maybe a lesser tone would be more helpful within your posts!, Paul.

I think I've been quite clear that I have been analysing the region just 'ahead' of the short wave upper trough over central south UK during the late afternoon period and not the region of precip expected across central regions overnight?

This post has been edited by Tony Gilbert: 21 June 2012 - 12:18

[PJB]

Subsidence is generally the same speak for NVA in my book. but if you want to speak facts.

You said "Fairly moderate mid level subsidence looks to cover the whole of the 'central south' near the secondary lobe of CAPE by 18Z" This is not true. There is no subsidence across the Central South by 18Z. There is large scale ascent through the afternoon as both Matt and I have stated above. Your post to Matt is actually condescending - I think he knows very well what Vertical Velocity is and its affect on the wider environment in terms of Lift and Momentum - so perhaps you should look at the tone you are writing in also.

Paul

[Tony Gilbert]

Its the same old story with you guys. My way or the highway! Maybe a separate and unbiased moderator should step in now?

Matts a big boy and you don't need to keep holding his hand when we have a difference of opinion. Why not just deal with you own issues, Paul.

NVA and NVV are completely different indices for which you are fully aware of. PVA is moving in from the west though ahead of this we have strong positive VV's which suggest subsidence plain and simple! It looks like you may be mistaking the returning occluded front as the region for which I was referring to . The frontal sector of this region of lift will be somewhere over Devon, NOTcentral south at the time for which I am stating. This is all pretty straight forward so where's the problem?

This post has been edited by Tony Gilbert: 21 June 2012 - 12:30

[Dave W]

Everyone calm down please! Theres a difference of opinion going on.. deep breath then try and see where differences are actually stemming from.

Hopefully this isnt condescending to anyone..

[PJB]

Tony - I think you are simply confusing widescale dynamic forcing and individual convective forcing. I dont really understand your point above but of course large scale ascent surpresses individual convection, its swamped naturally .. I am not sure what my own issues are i supposed to be dealing with. I dont think there is any need to get personal - but as usual you cannot accept any questionining or critiscm of your forecasts. you are so sensitive to other peoples opinions and points of view.

Fine if we dont agree - we dont agree. whats the big deal with us posting comments on your forecast. Last time i checked it was still a free country - Just !

Paul

[Tony Gilbert]

OK lets agree to disagree here before we all trash the thread. Though please everyone be under no illusion that helpful criticism should therefore be accepted both ways for all forecasters. I do not see this as being the case within this forum and it always seems to be me that bears the brunt. I rarely criticise anything anyone here has to say because if I do its like i've started gang warfare ? Bit too one sided maybe

Rant over!

This post has been edited by Tony Gilbert: 21 June 2012 - 12:49