24th Aug 2011 - Storm/convective Forecast Discussion And Reports

[Stuart]

Last Updated: 20:18 Tuesday, 23rd August 2011

Valid: 00:00 Wednesday, 24th August 2011 - 23:59 Wednesday, 24th August 2011

Areas Affected:SLIGHT RISK: Northern England and Wales

MODERATE RISK: Scotland and Ireland

Synopsis:A vertically stacked low pressure is forecast to the west of the UK throughout the course of Wednesday, bringing increasingly unsettled conditions to many areas, but particularly across northern and western regions of the UK. A frontal zone, in association with this surface low pressure, is evident across western areas of the UK during the day, but it is forecast to become more of a trough type feature. A returning polar maritime air mass is forecast across most areas of the UK, but particularly in the west.

Discussion:

The area of low pressure situated to the west of the UK is forecast to be a primary driving force for convective activity during the course of Wednesday across northern and western areas of the UK in particular. A band of showers or more organised precipitation is forecast across many western areas of the UK to start the day and showers are then forecast to become increasingly more frequent and heavier as the day progresses.

Areas within the slight risk zone are predicted to experience between 200j/kg and 400j/kg of SBCAPE during the course of Wednesday afternoon. The above mentioned frontal zone, in association with the surface low pressure, despite losing some of its forcing, it still forecast to be a primary focal point for convective activity and perhaps more organised bands of heavy showers. Instability across Northern England and Wales for example is likely to be modest and for the most part moderate or heavy showers are predicted, but the SLIGHT risk is covering the threat that a few isolated or scattered non-severe thunderstorms are likely to develop in a few places as the day progresses given the general all-around forecast across these areas. It should also be noted that NMM and WRF models in particular are signalling some locally large rainfall totals across parts of Wales and NW England during tomorrow afternoon, with perhaps a zone of more persistent heavy rain affecting Wales and perhaps SW England later in the day.

The MODERATE risk has been issued primarily in relation to the close proximity to the low pressure just to the west of the UK and a favourable signal from the WRF, NMM, NAE and UKMO MESO for significant convective activity. Clearly such close proximity to the low pressure is leading to steep lapse rates given cold mid and upper level temperatures and hence SBCAPE values of 500j/kg to 700j/kg and LI values of 0C to -2. Parts of Scotland and Ireland in particular are predicted to have a completely unstable atmosphere with surface temperatures of approximately 16C and dew points near 8C or 9C. Cloud tops may well reach up to 20,000ft in some instances and also there is forecast to be a zone of quite significant DLS (0 to 6KM) of between 30KT and 40KT across Ireland in particular. A widespread threat of moderate and heavy showers is expected through the day with a significant risk of some thunderstorm activity. For the most part these storms are likely to be single celled, but the DLS evident may well aid organisation somewhat with some banding and grouping of heavy showers and thunderstorms possible.

The primary threat of severe weather is likely to be from heavy, torrential downpours. Some hail is likely, but this is unlikely to be severe and given other variables, tornadoes or funnel clouds are unlikely, despite a minimal risk of funnel clouds on a local scale where any convergence zones perhaps develop. However, it should be noted that particularly in association with any cells nearing maturity then some temporary inclement conditions are highly possible within the MODERATE risk zone in particular.

http://www.ukasf.co.uk/storm-forecasts/82

[Jane Louise]

Good luck all.

[Phil UK]

Helloooooooo NSC boss.

Conditions are looking like al la 5th and moreover 7th August 2011. After the 'Frenchie' and 'Epic Fail' MCS of the previous few hours, just maybe a bit of luck may be on our side this time.

Good luck all.

Phil.

[The watcher]

Looking slightly favourable for me tommorow, but being near the East coast of NI the showers may be just too far inland to catch for me tommorow.

Since I'm off tommorow I can maybe take a trip up the Belfast hills if skies are semi clear, that should give me a good panoramic view.

[Jane Louise]

Helloooooooo NSC boss.

Conditions are looking like al la 5th and moreover 7th August 2011. After the 'Frenchie' and 'Epic Fail' MCS of the previous few hours, just maybe a bit of luck may be on our side this time.

Good luck all.

Phil.

Lol Phil, I'm hoping to be out the club by Saturday The storm risk has looked pretty good for the past few days. If I don't get a storm I would at least appreciate some rain . The MO have mentioned a risk of thunder tomorrow for here, and heavy rain for Friday. I guess we'll have to wait and see.

[Raidan]

Evening all,The low mentioned for wednesday at what time roughly are we looking at,early hours or during daylight hours into the evening?sorry if this has been said already but haven't got time to go through all the posts.

[Jane Louise]

Evening all,The low mentioned for wednesday at what time roughly are we looking at,early hours or during daylight hours into the evening?sorry if this has been said already but haven't got time to go through all the posts.

Hiya Raidan,I think we are probably looking at daylight hours but not 100% sure. Hopefully someone else will be along shortly to help you.

[The watcher]

It looks like a Diurnal event to me, so yes, daytime. Cape and LI reduce early evening, that's why I'm gathering it's Diurnal.

[ZONE 51]

I expect the convective stuff to stay quite intense until early evening, i dont see it weakening to early. anyway some interest could reach the east side later this could change, but i expect the trough to hold into the east for night hrs. this is different from monday night, another type of setup, i would expect some action to reach east side but cant say if the storms would remain but maybe thunder. (:

[MP-R]

Well just had a 2 minute torrential downpour so obviously the air is destabilising already. Fingers crossed for some action over the coming days.

[Coast]

What have we got for today then?

Stuart has covered the UKASF in the first post, ESTOFEX have Ireland in the firing line today:

Storm Forecast

Valid: Wed 24 Aug 2011 06:00 to Thu 25 Aug 2011 06:00 UTC

Issued: Wed 24 Aug 2011 00:16

Forecaster: TOMAS PUCIK

A level 1 was issued for parts of Eastern France, Germany, Switzerland, Northern Austria, Western Czech Republic and Western Poland mainly for large hail, severe wind gusts and to the lesser extent for the excessive precipitation.

A level 1 was issued for Southwestern Ukraine and parts of Northeastern Romania mainly for large hail and severe wind gusts.

SYNOPSIS

A large cyclonic vortex at all levels of troposphere over Atlantic seems to be a dominant player in the synoptic scenario with southwesterly flow between the forward flank of the low and a ridge stretching from the Central Mediterranean into Scandinavia. The ridge covers a plume of high Theta-E airmass with temperatures exceeding 20°C at 850 hPa. A frontal boundary encloses this plume of very warm airmass, with the most active zone - a cold front over France and Germany - lying under the moderate southwesterly flow with several vorticity maxima travelling over it towards north/northeast.

Very warm airmass is characterised by an environment of moderate to high latent instability, as suggested by Tuesday 12 UTC soundings from the region thanks to the steep mid level lapse rates and high dew points at lower levels of troposphere. Initiation will likely by linked to the surface frontal boundaries, passage of PV max (especially concerning France and Germany), orographically driven circulations and also outflow boundaries laid out by previous storms. It is very difficult to anticipate storm coverage and exact spots of initiation at the moment, especially for regions further away from the frontal boundaries.

SkyWarn and TORRO have nothing for the day so far.

Looks like Ireland starting late morning then moving East this afternoon, with North Western areas and Wales might be in the firing line. Looks quite potent over the West coast of Ireland this morning.

[Jane Louise]

This is from UKWW, I happen to be in the yellow box today lol. It'll be just my luck if anything happens i'll miss it especially if it's late afternoon whilst I'm at Drs grrr lol. Fingers crossed if anything sparks off it will happen this evening.

1am Weds

Moderate Risk of Thundery Showers Ireland 12Z-21Z

Slight Risk of Isolated Thunderstorms UK & Scotland 12Z-21Z

What a difference a day makes?; ...That is to say the models leading up to the 18Z update were showing a persistent broad zone of NVA (Negative Vorticity Advection) in line with the strongest instability over the UK. This suppressive condition seems to now look less obtrusive and hence warrants a convective forecast for parts of the UK and not just Ireland.

Slow moving Low pressure west of Ireland maintains a fairly unstable Swesterly flow across both Ireland & the UK with a rather sharp upper trough edging eastward increasing lapse rates through the convective period. Surface trough expected to develop over Ireland with a scattering of thundery showers likely over a very broad area. Prime threat CG's and small hail. Conversely much of the UK sees a rather weak cold front traverse eastward with a slight risk of isolated thunderstorms developing within a highly sheared environment. The lack of predicted CAPE is by far superseded by vertical shear as a strong divergent upper jet extends lift downwards toward lower mid levels enhancing any parcel lifted into its LFC. In addition to this both GFS and NMM now show a sharp decrease in humidity values at 600mb with the net effect of rapid cooling of cloud tops rising to this level. In short should see a number of fully developed cb's just behind the main cold front. Upper support would ironically seem to be optimised fully by the relatively weak winds at 850 mb. The increase in speed shear above this level looks to become absolute for a time; Net result if the forecast remains consistent will show one or two very defined isolated storm cells develop within the yellow box!!

Based on the lack of convergence and low level shear overall opinion would suggest isolated CG's prime threat with emphasis to Midlands, UK.

PS. The overall UK risk box looks be rather broad with the best or possibly only potential within the yellow box; So please don't expect a storm unless you are living within this box. The broad parameter is merely set as a fail safe to cover the smallest risk of a thunderstorm in a practical sense!

Edited by Tony Gilbert 24/8/2011 01:31

http://forum.ukweatherworld.co.uk/forum/forums/thread-view.asp?tid=44573&posts=2&start=1

[SteveB]

A day of sunshine & showers today, I can't see any storms happening.

[DeepSnow]

This is from UKWW, I happen to be in the yellow box today lol. It'll be just my luck if anything happens i'll miss it especially if it's late afternoon whilst I'm at Drs grrr lol. Fingers crossed if anything sparks off it will happen this evening.

1am Weds

Moderate Risk of Thundery Showers Ireland 12Z-21Z

Slight Risk of Isolated Thunderstorms UK & Scotland 12Z-21Z

What a difference a day makes?; ...That is to say the models leading up to the 18Z update were showing a persistent broad zone of NVA (Negative Vorticity Advection) in line with the strongest instability over the UK. This suppressive condition seems to now look less obtrusive and hence warrants a convective forecast for parts of the UK and not just Ireland.

Slow moving Low pressure west of Ireland maintains a fairly unstable Swesterly flow across both Ireland & the UK with a rather sharp upper trough edging eastward increasing lapse rates through the convective period. Surface trough expected to develop over Ireland with a scattering of thundery showers likely over a very broad area. Prime threat CG's and small hail. Conversely much of the UK sees a rather weak cold front traverse eastward with a slight risk of isolated thunderstorms developing within a highly sheared environment. The lack of predicted CAPE is by far superseded by vertical shear as a strong divergent upper jet extends lift downwards toward lower mid levels enhancing any parcel lifted into its LFC. In addition to this both GFS and NMM now show a sharp decrease in humidity values at 600mb with the net effect of rapid cooling of cloud tops rising to this level. In short should see a number of fully developed cb's just behind the main cold front. Upper support would ironically seem to be optimised fully by the relatively weak winds at 850 mb. The increase in speed shear above this level looks to become absolute for a time; Net result if the forecast remains consistent will show one or two very defined isolated storm cells develop within the yellow box!!

Based on the lack of convergence and low level shear overall opinion would suggest isolated CG's prime threat with emphasis to Midlands, UK.

PS. The overall UK risk box looks be rather broad with the best or possibly only potential within the yellow box; So please don't expect a storm unless you are living within this box. The broad parameter is merely set as a fail safe to cover the smallest risk of a thunderstorm in a practical sense!

Edited by Tony Gilbert 24/8/2011 01:31

http://forum.ukweath...posts=2&start=1

Im happy with that as for once I too am in the yellow box, though on its sothern edge so still doubt much will happen here, but I live in hope

[Phil UK]

Good morning peoples.

Going by Coast's maps, and I'd agree that west looks best but inland UK may have to wait for a few hours yet. Predominatly tomorrow or Friday I would have thought.

Now I know it's early in the morning, and we've not had the anticipated 'Frenchie' or his Sister MCS2 (Missed Coast's Slightly!) But if one comes in from the west and keeps strength well inland, then maybe... "Bristol Bruiser" or "Cheltenham Charged up", "Liverpool Lightning", "Manchester Monsoon"

Any better suggestions, feel free to add.

Just a bit of fun whilst we await potential, whether it comes to pass is another issue altogether.

But interesting morning sunrise, make of that image below what you will as Solihull is/was currently stuck between Frenchie's force and the curly wurly to the west of Ireland.

Phil.

Edited August 24, 2011 by Phil UK

[Jane Louise]

An update now from UKww, and Tony has even mentioned Glos

Update 8.40am Weds

As per earlier forecast with yellow box extended further NE.

For UK some limitations remain in place; namely limited convergence and weak low level shear. Conversely the stronger increase of shear within the mid levels will be more than capable of developing a mid level mesocyclone within any isolated stronger cell. Convective wind gusts are calculated at around 32kts (non severe). Prime risk CG's

Some scope for a tail end charlie for Gloucs late in the day as potential instability is utilised.

http://www.ukweatherworld.co.uk/forum/forums/thread-view.asp?tid=44573#M648017

Hmmm , could this late in the day be after 4pm lol I blinking hope so

[Coast]

Hmmm , could this late in the day be after 4pm lol I blinking hope so

I'm going to reservere judgement on any timing Jane Louise, but the CAPE doesn't look wonderful later, except for a really odd feature over Llanelli at 16.00 hrs!!!

[Jane Louise]

I'm going to reservere judgement on any timing Jane Louise, but the CAPE doesn't look wonderful later, except for a really odd feature over Llanelli at 16.00 hrs!!!

Ok thanks Coast. Hopefully more Opportunities on Friday.

[Staffordshire]

Been up all night sick as a dog (don't re-use KFC chicken in a caserole). But it's nice to see me in the yellow zone.

[Nick F]

Looking good for the north and west today ... will just have to make do with some drizzle here and some more rain and drizzle tomorrow!

Convective / Storm forecast - Issued 24/08/2011 10:00

Forecast Summary Map

Click for full size

Valid: 24/08/2011 10:00 - 25/08/2011 06:00

Headline: ... THERE IS A RISK OF THUNDERSTORMS ARE FORECAST TODAY...

Synopsis

Deep upper trough extends south from Iceland just to the west of the UK, driving a strong cyclonic SW flow across the UK, unstable across northern and western areas. At the surface, an area of low pressure to the W of Ireland drives an occluding cold front in across western areas this afternoon, lying roughly N across SW England, W Wales and W Scotland at noon, then by 00z – cold front further E from Humber to Devon, with wave developing from SW.

… EIRE, SW ENGLAND, WALES, W MIDLANDS, N ENGLAND and SCOTLAND …

Upper trough over E Atlantic edging eastwards will steepen lapse rates across northern and western areas today, which along with forced ascent along Eward advancing cold front, will produce a band of heavy showers and increasingly thunderstorms. A strong S/SW'erly jet running parallel with cold front moving in from the W and 40-60 knts of deep-layer shear indicated suggests organisation of convection/storms is likely – with a threat of strong wind gusts (up to 40 knts) lowering through increasingly dry mid-levels. Also there is a risk of hail, torrential downpours with risk of localised flooding, CG lightning and perhaps an isolated risk of a brief weak tornado – given the indication of increased low-level shear later this afternoon.

Storms today will be diurnal in nature, so will tend to fade after dark, though there is a low risk that elevated convection within developing wave rainfall moving up from SW England towards Midlands tonight may produce isolated lightning.

Issued by: Nick F - Forecaster for Netweather

[Raidan]

Sunny here,but hoping not for long and the heavy showers/storms get going soon,hoping for a Bristol Bruiser lol,Good luck all.

[Chris.R]

Looking good for me here in Liverpool I think, I'm about as far west as you can go in NW england. Already had 2 quite heavy showers this morning. Great forecast from Nick there!

[MP-R]

Beginning to cloud over here now. Quite a line of potent rain out west but no sferics attached. Looks like the place to be today though.

[Raidan]

Sun still shining here but black sky to the west,looks like we're in for a hammering!

[SteveB]

Looking at the radar, it looks like the channel is breaking up the line of showers.