[Tony Gilbert]

Posted 8.40am Weds

An early heads up regarding the potential for Thurs; WRF, GFS and NMM currently in agreement that we should see some relatively strong instability develop through the afternoon over central eastern and northern UK & Scotland. UKMO fax forecast chart depicts an initial pre frontal trough followed by a cold front. Moderate divergent jet stream aloft and the upper short wave trough to the west looks to generate further lift.

Certainly one to keep an eye!

This post has been edited by Tony Gilbert: 27 June 2012 - 07:57

[PaulKn]

Hi all, It seems like a Spanish Plume will (briefly) affect us through tonight and tomorrow, as warm and moist air continues to move from the south, and is overrun by reasonably steep lapse rates aloft, which have originated from Iberia and N Africa.
Steep lapse rates aloft will move into southern and some central parts of the UK tonight. Elevated thunderstorms typically occur on the periphery of the steepest lapse rates, as these tend to also be associated with a strong capping inversion. Thus, it might be suggested that on the leading edge of the plume, a few showers/storms could move into southern/SE coastal parts of England around the middle of the night before the cap strengthens. At the same time, as the cap weakens across SW England and later tonight, S Wales, activity might move into these regions. Given that these storms will be elevated above the boundary layer, one must look at the ‘cloud layer’ wind shear to decide whether these might be organised. Analysis of the forecast tephigram for Plymouth for the overnight hours suggests ~700 J/Kg of elevated CAPE and cloud layer shear of around 30-35 knots. This is enough for updraught rotation and perhaps elevated supercells and multicells. Hail to 2-3cms would be possible from this activity, and it may conglomerate into an MCS – a few strong wind gusts to 40-50mph are possible although elevated convection doesn’t tend to be a prolific wind generator.

During Thursday morning, the suggestion is that a cluster of showers and thunderstorms, perhaps an MCS, across SW England and S Wales will move NNE through Wales and parts of the Midlands into N England, with a continued threat of occasional large hail/strong winds. These should affect Scotland in the afternoon, with heavy rain becoming more of a threat as they become slow-moving.
Further south, the plume will drift eastwards across central and eastern England. This is where there is a higher potential for severe thunderstorms, but also some considerable uncertainty as to the evolution, as overnight/early morning activity as described above might be more (or indeed, less!) widespread than suggested. Current thinking is that to the south of the area of showers/storms/MCS mentioned above, sunny spells will develop within the plume axis, with scattered thunderstorms developing southwards along the plume from late morning through mid-afternoon. A modified forecast ascent for potential tx values (27C) within the moisture plume suggests around 2000 J/Kg of surface-based CAPE, and around 1700 J/Kg of mixed layer CAPE (using the lowest 50 hPa). The RAOB software suggests that hail close to 3cm diameter might result from this, and wind gusts to ~55 knots. Given that this is purely based on the thermodynamics, one might suggest that because the kinematic profile gives 0-6km shear values of around 30-40 knots (suggesting the risk of organised multicells and perhaps supercells), hail to 4-5cms is possible, with gusts to 65 knots. Also, reasonably strong low-level shear and veer and fairly low LCLs suggests a tornado risk. *If* a supercell can develop within this plume, a strong tornado is possible.

The final question is the southern extent of this activity – numerical guidance suggests the London area for this, so the highest risk of this surface based activity is approx a Berks – Leics – Lincs – Essex rectangle.

These are my current thoughts – bear in mind that what happens overnight tonight (if anything) might have a large impact on developments tomorrow, especially if a large amount of cloud forms as a result of overnight activity – this would limit heating tomorrow and reduce the risk of severe weather substantially.

[staffsstormspotter]

SKYWARN UK SEVERE THUNDERSTORM WATCH #033
ISSUED: 1500UTC WEDNESDAY 27TH JUNE 2012

SKYWARN UK HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR THE FOLLOWING REGIONS:
ALL AREAS UK

IN EFFECT FROM 0000UTC THURSDAY 28TH UNTIL 0300UTC FRIDAY 29TH JUNE 2012

MATURE STALLED LOW PRESSURE TO THE WEST AND WAVE IN THE BISCAY AREA ENCOURAGING A VERY WARM PRE-FRONTAL PLUME OVER THE UK, WITH STRONG INSTABILITY AND FAVOURABLE SHEAR FOR ORGANISED THUNDERSTORMS

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...STRONG GUSTS...FREQUENT LIGHTNING...FUNNELS/WEAK TORNADOES

DISCUSSION:
THERE IS GOOD MODEL AGREEMENT ON THE POSSIBLITY OF A SEVERE THUNDERSTORM INTO AND THROUGH THURSDAY WITHIN THE WATCH AREAS. A MATURE LOW PRESSURE SYSTEM STALLED TO THE WEST OF IRELAND BECOMES DEVELOPED THROUGH THE DAY BY A THE CYCLONIC JET ROUNDING TO THE SOUTH, WITH A WAVE DEVELOPED NEAR PORTUGAL MOVING THROUGH THE BISCAY AREA. TO THE EAST OF THIS, AN ELEVATED MIXED LAYER FROM IBERIA INTERACTS WITH DEEP MOISTURE CONVERGENCE OVER NW FRANCE AND IS ENCOURAGED NORTHWARD OVERNIGHT. THIS MAY GENERATE STRONG MIXED LAYER INSTABILITY OF WELL OVER 1000J/KG CAPE, WITH THE APPROACHING JETSTREAM NOSE PROVIDING SOME DIRECTIONAL SHEAR ORGANISATION FOR HIGH-BASED UPDRAFTS. THIS COULD LEAD TO MODERATE TO LARGE HAIL SIZES AND VERY HEAVY RAIN CORES ACROSS THE SOUTHWEST AND SOUTH OF ENGLAND DURING THE EARLY HOURS, SPREADING ACROSS THE MIDLANDS AND WALES BY LUNCHTIME AHEAD OF THE COLD FRONT.

BETTER MOISTURE CONVERGENCE THROUGH THE PROFILE AROUND IRISH SEA AREAS AND MIDLANDS TO NORTHERN ENGLAND, TOGETHER WITH FAVOURABLE WIND SHEAR, WILL PROVIDE A FOCUS FOR ORGANISED AND STRONG CONVECTION WITH SUSTAINED SUPERCELLS POSSIBLE. THERE ALSO EXISTS A RISK OF LARGE HAIL, FREQUENT LIGHTNING AND EXCESSIVE RAINFALL FOR THESE AREAS THROUGH THE AFTERNOON, ALONG WITH THE POSSIBILITY OF FUNNELS AND TORNADOES. THERE IS A POSSIBILITY OF THIS PATTERN EVOLVING INTO A MCS LATER IN THE AFTERNOON INTO SCOTLAND, THOUGH THERE IS ALSO EQUAL POSSIBILITY OF ISOLATED AND ORGANISED STORMS HERE WITH SIMILAR ATTENDANT THREATS.

FURTHER SOUTH ACROSS THE EAST MIDLANDS, EAST ANGLIA AND SOUTHEAST THERE IS LESS FORCING AND MOISTURE WHICH MAY PRECLUDE THE RISK OF SEVERE THUNDERSTORMS. HOWEVER INSTABILITY ALONG AND AHEAD OF THE FRONT IS DEPENDENT UPON DAYTIME HEATING AND MAY GENERATE ISOLATED STRONG CONVECTION WHICH, GIVEN A FAVOURABLE SHEAR PATTERN, COULD LEAD TO SHORT-LIVED SUPERCELLS CAPABLE OF PRODUCING LARGE HAIL AND STRONG GUSTS, WITH TORNADOES ALSO A POSSIBILITY. THIS WATCH MAY BE UPDATED OR UPGRADED. PLEASE MONITOR WEATHER AND TRAVEL INFO AS NECESSARY.

[Tony Gilbert]

POSTED 10pm Weds

Moderate Risk of Thundery Showers (though sparsely populated)UK & Ireland 03Z-19Z

Slight Risk of Elevated Storms 03Z-12Z Yellow Box

Moderate Risk of Severe Thunderstorms (Possible Supercell) NE UK 12Z-19Z Red Box

CONFIDENCE LEVEL; Medium

Probably not my speciality regarding this type of set up but here's my take. Pretty much agree with the two forecasts above so it is not necessary to elaborate too much with what has already been said. There are nevertheless a few additional point which particularly stand out for me. The past has shown me that the WRF might be more reliable than the GFS regarding such outlooks though I have tried to merge the two regarding considered zones for stronger potential. Earlier 1-8km DLS (Deep Layer Shear) was showing up to 50 kts. The last GFS run might now be suggesting even higher! This includes an increase of low level shear 0-3km within the red box zone. Forecast soundings for this region for mid afternoon take CAPE right up to the tropapause from surface. Both WRF and GFS are giving readings for Normalised Surface Solar Heating from mid day onwards East Midland where surface based storms are likely to initiate. Though as mentioned above this will be heavily dependent on how quickly the upper cloud moves away. The strong mid levels winds should achieve this.

With the most recent increase in shear at all levels we will have a defined risk of tornadoes occurring (possibly a strong tornado event) Though this detail will need to be looked at closer to the time!...All in all it is rare to see the models combine such strong shear with such strong instability. Similar past conditions can be associated with the Birmingham and London tornadoes.

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This post has been edited by Tony Gilbert: 27 June 2012 - 21:19

[John Mason]

This is certainly starting to look most interesting. Shear profiles are really quite impressive. Thanks for the details above, guys. All I can add is that the cloud formations ahead of the rainband moving up through Wales were dramatic tonight - oddly at about 2230 (was fishing down at Borth for the evening) when what looked in the twilight like a big gust-front began to approach. Attempted without much success to photograph it, not having my full kit to hand.

Cheers - John

[PaulKn]

TORRO TORNADO WATCH 2012/004

A TORNADO WATCH has been issued at 0740GMT on Thursday 28th June 2012

Valid from/until: 0740 - 2000GMT on Thursday 28th June 2012, for the following regions of the United Kingdom & Eire:

Parts of (see map)

Wales

Midlands

E Anglia

N ENgland

Southern and central Scotland

THREATS

Tornadoes; hail to 40mm diameter; wind gusts to 65mph; Frequent CG lightning

SYNOPSIS

Plume of high theta-w air has advected across much of Britain overnight with a large upper trough approaching from the west. At the surface, a cold front will move north-eastwards through the day.

Diffluent upper flow ahead of the trough is bringing forcing for ascent across portions of Wales already, and a number of thunderstorms have developed. At this stage they are likely marginally elevated above a coolish boundary layer, but strong cloud-layer shear suggests large hail to 20-30mm is possible with this early activity, with elevated supercells possible, with clusters of storms are likely. Gusty winds also possible.

This activity is expected to continue to move NNE and NE through this morning. Although there is a fair amount of mid-level cloud associated with the widespread large-scale ascent ahead of the trough, enough diurnal heating of the boundary layer is expected to allow storms to become surface based as the cluster of storms moves through the Midlands into N England, and later, Scotland. With 30-40 knots of deep layer shear, well-organised multicells and embedded supercell structures are likely. 20-30mm diameter hail and gusts to 60mph are possible. In addition, isolated tornadoes are possible.

In the wake of this activity, and across the more south-eastern parts of the WATCH area, further thunderstorms are expected to develop and the move north-east, although these will be more isolated with south-eastern extent. Modified ascents for afternoon temperatures suggest around 1200-1800 J/Kg of SBCAPE. WIth 0-6km shear remaining around 40 knots, and perhaps increasing a little, well-organised multicells and isolated supercells seem possible. With 0-1km SREH of ~150 J/Kg and fairly low LCLS, a few tornadoes may develop with this activity. Indeed, should an isolated supercell develop in an area with decent surface heating, a strong tornado cannot be ruled out. Hail to 40mm or so and wind gusts to 65mph possible with supercells too; otherwise, 20-30mm hail and 55mph gusts. If this activity can develop into northern parts of England too, it may tend more towards a squall line, with an associated wind threat.

The activity will end from the south-west through the afternoon and evening as the cold front moves in.

Forecaster: RPK

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[Tony Gilbert]

UPDATE 8.50am Thurs

As per earlier forecast with red box to include a slight risk of tornado development near coast, Though there is some small risk of tornado development anywhere across the Midlands and Northern UK today.

Upper jet stream continues to show the strongest lift generated within the front left exit zone (primarily Lancs/ Cumbs). Though there are reservations regarding how much instability is available in this region. Conversely Humber looks to maintain good potential for instability depending on level of solar heating available this afternoon. Convergence in this zone looks slightly less that recent models and jet stream looks delayed by around 3 hrs compared with earlier runs. Ultimately a small reduction in DLS.
Both WRF and GFS showing a strong SE surface vector along the coast undercutting a SW vector at 850mb. Potentially very pronounced shear possible within this zone (red box).

All in all a reduction in the spatial coverage for the SB storms later today right across the board though more concentration on the Humber region is now evident. A combination of conditions look to take place between 13Z-16Z. In addition to this I would not be surprised to hear a reports of waterspouts along that area of coastline as storm move out off land.

Confidence level ; Medium

This post has been edited by Tony Gilbert: 28 June 2012 - 08:05

[staffsstormspotter]

SKYWARN UK SEVERE THUNDERSTORM WARNING #034
ISSUED: 0900UTC THURSDAY 28TH JUNE 2012

SKYWARN UK HAS ISSUED A SEVERE THUNDERSTORM WARNING FOR THE FOLLOWING REGIONS:
MIDLANDS
NORTHERN ENGLAND
SCOTLAND

IN EFFECT UNTIL 2359UTC THURSDAY 28TH JUNE 2012

PLEASE REFER TO WATCH #033 FOR SYNOPTIC DETAILS

SEVERE THUNDERSTORMS ARE EXPECTED IMMINENTLY, OR OCCURRING, SOMEWHERE WITHIN THE INDICATED REGIONS IN THE TIME PERIOD SPECIFIED. THREATS WITHIN THIS WATCH INCLUDE, BUT ARE NOT LIMITED TO:
HEAVY RAIN...FLASH FLOODING...LARGE HAIL...FREQUENT LIGHTNING...FUNNELS/TORNADOES

DISCUSSION:
RECENT RADAR, SATELLITE AND SFERIC OBSERVATIONS HIGHLIGHT A CLUSTER OF STRONG THUNDERSTORMS HAS INITIATED AROUND SOUTHEAST WALES AND THE LOWER WEST MIDLANDS. THERE IS GOOD MODEL CONFIDENCE THAT THESE STORMS WILL CROSS THE MIDLANDS AREA WITH THE POSSIBILITY OF VERY HEAVY RAIN RATES CAUSING SUDDEN EXCESSIVE SURFACE WATER AND MODERATE TO LARGE HAIL. THERE ALSO REMAINS A RISK OF TORNADOES WITH A MARGINALLY CONDUCIVE SURFACE FLOW.

THERE IS ALSO GOOD MODEL CONFIDENCE OF SEVERE THUNDERSTORMS AFFECTING NORTHERN ENGLAND AND SCOTLAND DURING THE AFTERNOON AND EVENING, AS PER WATCH #033, AND FOR THIS REASON THESE AREAS ARE ALSO UPGRADED TO A WARNING. THIS ALERT MAY BE UPDATED, PLEASE REFER BACK REGULARLY. PLEASE MONITOR WEATHER AND TRAVEL INFO AS NECESSARY.

[PaulKn]

TORRO TORNADO WATCH 2012/004

A TORNADO WATCH has been UPDATED at 1320GMT on Thursday 28th June 2012

Valid from/until: 0740 - 2000GMT on Thursday 28th June 2012, for the following regions of the United Kingdom & Eire:

UPDATED TO INCREASE HAIL SIZE TO 60-70MM FROM SUPERCELLS ACROSS NE MIDLANDS AND INTO LINCS

[PJB]

Analysis of the storms indicate they formed initially above the boundary layer. Meteosat Rapid Scan 5 Minute imagery indicates they formed over SE Wales, English / Welsh border this morning and grew vertically quickly. Aloft the environment was very conducive to rapid cloud growth wih available WV imagery showing a well marked dry intrusion moving NE across Wales (now moving into N and E England)

The Cells grew even more over the Midlands and most likely become routed in the Bounday layer as the moved east the CAP was eroded and HRV imagery shows clear rightward moving cells over the Midlands late morning possiby indicative of Supercell activity. The 11Z Nottingham ascent - is indicative of the highly supportive upper air environment. Storm Motion of 237 deg. Scope for large hail and high amounts of MLCAPE, Deep Layer Speed Sheer and low level Helicty evident on the sounding. Meteosat TOps estimate cloud top temps of nr -57C, tops hitting the tropopause and indicating tops to around 225mb or around 38, 000ft.

Marked descent now moving into SW England, CS England and Wales and the surface passage of the cold front is very weak indeed.

Reports of Hail and possible Tornado across the Midlands are inline with the sounding below.

Storm cells is now over E Midlands and HRV Imagery, clearly shows the inflow moisture being sucked into the storm cells from the South and SW and growth continued.

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[smartie]

Radar and IR imagery suggests the presence of gravity waves in the boundary layer top. These could be playing an important role in tornadogenesis.
D

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This post has been edited by smartie: 28 June 2012 - 14:01

[PJB]

Selection of HRV Imagery, if anyone wants any HRV Imagery - please contact me direct as i have all the files locally saved ATM

Paul

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[smartie]

PJB, on 28 June 2012 - 14:31, said:

Selection of HRV Imagery, if anyone wants any HRV Imagery - please contact me direct as i have all the files locally saved ATM

Paul

Paul,
it would be interesting to have IR and false-color IR for the Humber storm say ~1300-1330 UTC as well as general VIS. Matt Clark has a radar snapshot showing a distinct v-structure ~1320.
Thanks,.
David

[PJB]

13:00 - 13:45 HRV / IR Images. Please remember these images are Copyright Eumetsat. Please do NOT post them elsewhere. I will post EIR Images Separartelyt

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[PJB]

EIR Images - I only have Europe wide ones at this time

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[smartie]

PJB, on 28 June 2012 - 15:30, said:

EIR Images - I only have Europe wide ones at this time

Thanks very much Paul.
You can clearly see the 'enhanced V' and 'warm trench' signatures here. Created by overshooting tops penetrating through and above the anvil.

Best,
David