Uk General Convective Discussion & Forecasts, 28th August 2012>

[Coast]

Still looking generally quiet convectively with a little blip around next Tuesday:

Now if this could be closer to the coast:

Then the NW might see something later in the day:

[AWD]

Looks like we could be back in business next week.

[legritter]

IM hoping next week could bring some pleasant surprises for one or two of us ,with low pressure well in charge and the sun having a bit of a bite in it . it looks like some high rainfall totals for some and the possibility of some autumnal winds . lets hope we can get some similar charts during the winter , bring it on , councill turned off my snow lamp ,so i will have to switch on outside security lights and peep through the corner of the curtains ,or stick me hand out of door and feel the Big flakes a falling cheers Legritter

[Coast]

IM hoping next week could bring some pleasant surprises for one or two of us

Tuesday Wednesday could be interesting, not a high level of expectation at this point, but certainly worth looking out for as the days move on:

Even into Thursday further South:

[Andy Bown]

The Cold Front is producing some very heavy showers across the Wiltshire area this afternoon, there was quite a 10 minute downpour in Warminster while I was in Westbury and I have 3.5mm in my gauge which has doubled the monthly total!!

[AWD]

Yes, its looking as good as it has done for a very long time now midweek next week, especially from an IMBY point of view too.

[Jane Louise]

Again, let's hope this potential doesn't disappear. From past experiences and many years ago I hasten to add , September storms were always good ones especially as soon as it got dark, so,it's fingers crossed again folks.

[AWD]

There is potential once the depression slows down over Ireland, for storms a few days midweek this coming week.

25th & 27th currently look the most decent, widespread days.

[Coast]

Am I going to see some convective elements to the storm tonight on the coast?

Storm Forecast

Valid: Sun 23 Sep 2012 06:00 to Mon 24 Sep 2012 06:00 UTC

Issued: Sun 23 Sep 2012 05:49

Forecaster: VAN DER VELDE

A level 1 was issued for the Iberian Peninsula, France and bordering regions mainly for severe convective wind gusts and tornadoes.

SYNOPSIS

An Atlantic low centered northwest of the Iberian Peninsula brings warm and moist, slightly unstable air with a strong southerly flow into western Europe. The warm front will extend across southern UK, Belgium, to Austria. Over much of western France elevated CAPE is quite weakly capped, but boundary layer mixed parcels (200-700 J/kg MLCAPE) have large CIN. Over western Spain into southwestern France CIN is weak and greater storm coverage may be expected. The cold front is the main trigger for convection and lies over western Spain and western France during the afternoon. Over 1000 J/kg MLCAPE is found over the Mediterranean close to northeastern Spain and southern France, but seems to be mostly capped over the former region, and reached by the cold front only after 02Z for the latter region.

DISCUSSION

...Iberian Peninsula, France...

The strong wind field associated with the low offers a favorable environment for storms to produce severe wind gusts and some tornadoes. The 1-3 km average wind speed is 20-25 m/s over Portugal, western Spain and southwestern France. It can be brought down to ground by storm downdrafts. The storm motion will be similar. As shear vectors ar aligned with the orientation of the cold front, linear systems are preferred, but will likely only produce severe gusts when the line becomes oriented more perpendicular to the shear (i.e. NW-SE), The magnitude of 0-1 km shear (12-17 m/s) and deep layer shear (20-25 m/s) is supportive of tornadoes and bow echoes. Low LCL heights with strong shear can help tornadogenesis and are found mostly over western Iberia and northwestern France - southern UK. The flow of moist unstable airmass over the Cevennes mountain range in southern France will create persistent storms during the pass of the cold front in the early hours of Monday, which may locally cause excessive convective rainfall amounts. In much of the level 1 area isolated large hail can be produced, especially by supercells rather than linear systems.

doubtful, but oh that rain.....

[ardsar]

Heavier rain now starting to arrive in Devon. Total rain so far only 3.8mm, but looking at the radar - its starting to intensify. Lightning detector also starting to pick up sferics from France.....Hope tonight will bring some coastal storms

[Robbie Garrett]

MetOffice now tapping into Thundery Showers this Tuesday.

http://www.metoffice.gov.uk/public/weather/forecast/?tab=fiveDay&fcTime=1348527600&locId=350928

[chionomaniac]

Quite a nasty embedded thunderstorm off the Cherbourg peninsular- where's that heading?

[lottiekent]

Quite a nasty embedded thunderstorm off the Cherbourg peninsular- where's that heading?

I was wondering that and thought maybe somewhere along the coast between Brighton and Hastings? Hopefully then to track a little further east to clip me here in Folkestone!

[chionomaniac]

I was wondering that and thought maybe somewhere along the coast between Brighton and Hastings? Hopefully then to track a little further east to clip me here in Folkestone!

It could do Lottie!

[Coast]

Big bellowing rolls of thunder in Eastbourne now!!!!

[AWD]

Looks like the GFS 12z has downgraded any storm potential this coming week now, instead keeping any thundery actiuvity in or over the channel, possibly clipping the south coast and Kent at times.

[Coast]

More thunder coming through overnight for me!

[Jane Louise]

More thunder coming through overnight for me!

Splendid enjoy Robin Sadly nowt forecast for me here.

[Coast]

Slim chance again today for me?

Storm Forecast

Valid: Mon 24 Sep 2012 06:00 to Tue 25 Sep 2012 06:00 UTC

Issued: Sun 23 Sep 2012 22:13

Forecaster: TUSCHY

SYNOPSIS

The predominant vortex over N-Europe shifts eastward, stepping aside for another cyclonic vortex, which approaches from the N-Atlantic. A strengthening depression from the Bay of Biscay gets incorporated into the circulation of the second vortex and the net result will be a major cyclonic vortex, centered over UK, which becomes the steering mechanism for quite some time. Over the Mediterranean, geopotential heights start to decrease with weak ridging present over the far E-Mediterranean.

Side-note:

Multiple structural changes occurred and will occur with the consolidating depression, which moves off the Bay of Biscay to the north/northeast. After initiated by a southbound moving upper low with a distinct tropopause fold during the overnight hours of the 21st to the 22nd, roughly 36h of a slightly less barocline atmosphere allowed for some low-end core modification, although negligible (and still classified as a true cold core low). As of writing (23rd), a plume of slightly modified tropical air (modified due to onshore trajectories into the depression's center), encircled most of the center and assisted in disorganized but slightly deeper convection with no distinct banding present. However this pushed the depression near the shallow warm core structure on available phase diagrams. During the afternoon hours of the 23rd however, an approaching frontal boundary from the NW marked the beginning of another round of enhanced baroclinity with models showing the evolution of a distinct tropopause fold over Ireland and UK during the forecast period, bending into the system. This increase of baroclinity results in a rapid deepening phase, which starts around the beginning of my forecast period and offers a central pressure drop of at least 10 K in probably less than 24 h while crossing UK from the SW with a strong turn to the NW during the night.

This results in a quite complex frontal and thermal structure of that low, as the majority of the models now propose some type of thermodynamic core modification. Current thinking is that a moderate warm-core seclusion takes place during the forecast although described history of that depression may offer a better defined LL warm core structure compared to truley extratropical cases. Latest ASCAT scans reveal a quite symmetric near surface wind field with uncontaminated 30-40 kt winds along the W/S and SE-ern fringes (much less defined along the NE quadrant) and this well defined wind field in the lower troposphere remains intact during much of the forecast period.

To complicate that scenario once more, ongoing baroclinic transformation will result in more typical vertical wind field distribution of the extra tropics along the southern fringe, as a strong mid/high-level jet moves in from the west and crosses N-France and Benelux during the night. Along the northern fringe of the depression however (e.g. N-UK and Scotland), no modification is expected until 06Z due to the dominant and still westward pointing wrap-around occlusion, which keeps the shallow warm core intact.

DISCUSSION

... Far E-France, parts of Germany and parts of Benelux ...

At 06Z, a strongly tilted warm front will be situated over the Netherlands and N-Germany with a slow down of its northward shift forecast (in response to the rapidly deepening low over the UK). A surface cold front is about to exit E-France with a rapid NE-ward motion, so a constantly narrowing warm sector over Germany will be the result with some type of triple point evolving over NW Germany during the night, as the occlusion process builds east.

At mid/upper levels, a NE ward swinging trough affects the area of interest. This trough weakens significantly during its shift to the NE with strong mid-level NVA forecast atop the NE-ward surging cold front. Strong frontal circulation of the cold front however may offset some of those more negative effects in addition to other models, showing the upper trough slightly stonger with better cyclonic curvature. CI should occur, given weakly capped warm sector and healthy looking cold front. Scattered DMC activity is forecast.

From the thermodynamic perspective, the (sub)tropical background of that air mass features weak mid-level lapse rates for all areas beside those along and just north of the Alps (where Alpine EML fans out to the NE, affecting SE Germany and parts of Austria during the forecast). Forecast soundings confirm MIMIC-TPW output with 30 to roughly 40 mm TPW present behind the warm front, as deep plume of moisture spreads NE-wards. Of concern is the cloudy warm sector, which keeps diabatic heating limited, although not much extra heating is needed for some CAPE build-up, given surface and 925 hPa dewpoints in the mid tens (with a few stations upstream (NE France) even reporting surface dewpoints in the upper tens). Models like WRF show some break up of the thick cloud shield, so even some diabatic heating is forecast and therefore, we agree in the quite robust CAPE forecast of 500 to locally 800 J/kg MLCAPE (EZ on the lower-end side). CAPE might be the most robust over far NW Germany and Benelux with a second maximum over far SW/SE Germany and N-Austria. In-between (e.g. W-C Germany), somewhat worse moisture/stronger BL mixing result in substantially lower CAPE. Finally, LL CAPE might be enhanced as seen in some forecast soundings.

Kinematics are good, especially at lower levels. With the eastward punching mid-high level jet from N-France probably arriving too late, DLS remains modest with 15 to locally 20 m/s, nevertheless enough for organized convection. Similar shear magnitude however is also forecast in the lower troposphere (3km and 1km bulk shear). Not only the speed shear, but also SRH is enhanced with peaks in excess of 200 m^2/s^2 in the lowest km (e.g. ahead/along of the cold front).

Numerous rounds of CI are forecast:

a) Sporadic warm front activity over NW-Germany (ongoing from the overnight hours). Forecast soundings show some MUCAPE with veered profiles, but nothing severe is forecast. This was the main reason for expanding the low prob. lightning area far to the NE over NE Germany. The risk wanes until noon.

b ) Warm sector convection. Coverage of convection is uncertain and depends on isolated better diabatic heating and forcing. Current thinking is that the Netherlands/NW Germany may see isolated activity during the morning hours onwards. However it will take some time before thunderstorms become surface based (probably until noon), so again no severe risk is forecast with that activity (at least until noon).

c) The main risk seems to evolve along the cold front and potential prefrontal convergence zone, which crosses the highlighted area during the daytime hours from SW to NE. Generally, the Netherlands/NW Germany (arrivial during peak heating) and SW Germany (better BL moisture) may see the highest probabilities for thunderstorms with less activity in-between. With worsening background forcing, a broken line of showers and thunderstorms is forecast, probably more solid over NW-Germany. Severe wind gusts will be the primary risk (with 25 m/s in 850 hPa) , but would not be surprised to see an isolated tornado report. The overall tornado risk might be steered by mesoscale processes/boundaries and locally better heating and therefore the overall risk is hard to pinpoint that far out. Over W-C Germany the severe risk might be low due to weak CAPE, but model forecasts diverge too much to split the level 1 into two distinct areas. Also, not much downward transport needed to mix 20-25 m/s winds down to the surface in showers/thunderstorms. The severe risk rapidly diminishes after sunset.

... N-UK and Scotland ...

Westward wrapping occlusion shifts only slowly to the north during the forecast period. Prolonged period of lift/forcing/influx of very moist air and slow storm motion indicate a high probability for heavy rain. Some MUCAPE is forecast, so we do not want to exclude a sporadic thunderstorm event (not reflected on our outlook map).

Beside the rain risk, a gradually intensifying easterly LLJ with 850 hPa winds up to 30 m/s results in severe wind gusts over Scotland (especially at more exposed areas). This risk is not included due to non-existent DMC activity.

During the overnight hours, thunderstorm probabilities increase over the English Channel,the S-North Sea and the N-Bay of Biscay as cold air at mid-levels spreads to the south and east.

SKYWARN UK SEVERE WEATHER WARNING #057

ISSUED: 1330UTC SUNDAY 23RD SEPTEMBER 2012 (SM/GJ)

SKYWARN UK HAS ISSUED A SEVERE WEATHER WARNING FOR THE FOLLOWING REGIONS:

SOUTHWEST ENGLAND WALES WEST MIDLANDS NORTHERN ENGLAND NORTHERN IRELAND SOUTHEAST ENGLAND

IN EFFECT UNTIL 2359UTC MONDAY 24TH SEPTEMBER 2012

REDEVELOPING BISCAY LOW WITH VERY MOIST TROPICAL AIR GENERATING INTENSE RAINFALL WITH ORGANISED EMBEDDED THUNDERSTORMS

SEVERE WEATHER IS EXPECTED IMMINENTLY, OR OCCURRING, SOMEWHERE WITHIN THE REGIONS INDICATED. THREATS WITHIN THIS WARNING INCLUDE, BUT ARE NOT LIMITED TO:

HEAVY RAIN...FLASH FLOODING...SHARP CATCHMENT FLOODING...STRONG GUSTS...FUNNELS OR WEAK TORNADOES

DISCUSSION:

THERE IS STRONG MODEL CONFIDENCE IN THE POSSIBILITY OF SEVERE WEATHER RISKS THROUGH SUNDAY AND MONDAY. AS THE LOW DEEPENS TO ~980MB WHILE TRANSITING NORTH, A FEED OF TROPICAL MOIST AIR AND CONTINENTAL DRY AIR WILL GENERATE VERY HEAVY RAIN THROUGH PORTIONS OF THE WESTERN MAINLAND UP TO NORTHERN ENGLAND AND NORTHERN IRELAND WHERE THE OCCLUDED FRONT AND CLOUD HOOK OVERPASS. ACCUMULATIONS OF UP TO 50MM IN A FEW HOURS IS POSSIBLE WITH PRIOR RAINFALL POSSIBLY CAUSING PRE-SATURATION OF SURFACES BEFORE THE HEAVIEST RAIN NEARER THE LOW CENTRE. A STRONG WIND FLOW WILL ALSO ENHANCE RAINFALL OVER EAST AND SOUTHEAST FACING HIGH GROUND, POTENTIALLY CAUSING LOCALISED FLOODING IN STEEP CATCHMENT AREAS. OVER THE SOUTHEAST, SOME WARM ELEVATED AIR WILL GENERATE STRONG CONVECTION WITHIN AND ALONG THE BACK OF THE RAINMASS, FURTHER ENHANCING RAINFALL AND POSSIBLY CAUSING FLASH FLOODING, MOSTLY DURING THE NIGHT BUT POTENTIALLY INTO MONDAY MORNING. CONVERGENT AND STRONG LOW-MID LEVEL WINDS WILL GENERATE DIRECTIONAL SHEAR ALONG THE FRONTAL ZONE THAT WILL SUPPORT ORGANISATION OF CONVECTIVE ELEMENTS AND MAY GIVE RISE TO VORTICES WHERE UPDRAFTS ARE BRIEFLY STRONG ENOUGH, ENHANCING GUSTS AND POSSIBLY GENERATING A TORNADO. THIS WARNING MAY BE UPDATED. PLEASE MONITOR WEATHER AND TRAVEL INFO AS NECESSARY.

Into tomorrow morning?

[ZONE 51]

Lightning in Channel at around 0015am.(.a few strikes where my symbol is placed)

Can expect more of a swing into the inland SE later as the storm low shifts and drags them in like a magnet

Edited September 25, 2012 by ElectricSnowStorm

[Coast]

ESTOFEX looking at more English Channel convection today, limited possibilities for most though I guess:

Storm Forecast

Valid: Tue 25 Sep 2012 06:00 to Wed 26 Sep 2012 06:00 UTC

Issued: Mon 24 Sep 2012 23:00

Forecaster: TUSCHY

A level 1 was issued for parts of E/SE France mainly for isolated strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for S-France mainly for isolated strong to severe wind gusts, large hail and heavy rain.

SYNOPSIS

An impressive 979 hPa depression, just about to move ashore along the E-C coast of UK at 22Z (24th), continues to be the main player for the following 24 h. According to the phase diagrams, the warm seclusion process will be finished during the start of the forecast and a very well structured shallow warm-core structure persists until the evening, before gradual weakening begins. Those spots with more or less undisturbed ASCAT measurements show roughly 40 kt near surface winds wrapping around the center with some zonal asymmetrie present. Despite the cut off of the warm/moist supply of modified subtropical air, it still takes some time for that energetic air mass to mix out, so a slow moving and intense/deep cyclonic vortex over the UK will be the major steering mechanism for most of Europe.

Lower geopotential heights and cool mid-levels affect most of Europe and result in unsettled conditions with showers/thunderstorms over numerous areas. Beside a few upper level disturbances, no major trough/rigde is seen during the forecast.

DISCUSSION

... Rest of the highlighted areas ...

A very broad low lightning area was issued, as sporadic activity may occur. The main activity will occur over offshore areas like the S-North Sea, the Bay of Biscay and the far E-Atlantic.

SkyWarn:

SKYWARN UK SEVERE WEATHER WARNING #058

ISSUED: 1230UTC MONDAY 24TH SEPTEMBER 2012 (SM/GJ/GS)

SKYWARN UK HAS ISSUED A SEVERE WEATHER WARNING FOR THE FOLLOWING REGIONS:

NORTHERN IRELAND SCOTLAND MIDLANDS EASTERN ENGLAND NORTHERN ENGLAND

SOUTHEAST ENGLAND

IN EFFECT UNTIL 1200UTC TUESDAY 25TH SEPTEMBER 2012

DEEP LOW PRESSURE ADVANCES NORTH THROUGH THE UK AND CONTINUES THE THREAT OF SEVERE WEATHER

SEVERE WEATHER IS EXPECTED IMMINENTLY, OR OCCURRING, SOMEWHERE WITHIN THE REGIONS INDICATED. THREATS WITHIN THIS WARNING INCLUDE, BUT ARE NOT LIMITED TO:

HEAVY RAIN...FLASH FLOODING...SHARP CATCHMENT FLOODING...STRONG GUSTS...FUNNELS OR WEAK TORNADOES

DISCUSSION:

STRONG MODEL CONFIDENCE CONTINUES WITH THE LOW EXPECTED TO DEEPEN FURTHER TOWARDS 975MB WHILE TRANSITING NORTH WITH FURTHER FALLS OF HEAVY AND PERSISTENT RAIN ARE FORECAST ACROSS THE WARNING REGIONS. RAINFALL TOTALS RANGE BETWEEN MODELS BUT CONFIDENCE OF BETWEEN 50-70MM HAS BEEN BOOSTED BY DISCUSSIONS FROM PARTNER AGENCIES AS RAINFALL CIRCULATES AROUND THE LOW PRESSURE GIVING PERSISTENT FALLS INTO NORTHERN ENGLAND AND IRELAND IN THE NEXT FEW HOURS PROMOTING THE RISK OF FLOODING.

AN INCREASE IN WIND SPEEDS TO GALE FORCE AROUND THE SOUTH-EASTERN CORNER OF ENGLAND IS EXPECTED OR OCCURING AND WILL DO SO FOR THE NEXT FEW HOURS. STRONG GUSTS OF 50-60MPH AROUND THE COASTLINES AND THROUGH THE CHANNEL CAN BE EXPECTED. THESE STRONG WINDS WILL ALSO TRACK NORTH TO AFFECT EASTERN ENGLAND AND SCOTLAND TO GIVE THE POTENTIAL FOR 50-60MPH MEAN WIND SPEEDS WHICH, WITH COMING FROM THE EAST AND TREES IN FULL LEAF IS LIKELY TO BRING SOME DAMAGE. STRONG WINDS AND STRONG GUSTS WILL AFFECT NORTHERN IRELAND INTO THE MORNING.

Some MLCAPE around along the Channel today:

Convergence over the extreme SE:

Low pressure Karin still whirling around:

Rain to the North (significant again)

Surface CAPE highest in the South:

As are the lapse rates:

Small tornado threat confined to a very localised area in Kent/Sussex?

Gusts of higher speeds mainly in the North and Scotland:

Simply shown in GFS format, maybe some thunderstorms along the South coast today:

A chance of one in the Humber later:

[Nick F]

Could be a few rumbles across the SE and E Anglia later, storm forecast for today:

http://www.netweather.tv/index.cgi?action=convective;sess=bf678aa17196f6b980aaf67b301e2ae1

[dave48]

Can see a good storm about 30 miles out to sea from here in Westcliff now the sun is going down, anvil is lighting up pink every few minutes. Should be better when the sun is down in 30 minutes or so,

[Coast]

Cells and anvils building nicely over Sussex tonight:

[Methuselah]

Nothing here...