Convective / Storm Discussion - 31st July onwards 2013

[smoggyjester]

Smoggy, watch the news 24 weather at 01:00!

. Got it on but its hardtallk at mo Edited August 1, 2013 by smoggyjester

[smoggyjester]

Wahoo Portsmouth about 5am for thundery showers everything is crossed lol

[Convective]

Absolutely throwing it down here all of a sudden. On a side note, the Estofex forecast shows widespread activity:

Discussion

...Ireland and Scotland...

Behind the eastward departing cold front, the post frontal air mass still features reasonably high BL moisture. This moisture also increases from the west before noon as a wrap-around occlusion approaches from the west. With cooling mid-levels atop, some modest MLCAPE build-up becomes likely in the order of 500 to 1000 J/kg. With surface pressure still below 995 hPa in the vortex's center, expect an ongoing brisk gradient flow in the LL with 850 hPa winds of 15 to 20 m/s. LCLs remain at or below 800 m and with some anticipated breaks in the clouds and attendant diabatic heating, it seems to be reasonable to expect modest LL buoyancy (as forecast by GFS). Final geometry of the cyclonic vortex in the low to mid-troposphere dictates strength of directional shear and therefore magnitude of SRH. However, there are signs of a broad overlap of 100 to 200 m^2/s^2 SRH-1, modest LLCAPE and adequate mid-tropospheric CAPE for longer lived and rotating updrafts ... mainly from Ireland to Scotland. Isolated tornadoes , marginal hail and strong wind gusts accompany strongest storms. Thunderstorms gradually decay onshore after sunset but keep going all night long south of Ireland beneath the base of an eastward racing mid-level trough with 500 hPa temperatures below -20 °C (SSTs around 16 to 17°C).

... Extreme N/NE France, N-Belgium/Netherland, parts of UK, the North Sea and W-Norway ...

Model data indicate a more or less undisturbed SW-erly flow regime over the area of interest. In different model runs/model data however, subtle waves were seen and one of those waves could indeed support enhanced CI probabilities on a regional and very isolated scale. With strong prefrontal WAA however (in 850 hPa, the 20 °C isotherm being advected as far north as the Netherlands), the air mass will stay capped from C-France all the way to the Netherlands. Foci for CI before 18Z will be areas like:

- extreme N-France, the E/SE coasts of UK and the coasts of Belgium and the Netherlands, where diurnal driven sea-breeze fronts may serve as trigger for initiation on an isolated scale.

- the ESE-ward shifting cold front itself. Despite ongoing weakening of this boundary, lift should still be adequate to break the cap in a few areas. This might be fostered by complex interaction with aforementioned mesoscale convergence zones along the coasts.

- a weak surface low, seen in a few high resolution model runs, which exits NE France during the afternoon hours to the NE, affecting the coasts of Belgium and the Netherlands. Enhanced BL convergence with that feature might also induce isolated CI.

Those storms evolve within a volatile kinematic environment of 25 to 30 m/s more or less unidirectional 0-6 km DLS, so this activity is forecast to speed to the NE over the offshore areas of the North Sea. Strongest shear will be displaced from the maximized and mostly capped CAPE plume along the front with up to 2000 J/kg MLCAPE forecast by GFS/EZ. Still 500 to 1000 J/kg will be available for this coastal/offshore activity, which is enough for long-lived and organized DMC. This activity is forecast to grow upscale into numerous storm clusters over the S-North Sea. Severe wind gusts and large hail will be the main hazard.

A concern right now is the chance of an isolated storm (e.g. tail-end Charlie of any cluster) with strong deviant storm motion to affect parts of the Netherlands and Belgium during the afternoon hours. A 18Z forecast sounding from the W-Netherlands show more than 2500 J/kg MLCAPE, strongly veered profiles, 25 m/s shear already in the lowest 3 km and a rich helical environment. In case initiation along the coast verifies, expect any storm to reveal explosive development into a mature supercell. Large to very large hail (hail diameter in excess of 5 cm), severe wind gusts and very heavy rainfall amounts likley accompany that activity. Combination of SRH-1 above 200 m^2/s^2 and LCLs around 1 km along the coasts also indicate an enhanced tornado risk in case this activity will be surface-based. CI has to be monitored closely and an upgrade may be needed if confidence in CI has increased.

Beyond sunset, a rapidly intensifying 850 hPa jet (15 to 20 m/s in its core) and increasing QG forcing from a NE-ward racing upper level short-wave all support constant upscale growth of the convection over the North Sea into an extensive thunderstorm cluster. With ongoing WAA, capped/elevated 500 to 1000 J/kg MUCAPE are forecast, but maximized BL moisture along the leisurely eastward moving cold front also supports less capped MLCAPE build-up over the E-North Sea into W-Norway. An organized line of storms should cross the North Sea during the night from SW to NE in response to the forcing and scattered strong to severe wind gusts can be anticipated over most parts of the C/E North Sea and over W-Norway. With rapidly increasing directional shear along the coast of W-Norway, embedded rotating cells with an isolated tornado risk can be anticipated. Beside that, isolated large hail will be possible, too. Effective PWs approach 40 mm, so flash flood producing rain will be another concern over SW/W Norway.

The southern fringe of that convective cluster has to be monitored in case of some nocturnal back-building towards the Netherlands and far NW Germany occurs. Despite a thinning CAPE plume and some BL stabilization, the risk for large hail, strong to severe wind gusts, heavy rain and an isolated tornado threat would still be present.

A broad level 1 was issued for that activity although confined areas might see level 2 conditions especially in case of an organized MCS (wind) event over the North Sea or isolated supercells over parts of Belgium/the Netherlands. An upgrade might be performed later-on.

[cheese]

Just woke up, looked outside and saw some Ac drifting by from the south. Interesting..

[staplehurst]

I'm rather surprised theres not much chatter for fridays offerings?!

Tomaz at 00:00 on news 24 seemed rather excited! Storms starting on the south coast and quickly developing north anywhere east of east wales, some sharp ones into the midlands and yorkshire by afternoon "if you get one boy will you know about it"!

 

happens too often when in reality not very much happens! 

[NO LONGER POSTING]

Storms fired in the channel and surviving the crossing!!!

[Supacell]

Looking at the radar these storms are moving at quite a rate. They could be here within an hour or two.

[the storm fisherman]

Hi all

just a quick one

I can confirm that the shower just passed here 1/2 hour ago had a couple rumbles of thunder some large rain drops but not much else

 

will post if anything more happens

Edited August 2, 2013 by the storm fisherman

[NO LONGER POSTING]

storms firing all over southern england???

[Rammie]

Nothing happening in North Somerset, but that's normal :-)

[flysurfer]

Some big cbs going up over maidst

one kent

[Sand Risnycatt]

Thunder in littlehampton West Sussex with light drizzle

[Guenhwyvar]

Yes been woken up by thunder..it's getting very dark.

[chionomaniac]

Hurray thunder and torrential rain.

[NO LONGER POSTING]

Hurray thunder and torrential rain.

 

Nice little storm heading North very slightly north East

[Norfolk Sheep]

Thunder reported in Worthing - Smiley Carol didn't mention that place over the channel or the H word; we shoud be truly grateful

[East_England_Stormchaser91]

Are these storms SB or elevated? The sun being out crystal clearly should enhance further development as these head NE?

P.S wow, alto cas heading rapidly S-N !! never seen them move so quickly!

Edited August 2, 2013 by East_England_Stormchaser91

[Nick F]

Mostly blue skies here in south Kent though can see billowing Ac cas towards the southwest and west. Typical go down to Kent and storms erupt back at home over Croydon, then when I'm in London all the storms are down here!

 

Anyway, Netweather storm forecast for today issued last night:

 

 

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

Edited August 2, 2013 by Nick F

[Biggin]

Mostly blue skies here in south Kent though can see billowing Ac cas towards the southwest and west. Typical go down to Kent and storms erupt back at home over Croydon, then when I'm in London all the storms are down here!

 

Anyway, Netweather storm forecast for today issued last night:

 

convmap_020813.jpg

 

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

Yep can confirm a few rumbles here atm typical eh ???

[Supacell]

Are these storms SB or elevated? The sun being out crystal clearly should enhance further development as these head NE?

P.S wow, alto cas heading rapidly S-N !! never seen them move so quickly!

 

I would imagine they are elevated. Chance of them becoming SB later though and then parameters look ideal for some possible severe thunderstorms across Lincolnshire and East Anglia.

[East_England_Stormchaser91]

I would imagine they are elevated. Chance of them becoming SB later though and then parameters look ideal for some possible severe thunderstorms across Lincolnshire and East Anglia.

Crikey! Right on my patch too, Ill keep my camera handy !

[Tom Jarvis]

storms are definately elivated the one from brighton only missed me by a few miles but there was some nice deep rumbles of thunder but couldnt see any lightning

[nanu]

Lost of thunder just south of me... might get clipped

[Harry]

In the car on the A2 saw a towering Cb and impressive dark sky. Saw a number of CGs too

[Harry]

Flash and crack overhead in Blackheath :-)

Edited August 2, 2013 by Harry