Uk General Convective Discussion & Forecasts, 13th March 2013 onwards

[Jane Louise]

Looking forward to Saturdays potential and the rest of the 2013 storm season.

[MP-R]

It was on this Saturday that I recorded my first thunderstorm of the year last year. As it is, there has already been one this year at the end of January (Bristol)

As for Madrid, experienced a brief but very nasty thunderstorm last Thursday evening as a proper gust front passed over with horrendous rain, hail, thunder and lightning overhead and wind that managed to carry a few bins across the road and take off a few branches.

[Raidan]

26th of Jan to be precise MHR,earliest I've ever seen a storm in any year.

[Nick F]

Yes, looks to be some convective potential both Saturday and Sunday across south Wales and southern England in particular, Lapse rates will steepen widely across the UK in the post cold front rPm airmass on Saturday. However, moisture will be greatest across southern England, (temp/DP - 7/6C) and couple this with a little limited surface heating where the sun pops out to warm the ground - and we should see the best chance of strongest convection across the south both Sat and Sun in the slack cyclonic circulation. Hail showers with the odd rumble of thunder look likely, and hopefully should be some good cloudscapes in the polar air if we see blue skies in between showers.

[Coast]

Yes, looks to be some convective potential both Saturday and Sunday across south Wales and southern England in particular

Liking that prospect!

A bit of higher level cape follows the low into SW England:

and quite a lot of surface cape

Convergence:

Those steep lapse rates that Nick was talking about:

A lot of rain, maybe squally:

and then the gusty winds and helicity which may just gives something rotational:

Edited March 15, 2013 by Coast

[Nick F]

Looking at the CAPE charts this morning, SW England probably the sweet spot tomorrow near centre of the low/cold pool. Then SE England, E Anglia, LIncs, Yorks on Sunday, as the low drifts NE

[Raidan]

With the SW possibly being a sweet spot tomorrow for thundery activity where in the SW would be best placed?Would I be better off being by the Bristol channel or staying in Bristol its self?

[liambailey09]

I've been looking forward to this thread coming back for a while now. Looking forward to the weekends prospects, and into next week. I have tomorrow off due to work being slow but looks like Sunday would have been a better day for my location!

Good luck everyone, and here's to a summer of sunshine, warmth and thunderstorms

[AWD]

With the SW possibly being a sweet spot tomorrow for thundery activity where in the SW would be best placed?Would I be better off being by the Bristol channel or staying in Bristol its self?

Thundersnow?

I'm thinking Bristol Channel Streamer?

Edited March 15, 2013 by AWD

[Nick F]

With the SW possibly being a sweet spot tomorrow for thundery activity where in the SW would be best placed?Would I be better off being by the Bristol channel or staying in Bristol its self?

Think inland will be just as good as near the Bristol Channel, the seas are reaching their coldest point of the year now - so aren't creating as strongly buouyant updrafts when cold air is aloft as say in December. A moist airmass, steep lapse rates, large scale forced ascent under the low and a little warming at the surface in the now slightly stronger sun will all help those cbs build across the SW tomorrow - regardless whether near seas or not.

Edited March 15, 2013 by Nick F

[AWD]

I think thunder and lightning is unlikely tomorrow going by the latest forecast lapse rates for the SW;

But with warmer air aloft and colder air further down, then sleet/snow/hail are all a real possibility. Latest GFS run showing the favoured area tomorrow afternoon, with a few hundred joules of CAPE developing over the SW;

Edited March 15, 2013 by AWD

[AWD]

Low to mid level lapse rates pretty steep across most of the western half of the UK, so no issues there. The skew-T shows the temp. falling away nicely well past 500 hPa, so adjusting the temp. for an air parcel to allow for max insolation where the sun can get to work should see convective updrafts to build to around 18-22,000 feet - sufficient height for lightning.

Think there'll be a little too much residual cloud cover across more central and eastern parts from the front, so instability looks to be inhibited here. Might be a chance later on the day for the West Mids though if there are any breaks.

Thanks for that. Good provision of evidence to suggest some support for electrical activity.

Latest FAX for tomorrow shows a trough to the SW, which is the disturbance that looks to kick the atmosphere and provide the SW and Wales with shower cells;

Similar to the GFS too;

Suggestive of slow moving downpours in the centre of the depression, helped on by solar heating.

Edited March 15, 2013 by AWD

[Stuart]

Synopsis:

Upper trough continues to migrate slowly eastwards across Britain on Saturday, accompanied by cool mid-levels. Another day of convective weather is expected in the post-frontal environment.

Discussion:

... SW/CS ENGLAND, WEST COUNTRY, S WALES, MIDLANDS ...

Cooling aloft will steepen lapse rates, and accompanied by diurnal heating will generate 400-600 J/kg CAPE. As a result, scattered showers and a few pulse-type thunderstorms are expected to develop, moving north/northeastwards towards the Midlands later in the afternoon. A shortwave trough will enhance convection organisation during the afternoon, with small hail possible in some stronger cores.

40kts DLS and 20kts LLS will allow convective cells to be fairly well-organised, and also increases the chance of a weak tornado. Some 150 m^2/s^2 SRH accompanied by favourable slightly-backed surface winds suggest that a low-topped supercell is possible for a short time over the West Country and/or across the M4 corridor, capable of producing hail up to 2.0cm in diameter.

In either case, sferic activity is still expected to be fairly minimal/sporadic/isolated. Most notable feature is likely to be hail which may cause some local disruption on roads etc.

... NORTHERN IRELAND, W IRELAND, NW ENGLAND, N WALES ...

Isolated convective showers are possible here during Saturday, but the coverage and/or chance of sferic activity is considered too low to issue a threat level.

http://www.ukasf.co.uk/storm-forecasts/228

[Jane Louise]

And not forgetting Estofex

Storm Forecast

Valid: Sat 16 Mar 2013 06:00 to Sun 17 Mar 2013 06:00 UTC

Issued: Fri 15 Mar 2013 21:50

Forecaster: TUSCHY

A level 1 was issued for SW UK mainly for an isolated large hail and tornado risk.

http://www.estofex.org/

[Coast]

And not forgetting Estofex

Good old ESTOFEX!!

Storm Forecast

Valid: Sat 16 Mar 2013 06:00 to Sun 17 Mar 2013 06:00 UTC

Issued: Fri 15 Mar 2013 21:50

Forecaster: TUSCHY

A level 1 was issued for SW UK mainly for an isolated large hail and tornado risk.

SYNOPSIS

A deep channel of low geopotential heights remains anchored over Europe. A cold air mass is in place over most of Europe with bitterly cold conditions ongoing over E/SE Europe. A pronounced vortex enters the scene from the west and becomes quasi-stationary over Ireland, UK and Scotland. A slightly warmer and increasingly more humid air mass starts to affect parts of Portugal and Spain and another pronounced air mass boundary sets-up over N-Portugal/Spain.

DISCUSSION

... Aegean Sea and W/SW Turkey ...

With the cold mid-level air mass still present atop the Aegean Sea, expect another day with active shower and thunderstorm development before a southbound moving cold front cuts off that risk from north to south during the overnight hours. Until then, scattered DMC is forecast with marginal hail and gusty winds. Once again, the waterspout risk will be somewhat elevated especially along the coasts, but decreasing LL convergence signals during the afternoon hours should bring the overall risk down during the day. During the night, only isolated thunderstorms are forecast, which exit the forecast area to the E.

... France and parts of the UK ...

Seasonably moist post frontal cold sector air mass beneath modest mid-level lapse rates may result in a few short-lived thunderstorm events. The main foci for CI will be those regions, where postfrontal air mass (with cellular convection) advects onshore during peak daytime heating (e.g. NW France and SW UK). Those spots will see some SBCAPE build-up. DLS remains weak over most areas and therefore only gusty winds and marginal hail/sleet is forecast.

An exception will be SW-UK, where a low-end severe risk may materialize. The placement of a near vertically stacked low just to the south of Ireland during the afternoon hours assists in good onshore moisture advection beneath an ongoing diffluent mid/upper-level flow regime. A lot depends on how strong the lower part of that vortex will be regarding the magnitude of backing of the LL wind field. Still, with 15 m/s DLS and enough time for diabatic heating forecast, a few strong thunderstorms may evolve during a short time-frame between noon and sunset. Forecast soundings show good speed shear with improving directional shear next to 300-500 J/kg SBCAPE, so storms will be capable of producing isolated large hail. Some SRH-1, low LCLs and LL CAPE in excess of 100 J/kg may also result in a very isolated tornado event. The risk rapidly diminishes after sunset.

During the overnight hours, the eastward moving cold front interacts with a prefrontal moisture surge from the W-Mediterranean (advected moisture becomes more elevated atop the stable nocturnal BL). At the same time, mid-levels cool down a bit which only increases mid-level lapse rates atop the surface cold front, so I would not be surprised to see the initiation of a few elevated thunderstorms along the eastward moving cold front. However, this front becomes masked betimes with washed out temperature gradients and a near parallel alignement to the background flow. Hence, only a sporadic thunderstorm risk may occur

Onto the specific charts:

Mixed level CAPE creeping up the Channel approaches later:

Significant surface CAPE and a spout risk for Wales:

Shear and convergence:

Lapse rates are decent enough:

Helicity on the Eastern side of the low:

Gusty wind all day:

Could be a few bangs and flashes later in the SW:

[Supacell]

From reading the forecasts and looking at the charts I would say that today will see an area of showers move into the SW during the morning and slowly spread north and east into south Wales, West Country and the West Midlands during the day, reaching the West Midlands by evening. The biggest threat would be hail, with larger hail possible during the day in the area highlighted by the level 1 by estofex. Lightning i think will be at a premium, although could be the odd flash and rumble amongst these showers (again under the level 1 area).

Not a great overall convective outlook in the grand scheme, but definitely not bad for mid March.

Edited March 16, 2013 by Supacell

[AWD]

Cells already building over Cornwall with some decent radar returns.

With the influence of solar input over the next few hours, possibilities we could see some rather potent downpours as the cloud base rises building some decent Cb's.

Here's hoping anyway.

[Nick F]

Issued: 16/03/2012 11:00

Valid: 16/03/2013 11:00 - 17/03/2013 06:00

... THERE IS A RISK OF THUNDERSTORMS FORECAST ...

Upper and surface low sit across the western side of Britain with an unstable rPm airmass moving across SW UK behind cold front clearing SE England and E Anglia, a shortwave trough moves through SW UK enhancing instability this afternoon.

... SW UK ...

Cold mid-levels in association with upper low across western Britain will create steep lapse rates across much of the UK in polar maritime flow following cold front clearing east. Convective potential will be greatest across SW England and S Wales where we have the combination of insolation, greatest moisture and the passage of shortwave trough which will increase instability to around 100-300 j/kg CAPE this afternoon. The shortwave is evident on WV imagery and as Cb clusters on vis satellite in the SW approaches.

Heavy showers/thunderstorms are likely throughout the day here. Deep layer shear values of 20 - 30 knts and veering winds at the surface around surface low will aid in organisation of convective cells - with a risk of hail to 1cm in diameter, gusty winds and torrential rain leading to localised a risk of flooding. An isolated weak tornado cannot be ruled out either, particularly where winds back ahead of approaching shortwave trough. But given time of year and low-end instability, risk does not reach severe criteria.

Issued by: Nick F - Forecaster for Netweather

Edited March 16, 2013 by Nick F

[Raidan]

Expected to wake up to a dull overcast sky but the sun is out so at the moment I feel maybe something could happen,going to be a day of radar and sky watching,good luck all hope you get something.

[East_England_Stormchaser91]

Looking not bad for SW areas today. Already some decent looking showers showing on radar and look to have a bit of rotation amongst them as well. Good luck you lot, you could hit first blood this year with some thunder, if we take thundersnow out of the equation!

Edited March 16, 2013 by East_England_Stormchaser91

[Karl83]

Could this mean a good chance of a funnel in the channel today?

[ardsar]

Had one or two showers this morning in Plymouth. Nothing too significant as of yet. There have been a few sferics across the channel already this morning, but nothing organised.

[Raidan]

Go to Bristol channel or not go Bristol channel that is the question,can't decide what to do.

[AWD]

Go to Bristol channel or not go Bristol channel that is the question,can't decide what to do.

Go to Portishead, at least you can have fish & chips while waiting any action.

In all seriousness though, the cells are developing rather randomly so I don't think any one specific location in our vicinity is favourable over another.

[Trikx198]

Reporting some small hail about 10 minutes ago in Bristol