Monday 5th Sept - Storm/Convective Forecast Discussion & Reports

[Nick F]

I see there is a thread ongoing for wet and windy weather likely on Tuesday, as isobars pack tightly towards the low arriving close to the NW. But there is some potential for storms on Monday too, so thought I would start a new thread with a storm forecast:

Convective / Storm forecast - Issued 04/09/2011 23:30

Forecast Summary Map

Click for full size

Valid: 05/09/2011 06:00 - 06/09/2011 06:00

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

Synopsis

Upper trough crosses the UK during Monday morning, clearing into the N Sea during the afternoon. At 12z Mon, surface low expected centred east of Orkney Islands, with an unstable returning polar maritime W to SW flow across UK.

… SCOTLAND, ENGLAND, WALES …

Modestly unstable W to SW'erly flow across the UK on Monday will allow numerous showers to develop widely. GFS indicates 100-300 j/kg MLCAPE late morning - early afternoon across S Wales, S England and also across Scotland, N and E England - so here we may see isolated to scattered thunderstorms develop aided by insolation ... t-storms possibly most numerous across the Scotland and N England area where steepest lapse rates are indicated, though a few isolated storms are possible elsewhere too. Increasing susbsidence from the west by late afternoon ... as upper trough clears the E will likely reduce convective potential, and thus storms, by evening. Vertical shear will be fairly strong across England and Wales - up to 40-50 knts deep layer and 15-20 knts of 0-1km shear, so potential exists for convection to organise to bring a risk of strong wind gusts. Also storms maybe accompanied by hail, heavy rain ... and an isolated brief and weak tornado can't ruled out. But overall severe threat seems too low to warrant a categorical risk - given absence of warm moist airmass.

[ZONE 51]

GFS - 18z

Lifted index showing unstable air in a wide area monday by afternoon-yellow areas indicating most unstable over Northern areas with here the highest risk of thunderstorms breaking out, with light blue less unstable for most areas but enough for convective activity and if (they should do!)downpours can develop into larger clouds then we would also see thunderstorms for other areas but more likely to be well scattered.

500hpa chart showing some very cold upper air on monday, this brings the risk of incredible iced anvils and also hail (;

Edited September 5, 2011 by ElectricSnowStorm

[Coast]

Nothing at all from ESTOFEX but that may be down to staffing rather than a lack of storms in Europe. UKASF have a slight risk and a forecast as follows:

Forecaster: Dan

Last Updated: 22:19 Sunday, 4th September 2011

Valid: 00:00 Monday, 5th September 2011 - 23:59 Monday, 5th September 2011

Areas Affected:

SLGT: Northern Ireland, W Ireland, C, E + S Scotland, N England, N Wales, Midlands, East Anglia, SE England

Synopsis:

Sharp upper trough axis moves eastwards across the United Kingdom during the first half of the forecast period, followed temporarily by a weak upper ridge. A deepening upper low will approach from the Atlantic late in the forecast period.

Discussion:

Two main events of interest exist:

.. NORTHERN IRELAND, C, S + E SCOTLAND, N, C + E ENGLAND, N + E WALES ..

Cold air aloft will overspread these regions during the morning hours associated with the base of the upper trough. Diurnal heating will steepen lapse rates significantly, with up to 600J/kg CAPE possible. Scattered showers will become widespread across much of the British Isles, but attention is turned to the highlighted areas for quite a few pulse-type storms to develop, especially given ELT's of -35C.

The window of opportunity is rather restriced, however, with the best of any lightning activity expected from late morning until early afternoon. This is primarily due to the clearance of the upper trough axis by the mid-afternoon as the next Atlantic low pressure system rapidly advances from the west, and some subsidence is likely beforehand as a weak ridge develops between the two areas of low pressure.

Thus during the afternoon any lightning activity will become increasingly confined to eastern coastal counties, and then eventually mainly over the North Sea. As a result of this, and the fact that maximum daytime heating is likely to be acheived after the best potential has passed, we have issued a SLGT level due to the marginal conditions. Given cold air aloft, hail of up to 1.5cm is possible in stronger cells. DLS of ~30kts and LLS of up to 20kts would increase the potential for tornado development. Whilst the best shear increases into the evening hours, the most favourable conditions for thunderstorm development will have passed by this stage, limiting severe weather potential somewhat.

Of particular concern is the strength of the winds - it is likely that steering winds will be too strong for significant organisation and thus convection may become quite messy at times due to anvils/cloud tops being sheered. This may limit lightning coverage, and thus provides another reason for only a SLGT level at this stage.

.. NORTHERN IRELAND, W IRELAND ..

The next potential event for severe weather is in the evening and overnight. Some uncertainty exists due to the disagreement in timings and orientation of the cold front between the GFS and NAE models, and the final position of the Jet Stream, which will then determine the position of the right entrance/left exit. Nevertheless, it is quite likely that convection will develop/be embedded along the cold front with potential for sporadic lightning over W + N Ireland and Northern Ireland.

DLS >50kts accompanied by LLS >45 kts would suggest a significant tornado threat along the cold front in particular, with the potential for squall line/LEWP development. Indeed hail >2.0cm in diameter is possible as a localised event, accompanied by very strong and gusty winds, potentially damaging winds/SLW events.

This situation is borderline SVR threat level given the criteria it meets, but we will monitor conditions over the next 24 hours and may increase the threat level if necessary. This environment then moves southeastwards across England and Wales on Tuesday which will also need monitoring.

SkyWarn say:

e Weather Risk:

Low

Mostly cloudy with light rain, heavier showers across Wales and Northern England. Becoming very wet and windy into Tuesday, mainly Western and Southern UK.

The UKMO chart from 21st OWS is completely different:

The wind seems t be punching right through into SE England:

Ireland in the firing line today:

Not much CAPE about though

More convective rain, increasing in the afternoon

Here's where it starts to get interesting as we enter the evening and overnight:

The thunderstorm potential looks low to me, but maybe something in that NW corner today. Going to get windy though and maybe a funnel or two early evening and then windy windy overnight into tomorrow?

[IanR]

Had one crack of thunder here so far this morning

[user700]

Very heavy rain here and one loud crack of thunder.

Edited September 5, 2011 by danieluk

[Lauren]

Been pretty gusty here today.

[sunny scunny]

It's very dark here with heavy rain currently.

[Chris K]

Have had some beasty looking dark clouds pass eastwards just to the South of Bristol. I've personally had about 2-3 heavy showers that's about it.

[stormforce 10]

been raining heavily last half hour here on already saturated ground.

[sunny scunny]

The temp dropped to 12c during the last shower about 30mins ago.

[jk]

Two water spouts spotted at Barmouth, Unable to give anymore info was just told this by a person who saw them hopefully someone managed to film them.

jk.

[loopydreamer]

Two water spouts spotted at Barmouth, Unable to give anymore info was just told this by a person who saw them hopefully someone managed to film them.

jk.

woah kool! I have never actually seen a waterspout with my own eyes! seen pics and they look awesome!

[stormforce 10]

wind starting to pick up, raining quite heavily for the past hour.

[jk]

woah kool! I have never actually seen a waterspout with my own eyes! seen pics and they look awesome!

People did take videos of them just have to wait until they post them online or if a news station picks up on them not sure how newsworthy they are.

[Dom W]

Had some heavy showers earlier this afternoon - I think there was some thunder somewhere in the North East earlier too.