Convective Outlook - UK/Ireland - Fri Aug 20th

[martinastro]

Valid: 20/08/2010 00:00 - 21/08/2010 00:00
Headline: ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FORECAST...

Synopsis
Upper/surface low to the NW of Ireland on Friday will drive a cyclonic SW flow across the UK, with a cold front lying Peterhead - Middlesborough - Plymouth at 12z moving east with sfc low lying N of Scotland by midnight, while cold front develops wave from the SW as it moves across S England.

… EIRE, WALES, W MIDLANDS, N ENGLAND and S SCOTLAND …

Warm moist airmass advecting NE with high theta-w values (wet bulb potential temps of 16C+) may destabilise with an isolated t-storm or two possible across Wales and Ireland tonight as forcing increases with arrival of cold front and shortwave trough from the SW. Any storm may pose a risk of an isolated strong wind gust and torrential rain with localised flooding.

During Friday, CAPE is modelled to increase ahead of cold front across parts of Wales, W Midlands and N England ... therefore convection may develop with approaching cold front from the W by midday over these areas with a few thunderstorms possible. Any storms that develop will benefit from 30 - 40 knots of 0-6km deep layer shear ... so organisation into multicell/line segments may occur with threat of strong convective wind gusts to 45-50 knots. Strong 0-1km low-level shear up to 30-35 knots is also modelled to develop as winds back ahead of approaching cold front, so there may exist risk of an isolated tornado - if a strong storm with rotating updrafts can develop ... though this could be restricted somewhat by cloud cover/lack of insolation to develop enough instability. However, will issue a SLIGHT risk for parts of N/E Wales and N England given favourable severe kinematics and potential instability.

Thunderstorms will also be possible in the morning and early afternoon across Eire and Scotland post cold front, again with a risk of strong convective wind gusts and an isolated tornado, given strong deep layer and low-level shear in place. Credit Nick Finn from Netweather

[Padraig OBrien]

some thunderstorms down in SW Rep Of Ireland atm really letting off the sferic now

[martinastro]

TORNADO WATCH 2010/004

A TORRO TORNADO WATCH has been issued at 2315GMT on Thursday 19th August 2010

Valid from/until: 23:15- 08:00GMT on Thursday 19th/Friday 20th August 2010 for the following regions

Parts of (see map)

Eire

N Ireland

S Scotland

Isle of Man

THREATS

Tornadoes; wind gusts to 65mph; CG lightning; heavy rain

DISCUSSION

Strong upper low is moving NE to the west of the British Isles, with an associated short-wave crossing the watch area. A broad, very moist tropical maritime airmass exists at low levels, behind a warm front, which is currently moving through Scotland. A cold front to the west of Eire/N Ireland will start to move eastwards later tonight, and tend to surge eastwards later. Instability exists in the warm sector, partially aided by mid-level cooling associated with the upper trough. Thunderstorms are occurring already across SW Eire, and further showers and a few thunderstorms are likely overnight. Strong deep layer shear (40-50 knots) should promote cell organisation, perhaps supercells. Low-level shear of 25-30 knots suggests low-level mesocyclones are possible. Tornadoes and damaging winds are possible across the area, and the magnitude of the low-level shear coupled with a very moist boundary layer suggests a small chance of a stronger tornado.

The risk will extend into Scotland through tomorrow morning, and a new watch is likely to be needed tomorrow morning.

Forecaster: RPK.

[martinastro]

Updated TORRO forecast for Britain today...

http://www.torro.org.uk/site/forecast.php

[JohnC]

RainToday showing very heavy rain out of Wales towards Birmingham passing  just to the north of Gloucester but atm (12 noon) we have heavy rain but that will pass through soon.