MODERATE RISK OF SEVERE STORMS

Issued 2014-07-19 08:25:26
Valid: 19/07/14 0900z to 20/07/14 0600z
THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FORECAST

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Synopsis
Long-wave trough just west of western Europe continues to slowly advance eastward today, southerly mid-upper jet streak continues across France and British Isles ahead of this advancing trough, with a plume of warm moist air continuing to advect north, though beginning to be displaced east later in the day. A slack area of surface low pressure covers the UK, with a frontal zone lying SW Scotland down to Dorset at 12z (noon), with a warm/moist surface airmass that will be very unstable to surface heating to the east of frontal zone.

… ENGLAND and WALES …

Overnight storms will continue to spread north across England and Wales this morning, with further elevated storms moving north out N France in next few hours. Dependent on insolation breaking through cloud cover from morning convection later, any surface heating will allow temperatures to soar to the mid or upper 20s deg C this afternoon across S England. Combine this with dew points of 19-20C (already 19C observed in the SE) – large amounts of CAPE are indicated by models. GFS shows up to 1600 j/kg MLCAPE and up to 2000 j/kg SBCAPE this afternoon across SE England.

Breeze convergence ahead/near frontal zone and large scale ascent of airmass with approaching upper trough will likely erode any CINH/cap in place this morning, to allow storms to develop should surface heating take place. Strong mid to upper southerly jet and winds backed SE towards the surface will generate sufficiently strong vertical shear for any storms that develop to quickly organise into muticell and even supercell structures given large CAPE too. Such storms may produce large hail and damaging wind gusts.
Given potential for surface-based storms, any old outflow boundaries from morning storms and influence of backed winds near convergence zones, areas of strong low-level shear may develop – which may be sufficient for rotating mesocyclones on supercells that develop – which may produce one or two tornadoes too.

Therefore, have issued a Moderate risk across southern, central and eastern England – where surface heating is most likely and thus risk of severe thunderstorms.

SEVERE THUNDERSTORM WARNING ISSUED

SKYWARN UK SEVERE WEATHER WARNING #026
ISSUED: 1500UTC FRIDAY 18TH JULY
SKYWARN UK SEVERE THUNDERSTORM WARNING NO:26 ISSUED AT 1500UTC FRIDAY 18th JULY 2014.

VALID FROM 1800UTC FRIDAY 18TH JULY 2014 TO 1200UTC SUNDAY 20TH JULY 2014

SKYWARN UK FORECASTS THE POTENTIAL FOR SEVERE WEATHER AS FOLLOWS: TORRENTIAL RAIN, FLASH FLOODING, THUNDERSTORMS WITH REGULAR C-G LIGHTNING ALSO THE POTENTIAL FOR SUPERCELL DEVELOPMENT. THIS WARNING COVERS THE MAJORITY OF THE UK BUT IN PARTICULAR THE SOUTHERN HOME COUNTIES, SOUTHEAST ENGLAND, THE MIDLANDS, EAST ANGLIA AND LINCOLNSHIRE. CIN MAY WELL INHIBIT STORM INITIATION IN THE FAR EAST OF EAST ANGLIA.

DISCUSSION:
THERE IS NOW GOOD MODEL AND PARTNER AGENCY CONFIDENCE IN THE POSSIBILITY OF SEVERE WEATHER THROUGHOUT THE PERIOD. A WARM, MOIST AND UNSTABLE AIRMASS WILL EXTEND INTO SOUTHERN COASTAL REGIONS OF ENGLAND LATE FRIDAY AFTERNOON AND EXTEND FURTHER NORTH AND EAST DURING THE EVENING FROM A TROUGH ASSOCIATED WITH THE COMPLEX LOW OVER THE UK AND NEAR CONTINENT. CAPE OF OVER 1500j/Kg WITH LI VALUES OF -2 TO -5 IS MODELLED OVER A WIDE AREA. OVERNIGHT FRIDAY AND INTO SATURDAY MORNING FURTHER ELEVATED PLUME DESTABILISATION WILL OCCUR AND IF UPDRAFTS ARE STRONG ENOUGH STORMS WILL INITIATE CLOSER TO THE SURFACE. DURING SATURDAY THIS COULD BRING A WIDESPREAD RISK OF TORRENTIAL RAIN WITH FLASH FLOODING. THERE IS ALSO A POSSIBILITY OF LARGE HAIL ANYWHERE WITHIN THE WARNING AREA. SOME OF THESE AREAS COULD WELL SEE DAMAGING WIND GUSTS DURING ANY STORMS. THERE IS ALSO POTENTIAL FOR SUPERCELL DEVELOPMENT GIVEN THE DIRECTIONAL SHEAR AND CAPE VALUES . THIS IS AN EXTREMELY VOLATILE SITUATION AND THIS WARNING MAY BE UPDATED TO INCLUDE OTHER AREAS. PLEASE MONITOR ALL WEATHER AND TRAVEL INFORMATION AS NECESSARY AS THERE IS A SIGNIFICANT RISK OF DISRUPTION.

SPOTTER ACTIVATION IS REQUESTED AND SPOTTERS ARE REQUESTED TO REPORT ALL FACTORS EXCEEDING ACTIVATION CRITERIA.