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
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.