Upper-level trough extending SE across far western Europe will disrupt into a cut-off upper level low over the Bay of Biscay today. At the surface, an area of low pressure centred over Wales this morning will drift slowly south and fill, with associated slow-moving occluded frontal system lying across EIRE and central UK, while cold front moves slowly east across eastern England with an unstable Polar maritime (Pm) airmass edging east across Wales, Midlands, southern and western England later this morning and into the afternoon.
... S IRELAND, WALES, W ENGLAND, MIDLANDS and C/S ENGLAND ...
Clearer skies with sunny spells following cold front with its bands of rain moving east across England this morning coupled with increasingly cold mid-level temperatures towards the west will create steep lapse rates across the above areas with surface heating in sunshine. This will support the development of scattered heavy showers and isolated weakly-electrified thunderstorms into the afternoon and early evening, before diurnal heating wanes after dark and storm risk fades. Any showers/storms may produce hail, gusty winds and localised flooding. No severe weather is expected - given weak shear where strongest instability is indicated - coupled with cool surface airmass and shallow instability.
What do the risk levels mean?
Thunderstorms - Severe thunderstorms unlikely to occur, slight risk of hail, gusting winds and localised flooding.
Slight Risk - A slight risk of severe convective weather exists. Hail >2.0cm diameter, wind gusts exceeding 50mph or 5-10% risk of a tornado within 50 miles of a point, or > 30% risk of localised flooding.
Moderate Risk - A moderate risk of severe convective weather exists. Hail 4-5cm in diameter, or wind gusts of 60-70mph, or a 10-15% chance of a tornado within 50 miles of a point, or >50% risk of localised flooding.
High Risk - A high risk of severe convective weather exists. Rarely, if ever used on the UK. Hail >5cm, or wind gusts in excess of 80mph, or >20% risk of a tornado within 50 miles of a point, or >70% risk of localised flooding.