Upper trough will continue to push SE into mainland northern Europe today, the back edge of the trough influencing eastern England, while heights rise off the Atlantic across northern and western Britain. At the surface, an area of low pressure will drop SE from the Norwegian Sea toward the North Sea by midnight, associated frontal system ahead of the low will bring cloud and rain to Scotland, mostly dry EIRE + Wales, with an unstable and showery Nly flow across England.
... ENGLAND ...
Cold air aloft towards SE UK (500mb temps AOB -35C) in association with departing upper trough will create steepest lapse rates here and the strongest likelihood of thunderstorms as diurnal heating increases instability by and through the afternoon. Isolated lightning from heavier showers can't be ruled out anywhere across England though. Any storms will be weak, given dry polar airmass and lack of warmth, but will produce small hail or graupel locally and perhaps gusty winds too.
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.