Deep convection will be ongoing at the start of the forecast period over the Celtic Sea, associated with a slowly-filling cut-off upper low. This convection will tend to weaken during the morning hours, with the focus then shifting northwards across the Republic of Ireland and eventually Northern Ireland. Under the trough axis, shear will be fairly weak and so pulse-type showers/storms are most expected - hence any lightning will tend to be short-lived and sporadic. Worth noting that subsequent model guidance have tended to make showers increasingly isolated compared to earlier runs, causing some uncertainty over likely coverage.  Showers/storms will gradually decay through the evening, though further showers are likely close to Atlantic coasts overnight.

On the forward side of the upper trough, a frontal wave will develop over France, pushing rain northwards into SE England and East Anglia. The exact track / shape of this wave remains uncertain, and hence timing and areas affected. Either way, on the eastern flank some weak instability may be present, with perhaps an isolated lightning strike - hence the inclusion of a LOW threat level.