Meanwhile, diurnal heating and deep mixing of the boundary layer over France is expected to be sufficient to erode the ~900mb cap and yield >1,500 J/kg CAPE. Numerous waves of thunderstorms are likely to erupt over France from around 11-12z onwards, and this casts considerable uncertainty as to how things evolve during the evening and night over the UK. Such clusters of thunderstorms in France will ultimately work over the atmosphere and alter the thermodynamic profile (thus depleting CAPE), and it is rather unlikely model guidance will be able to (even at short lead times) capture this particularly well. If, initially surface-based, thunderstorms do develop near the Brest/Cherbourg peninsula and drift offshore, they may slowly weaken as they approach the south coast (but could be very electrically active initially, especially near the Channel Islands for example). In either case, the gentle southeasterly flow at ~850hPa will maintain gradual advection of unstable air from France towards S/SE England and into the Midlands through the evening and night.

During the late evening more substantial height falls aloft are likely, and with increasing synoptic-scale lift this will probably promote greater forced ascent and a better potential for elevated showers/thunderstorms to develop along the residual instability plume, especially by late evening and into the early hours. Storms initially could be potentially quite electrically active given the magnitude of instability, with cloud tops exceeding 30,000ft, but likely eventually morphing into larger areas of rain with a reduction in lightning activity as this lifts slowly north across the Midlands and East Anglia later in the night. Therefore, unlike many events where thunderstorms are advected into southern England from the Channel, the potentially late initiation suggests cells could develop close to the coast or even inland instead (but there will naturally be an error bar of a couple of hours which will make the difference between offshore vs inland initiation). Cells will be rather slow moving given the fairly weak steering flow, and this could lead to spot totals of 40-70mm and the risk of surface water flooding - a SVR has been issued because of this risk.