Diurnal heating of a warm and relatively moist surface airmass will likely develop some marginal instability across these areas - though in reality, surface-based parcels likely capped by a warm nose around 850mb suggesting lightning rather unlikely given limited depth to any convection. Given also little signal from most NWP output (ignoring GFS with its dewpoints of 16-18C vs EURO4 with 11-13C), have refrained from issuing a LOW threat level for now - but will need monitoring. Some residual elevated instability may also produce the odd shower over the Midlands into East Anglia too.

Upper trough west of Iberia will continue to disrupt southwards, backing the flow and allowing advection of high WBPT airmass from Biscay to occur into southern England on Wednesday night. Falling heights aloft will result in an increase in coverage of elevated thunderstorms, though the exact forecast evolution is marked with some uncertainty.

The vast majority of guidance suggests a few thunderstorms will develop over the English Channel and/or S/SW England 23 - 02z, expanding in coverage through the remainder of Wednesday night as the focus shifts northwards and eastwards over S Britain as the new warm front lifts north - hence in a broad sense, probabilities per given location increases as you head farther north and east across the SLGT area. However, the ECMWF has been fairly consistent in keeping the main upper forcing a little farther south, and hence any thunderstorms may only graze the extreme south coast towards the end of the night. At the same time, it also allows more activation of the frontal boundary towards Northern Ireland and western Scotland. Given the ECMWFs history of generally having a good handle (compared to many other models) on medium-level instability events, this casts a little doubt on how widespread thunderstorms may be over southern England - though the degree of instability and very steep mid-level lapse rates would suggests any thunderstorms that do occur would be prolific lightning producers, perhaps capable of producing some hail and gusty winds. A MDT may be issued should confidence improve.