Ahead of this band of rain, some early elevated convection / showers will be possible over East Anglia and the East Midlands. Assuming sufficient cloud breaks can occur by late morning and into the afternoon, surface heating could lift 2m temperatures to 21-22C with dewpoints of 16-17C possible. A strengthening northeasterly wind will allow advection of this warm, moist low-level air west/southwestwards towards the northern Home Counties and south Midlands/M4 corridor, where an area of enhanced convergence may evolve by the afternoon hours. If all ingredients come together, surface-based thunderstorms could develop just ahead of the main rain band, perhaps also aided by some modest orographic forcing in the area. Forecast profiles exhibit fairly unidirectional winds, although some backing of the low-level winds will be possible. Speed shear is fairly low due to similar strengths at most levels, but stronger flow in the upper troposphere is available if deep convection can grow tall enough. Moist profiles (with a large proportion of the cloud layer below the freezing level) and back-building could result in some locally large rainfall totals. Current NWP guidance suggests anywhere from 50-70mm to as much as 100mm may be possible very locally, with hourly accumulations of 20-30mm. A SVR has been issued primarily for the risk of exceptionally high rainfall totals and associated flash flooding, but this will be fairly localised and not affect the whole area highlighted. If surface-based convection can develop, then low cloud bases and vorticity stretching near convergence zone could produce an isolated tornado. However, the situation is very finely-balanced, and insufficient surface heating or a faster northward motion of the rain will probably eradicate this potential.

Also, depending on quickly rain can clear from Cen S / SE England (and indeed how extensive such rain/cloud is in the first place), there may be scope for surface heating here later in the afternoon that could foster surface-based convection too. In some ways, faster motion of rain northwards decreases the potential in S Midlands but increases it in SE England, and vice versa. Any thunderstorms will probably weaken eventually towards mid/late evening, but a new wave of rain with embedded elevated convection will likely push out of France towards southern England during the overnight period as the next PVA lobe rotates around the main upper low. Again, lightning may be rather infrequent/isolated given moist profiles and skinny CAPE.