The main focus for lightning, however, will be associated with the post-frontal environment as cool air aloft overspreads a moist low-level airmass with dewpoints typically 15-17C widely. The coldest mid-level air will be associated with the upper low, over Ireland during the day, and swinging across southern Britain during the evening/night. Modest surface heating is likely to yield 500-900 J/kg CAPE, with the potential for scattered showers and a few thunderstorms to develop - especially aided by low-level convergence and orographic forcing, but also PVA lobes within the base/forward side of the upper low. Speed shear is a little weak below 500mb, but some increasingly strong flow aloft will yield reasonable cloud-layer shear, especially when coupled with backed low-level winds. Overall, the environment may be conducive for some cell organisation and hence longevity, and there may be scope for an isolated supercell in the West Country in particular (e.g. Somerset / Dorset / Wiltshire). If an organised thunderstorm can develop it could pose the risk of some marginally-severe hail and perhaps an isolated tornado. Otherwise, the main hazard, aside from lightning, is the risk of local surface water flooding.

The forecast evolution in Northern Ireland is less clear, with the potential for frontal rain to potentially linger and limit instability. However this will be monitored and an additional SLGT could be issued if confidence increases. Elsewhere, while instability will gradually reduce during the mid/late evening onwards due to nocturnal cooling of the boundary layer, forcing aloft and residual humid airmass will still pose the risk of some showers and the odd thunderstorm overnight - especially across East Anglia and SE England - as the main upper low shifts eastwards across southern Britain.