A broad westerly flow covers the British Isles on Tuesday, under an upper ridge. However, low-levels remain reasonably moist courtesy of the remnants of a decaying frontal boundary, which combined with diurnal heating and low-level convergence will encourage convective development during Tuesday daytime.

Numerous showers will develop almost anywhere from the M4 northwards, though with weak instability and capping at 600mb lightning is rather unlikely. Hence for most areas, the risk of lightning is considered very low / around 5%. However, a relatively slack surface pattern may promote the development of a few funnel clouds. Any showers that do develop will tend to decay during the evening hours as daytime heating subsides.

On Tuesday night, frontal rain will spread north across most parts of the British Isles, as an area of low pressure approaches from the Atlantic. During the early hours, much stronger forcing aloft, combined with increasingly steep mid-level lapse rates (as cold pool aloft approaches) and a few hundred J/kg CAPE may result in some embedded convective elements on the rear side of the cold front - and hence it is this, and the post-frontal environment, that is more conducive to producing lightning during this forecast period. Main area of interest is SW England, Cen S England, S Wales, Celtic Sea and perhaps the extreme south of the Republic of Ireland.