Frontal rain will migrate northwards across the British Isles on Wednesday morning, with a clearance then developing across southern areas coinciding with peak daytime heating. Cold air aloft combined with surface heating will generate 300-600 J/kg CAPE within an environment with notably steep mid-level lapse rates. Scattered showers are expected to develop, initially over southern England on Wednesday morning, the risk then extending northeastwards across parts of the Midlands and East Anglia into the afternoon. A few thunderstorms will be possible, particularly focussed where low-level convergence is maximised (especially in a zone from SW England across the M4 towards East Anglia, bounded by the SLGT area). Shear is somewhat limited given similar wind speeds / direction through depth. Hail up to 1.5cm in diameter is possible from the most intense cells.

Some uncertainty exists over the potential for more cloud and showery outbreaks of rain that could develop over SW England during the afternoon hours, associated with an area of low pressure running eastwards nearby (over the western English Channel). NWP guidance differs in its handling of this feature, and if this cloud/rain does push into SW England (and further east later in the afternoon) then this may reduce the overall thunderstorm potential, especially in the southern portion of the SLGT. Meanwhile the far north of the SLGT area (i.e. Lincolnshire) is subject to slow cloud clearance following morning rain - some model guidance restricts instability to The Wash southwards.

Any showers/storms that do develop will tend to weaken during the second half of the evening as nocturnal cooling of the boundary layer commences - though some forcing from low-level convergence may persist along the axis of the elongated area of low pressure well into the night.