Largely dynamic (frontal) rain will already be affecting the English Channel and southern counties of England at the beginning of this forecast period on Friday morning, largely driven by increased forcing aloft from an approaching shortwave. Given the presence of some (limited) mid-level instability, embedded convection is likely and a few sporadic lightning strikes possible - more especially over the English Channel / Channel Islands / northern France. A few elevated showers may even develop ahead of this main band of rain. In either case, the intensity of this feature is likely to gradually weaken during the morning as it drifts further north across southern England and south Wales. A few showers may develop over W/SW Ireland during Friday afternoon on the leading edge of the more organised band of rain, and a low-end SLGT has been introduced to cover this risk.

The attention then turns to the late afternoon / evening hours, as the left exit region of a mid-level jet streak approaches NW France, the resultant upper divergence creating a lifting mechanism and combining with increasing mid-level instability due to low-level warm advection and cooling aloft. Instability will increase across northern France and the English Channel by this time, with 400-700 J/kg CAPE, and scattered elevated showers and thunderstorms are expected to develop from ~13z onwards, initially close to northern France but expanding in coverage while migrating northwards across the English Channel towards and during the evening. Initially, lightning could be fairly frequent with the strongest cells (northern France / English Channel), given a favourable overlap of instability and increasing shear through the cloud-bearing layer. 

However, main concern surrounds how much lightning activity will actually occur over inland parts of southern Britain - guidance continues to suggest that much of the lightning activity over the English Channel will weaken significantly as it approaches the south coast and move inland (aside from perhaps SW England). This ultimately affects how far north to place the northern edge of the SLGT area. Adjustments may be required to the shape and extent of the SLGT area as new data becomes available. The bifircating steering flow, with cells heading N/NNE over the eastern English Channel but NW over the western English Channel, also makes it difficult to assess which areas have the greatest risk of lightning.