Elevated convection will begin to spread into southern England by mid to late morning and this may produce lightning for south-central England for a time around midday and early afternoon, perhaps re-invigorating with some surface based convection into the E Mids during the late afternoons and early evening. 

However, the main risk areas will be across Wales and NW England...Diurnal heating with surface temps climbing into the mid to upper 20s and Tds of 15-17C will develop across the Midlands and into Wales and NW England during the afternoon. This combined with surface convergence and 600-800 J/kg of CAPE will help to trigger some surface based showers and thunderstorms across the Slight risk area by mid to late afternoon. These thunderstorms have the capability to produce some frequent lightning as they develop through the late afternoon and into early evening.

Isolated thunderstorms are expected to occur elsewhere outside of the the Slight risk area, more especially across SW England, eastern parts of Ireland and perhaps into central and eastern parts of Scotland by the late evening and overnight.