As a sharpening Atlantic upper trough continues to dig southwards towards Iberia, while pivoting slowly northeastwards, warm advection will occur on the forward side from France towards S/SW Britain, especially so on Friday evening/night. Cooling aloft and gradually falling heights with a shortwave running northwards on the eastern flank of the upper trough, atop this increasingly warm, moist airmass, will encourage convective initiation across northern France and the English Channel on Friday evening, with a few elevated showers and perhaps thunderstorms forming (given steep mid-level lapse rates) and then moving towards the N/NNW on Friday night.

NWP guidance continues to offer differing solutions - UKMO suite consistently taking most mid-level activity across SW England and S Wales, whilst a blend of GFS/ECMWF prefer a more easterly path towards Cen S England then into the Midlands by Saturday morning. Models notoriously struggle with elevated convection, both in placement and extent/intensity, and so it is unsurprising to see the differences at this range. As is often the case, bifurcating flow aloft could potentially produce 2 primary axes of interest, one perhaps towards Devon/Cornwall and another towards IoW/Hampshire/Dorset, with a relative minima in between. 

For now, too much uncertainty exists to narrow down areas of interest, hence the broad LOW, although it is noted that most guidance since Wednesday evening have tended to shift a little farther westwards. Instability also looks marginal which might limit how electrically active such mid-level showers will be. Given the complexity, this forecast may be updated nearer the event.