Due to a combination of both warm air aloft, having advected from France overnight, and a cooler easterly surface flow forecast profiles are capped to surface-based convection unless temperatures can reach at least the mid 20s Celsius - which seems rather unlikely. The main exception is W / SW Scotland where capping aloft is much weaker, and the easterly surface flow aiding fohn effect to the west of high ground may allow a few heavy showers or weak thunderstorm to develop for a time in the afternoon / early evening, and a low-end SLGT has been introduced.

Meanwhile, a day of intense heating is anticipated across France with TMax of 25-30C. This is likely to result in a deeply mixed boundary layer, and a gradual eroding of the 850hPa capping inversion. A longwave trough axis associated with a large upper low over the Atlantic will swing northeastwards across Biscay and into northern France by evening. Cooling aloft/lift associated with this will likely aid in deep convection erupting over France by late afternoon and into the evening. Some of this may be surface-based initially, but even with nocturnal cooling of the boundary layer and/or cooler English Channel the substantial warm nose at ~900hPa could also foster elevated convection both over France and also the English Channel. It should be stressed even at this relatively short lead time there is a large amount of uncertainty as to exactly how things will evolve during Sunday evening and night in terms of coverage and location of areas of showery rain and thunderstorms, but there is certainly an increasing risk as the trough continues to migrate northeastwards.

Given the south/southwesterly upper level flow, rain from convection will likely fall to the northwest of storm relative motion, and therefore it is most likely new cell development will occur to the east/southeast where the best supply of unstable air resides. As such, and considering climatology, a corridor from northern France to SE England is most favoured for the greatest lightning coverage and so have tentatively included a MDT. However, the instability axis expands much further west and so it is certainly plausible additional elevated thunderstorms could develop perhaps as far west as E Devon to SE Wales. CAM guidance, albeit inconsistent, has been periodically suggesting rather outflow-dominant storms with cold pools over the English Channel and adjacent portions of France, and wind gusts of 50-60mph. The net result is an expanding rain shield, with new cells building progressively eastwards on the eastern flank. Some marginally-severe hail may be possible from the strongest cells in the vicinity of the English Channel.

The instability axis will slowly migrate northeastwards through the night, and so upticks in heavy showers or few thunderstorms will continue to be possible through the night into the Midlands and East Anglia too, possibly Yorkshire towards 06z Monday, but attempting to pinpoint specific areas is rather challenging.