Between an upper ridge over western Europe/North Sea (surface high over Scandinavia), and a longwave upper trough to the west of Biscay, a broad southeasterly flow will continue to advect relatively high Theta-E northwestwards across the British Isles from France. Showery rain/elevated convection will likely be ongoing at 06z Saturday across the Channel Islands, western English Channel, SW England, SW Wales and parts of Ireland in the vicinity of a NW-SE orientated occlusion. The extent of cloud cover associated with this activity as it drifts slowly northeastwards (individual clusters moving northwestwards) may, to an extent, impact the potential for deep convection later in the day, especially over Ireland. Ahead of this zone of cloud and showery rain, strong heating across the Midlands, Wales and southern England with Tmax of 24-27C anticipated widely and locally 30-31C in/around London will lead to a deep and well-mixed boundary layer. Analysis of forecast soundings suggest near-surface temperatures of 26-27C or higher will generally be required to overcome low-level capping, while very dry air persists in the mid-levels due to subsidence associated with the nearby upper ridge. This casts some uncertainty over how much deep convection will be able to develop, and it is plausible convective cloud may undergo several attempts at gaining height before become sustained, delaying true initiation until later in the afternoon. Nonetheless, profiles are anticipated to moisten fairly rapidly in the mid-levels across Wales and the West Midlands towards and beyond mid-afternoon as a shortwave approaches from the south (so here earlier initiation perhaps most favoured), while at the same time a surface low is signalled to develop over the SW Midlands. A brisk southeasterly wind across central and eastern England will aid moisture pooling along a strengthening NW-SE low-level convergence zone roughly in the vicinity of the M40-M6 corridor, with near-surface dewpoints of 15-18C signalled. This, in conjunction with orographic forcing in Wales and the West Midlands, will likely provide the focus for scattered, relatively high-based thunderstorms to develop in a fairly explosive manner, especially late afternoon and into the evening hours, growing upscale into a larger cluster across N Wales and parts of NW England.

Forecast soundings suggest the potential for MLCAPE on the order of 1,000-1,500 J/kg generally, but perhaps locally 2,000 J/kg where higher dewpoints exist, with cloud tops potentially to 40,000ft (-60C or colder). While flow aloft will be fairly weak (~15kts at 500hPa, for example) a reasonable degree of low-level wind shear is evident with slightly backed near-surface winds. Despite limited shear, the magnitude of instability may be sufficient for the strongest cells to become better organised with potential for some transient supercell characteristics. All-in-all, the environment will likely be conducive for some very active, slow-moving thunderstorms that may train over similar areas given steering flow roughly parallel to CZ, greatly increasing the risk of significant surface water flooding (30-40mm in less than 1 hour), especially over urban areas. Hail will likely be sub-severe for the most part, but any stronger/sustained cells could potentially produce hail ≥2cm in diameter. A SVR has been introduced for the risk of flash flooding, and to a lesser extent for locally severe hail. The deep, well-mixed boundary layer/inverted-V profiles suggest the potential for rather gusty winds, 40-50mph in the strongest cells. Greater coverage of thunderstorms is anticipated across the W/NW Midlands, C/N Wales and into NW England, where associated MDT and HIGH risk areas have been introduced, while activity will be more isolated/conditional further southeast towards London. That said, similar setups in the past have sometimes produced a rogue thunderstorm to the NW of London in the mid-late evening.

While nocturnal cooling at the surface occurs through the evening/night, substantial CAPE will persist given the existing deep mixed layer in the low/mid-levels, and with strengthening 600-850hPa flow through the evening this will advect more rapidly northwestwards across the Irish Sea, and eventually towards SW Scotland/Northern Ireland, and later W/NW Scotland. As such, there may be an uptick in elevated thunderstorm activity over these areas during Saturday night as the shortwave over Wales at 18z continues to migrate northwestwards.

Elsewhere, a couple of surface-based showers/thunderstorms will be possible over north Devon, the Pennines and SW Scotland during the afternoon/evening hours, and perhaps also a few over Connacht although this risk is much more dependent on the degree of surface heating given proximity to cloud and areas of showery rain at times through the day. There is also the chance of elevated thunderstorms emanating from northern France and/or developing in-situ across the Channel Islands/Cen S and SE England along the instability axis during the overnight period as mid-level vorticity from earlier French convection arrives from the south, but confidence in much activity here is rather low at the moment and so have refrained from including this risk for now.