Outbreaks of rain across the eastern half of Ireland/Northern Ireland, perhaps with embedded convection/thunderstorms, will slowly clear northeastwards through the day. The extent of cloud in its wake is one cause for concern, especially in Ulster and NE Leinster given a later clearance. Elsewhere, convective overturning should allow some reasonable cloud breaks to occur, increasing surface heating as the day wears on. This will lead to a very unstable environment given proximity to the upper low to the west, and associated cold pool aloft. Assuming near-surface temperatures are able to reach the low 20s Celsius, with dewpoints in the 15-17C range, then modified forecast soundings suggest MLCAPE on the order of 1,000-1,500 J/kg could be available. Given the moist boundary layer, SBCIN will be relatively small and showers could develop fairly quickly once sufficient breaks develop in the morning cloud cover. However, rather skinny CAPE below 700hPa suggests slow parcel accelerations initially, and so it may take some time with additional heating required before heavy showers/convective cloud can become sustained. Nonetheless, a number of heavy showers and active thunderstorms are likely to develop through the afternoon and evening hours, especially away from the south coast and extreme east coast, aided by increasing low-level convergence within the fairly slack regime. 

The proximity to the upper low will also be favourable for broad ascent as a series of PV lobes slide northwards across Ireland on its eastern flank, in tandem with the left exit region of a powerful jet streak over the Celtic Sea. Modest flow aloft and backing of the low-level winds will yield 15-20kts effective bulk shear, allowing some organisation of cells and perhaps promoting upscale growth in multicell clusters. The broad thermodynamic/kinematic environment also suggests the potential for large hail in the most organised/strongest cells, potentially 2-4cm in diameter. While cloud-layer shear is modest, it may just be sufficient when coupled with the magnitude of CAPE for an isolated supercell, this perhaps most likely in counties adjacent to the Northern Ireland border and into NE Leinster where the low level flow is most backed. Considering the fairly low cloud bases and rather sharply veering winds with height in the lowest 1km (albeit with relatively weak inflow) an isolated tornado cannot be ruled out - although naturally the risk in any one location is very low. A SVR was introduced primarily for the risk of large hail.

In a similar fashion to the past couple of days, diurnal heating may aid in the development of a few heavy showers or thunderstorms over high ground and/or near to coasts along zones of enhanced low-level wind convergence. While coverage is uncertain, any showers/storms that form in Wales/SW England will generally drift to the NNE, while in Scotland the steering flow will encourage cells to move to the NW. In all cases, the potential may be negatively impacted by extensive cloud/showery outbreaks of rain associated with an occlusion, and this casts some uncertainty over how much shower/lightning activity may actually occur. Worth noting, any deep convective cloud that can develop over Wales and SW England in particular will have the potential to tap into some very strong southwesterly flow aloft associated with a jet streak in the vicinity (albeit perhaps also inducing some upper-tropospheric capping and limiting convective depth).

CAM guidance has been fairly consistent in generating a wave of elevated thunderstorms over northern France and the mid/eastern English Channel on Saturday night, atop a fairly notable warm nose in the 900-950 hPa layer and in an environment with 20-30kts effective shear, as a developing PV lobe arrives from Biscay. 

Elsewhere, odd showers/thunderstorms may be possible almost anywhere through Saturday night given residual instability within the broad Theta-W plume and subtle features aiding forced ascent, but attempting to pinpoint specific areas at greater risk is rather difficult. It is plausible an uptick in activity could occur over Wales, the Irish Sea, eastern Ulster, SW Scotland and more broadly across parts of the Midlands and northern England, but low confidence on how widespread or frequent any lightning may be.