Pulses of elevated convection are likely to be ongoing first thing Saturday morning over NE England associated with a PV lobe and on the leading edge of a Theta-E tongue, drifting generally northwestwards across southern Scotland, while probably weakening during the afternoon as the upper forcing overruns and clears to the west. By the afternoon, residual mid-level instability plume will straddle central portions of Scotland. In general, the greatest risk of lightning (if any) will be on the eastern edge of such convective precipitation over the North Sea (close to E / SE Scotland), where forecast profiles reveal some fairly substantial instability rooted from the 900mb level. Beneath, a cool and moist boundary layer with extensive low cloud and sea fog is likely. A low-end SLGT may be considered for this area. Higher surface temperatures (20-23C) and cloud breaks are expected in western Scotland, but rather saturated profiles and skinny CAPE here suggests the risk of lightning with any cells that develop here is substantially lower. That said, an isolated thunderstorm cannot be ruled out if any minor impulses engage with this residual plume. 

Late on Saturday night a renewed uptick in elevated convection is expected over the North Sea, close to NE England and SE Scotland and so there could be a few sporadic lightning strikes here at the end of the night.

Many areas will have fairly extensive low cloud with hill fog on Saturday morning, but this will be gradually eroded with time. Strong surface heating combined with advection of a lower Theta-E airmass from France into southern Britain will result in a deep, well-mixed boundary layer and consequently despite surface temperatures increasing through the day, dewpoints are expected to decrease below 10C. This may initially develop a differential heating boundary / quasi-dryline that pushes north across the Midlands during the morning (dewpoints 14-16C to the north of the boundary, dropping to 7-9C behind) and then on into Wales and southern parts of northern England by the afternoon. Essentially, once this boundary moves through air parcels are unlikely to be able to get past a substantial warm bulge in the mid-levels (around 600-650mb), serving to inhibit deep convection. Current thinking is this boundary will be located between Gloucester-Ipswich at 10z, reaching the latitude of Birmingham by 13z and perhaps Chester by 16-17z - by this stage it will be battling a strengthening easterly wind north of the boundary, which will slow its forward motion. The net result is by peak heating, much of the Midlands will be behind this boundary with much lower CAPE, leaving primarily northern England and the far north of Wales with dewpoints of 14-16C and potentially yielding anywhere between 700-1,500 J/kg CAPE.

The extent to which instability increases will be heavily dependant on cloud breaks allowing sufficient surface heating. This is more questionable across portions of northern England, while north Wales (and perhaps far NW Midlands) exhibits better prospects for lengthier sunny spells to help increase surface temperatures. This area will also be in greater proximity to a strip of vorticity aloft, which combined with upslope orographic forcing and sea breeze convergence may aid in the development of deep convection. It is possible that numerous towering cumulus could develop across the north Midlands / northern England / north Wales, but may struggle to both lift beyond the warm mid-level nose, and become sustained given the very dry mid-level profile. If deep convection can become sustained, which seems most likely over north Wales initially, then 20-30kts uni-directional shear may be supportive for some cell organisation, capable of producing large hail 2-3cm in diameter and frequent lightning. The deep boundary layer with inverted-V profile suggests some rather gusty outflow winds may be possible too.

During the evening hours, as the upper low by now near Cornwall continues to slowly lift north, so the vorticity strip will overspread more of NW England and this may in part act as a better lifting mechanism for a few additional thunderstorms to develop and then drift offshore across the Irish Sea towards the Isle of Man.

Theta-E, having reduced during the morning / early afternoon, is expected to increase during Saturday late afternoon into the evening hours as advection from France continues. This will be occurring beneath the aforementioned vorticity strip, and as the 750-850mb layer moistens this may encourage deep convection to develop and hence brings the risk of scattered showers and a few thunderstorms from late afternoon and into the evening hours, tracking northwestwards. Uncertainty exists in both just how much moistening occurs in this crucial layer, and whether it will be significant enough to pass the relatively deep warm/dry mid-levels to generate sustained deep convection. Nonetheless, there could be an increase in lightning activity from SE England northwestwards into the Midlands and perhaps on into Wales during the evening hours.

Assuming early cloud and rain clears westwards at a reasonable pace, significant surface heating of a moist low-level airmass (dewpoints 12-15C) will yield 400-800 J/kg CAPE beneath increasing PVA aloft. Low-level convergence and orographic forcing will aid in the development of scattered thunderstorms, which will drift generally westwards in an environment with modest shear (15-25kts). Some concerns also exist about warm/dry mid-levels, which could inhibit depth of convection potentially. The SLGT may be expanded northwards if trends suggest other areas may also be at risk - much of which depends on the clearance of early cloud/rain. Additionally, elevated showers/thunderstorms from the Irish Sea could also drift inland on Saturday evening/night, but some uncertainty over the exact details at this stage.

Instability will be lower here due to the proximity to the upper low and associated wrap-around occlusion cloud and showery rain, reducing surface temperatures. However, a few isolated lightning strikes are not ruled out from any stronger pockets of convection that may develop.