UTC Latest guidance continues to trend towards a reactivation of the frontal boundary this evening/overnight with showery outbreaks of rain developing over the Midlands into East Anglia - questionable as to how much (if any) lightning activity will be present with this, but given some marginal elevated instability have extended the LOW threat westwards to cater for this potential. That aside, still some uncertainty as to how things will evolve this afternoon with regards to surface-based thunderstorm potential over East Anglia / SE England - early initiation (11-14z) is still signalled by some NWP output.
Broad upper vortex will approach from the Atlantic, preceded by a strengthening southerly flow encouraging advection of a warm, moist airmass at low-levels ahead of an eastward-moving cold front.
There is the potential for some elevated convection to develop over the English Channel and drift into SE England late Friday morning. Then, given enough insolation, combined with low-level convergence, a few scattered thunderstorms will be possible in the warm sector during the afternoon and evening hours across E/SE England. This is highly dependent on the speed of the cold front, which carries some uncertainty - in general, a quicker arrival will narrow the window of opportunity for phasing with maximum daytime heating.
Nonetheless, given 600-800 Jkg-1 CAPE and reasonable DLS a few thunderstorms seem plausible at least, in an environment favourable for organisation into multicells and perhaps even a supercell. Initiation could be as early as 11-14z. Forecast profiles look fairly saturated, so hail perhaps not such an issue - though the strongest cells could produce local incidences of hail up to 1.5cm in diameter. Surface water flooding perhaps more of a significant risk given PWAT 30-35mm - scope then for 25-30mm to fall in a short space of time.
There are also suggestions amongst NWP guidance of an additional cluster of elevated deep convection to occur during the evening and night hours over similar areas.
A few scattered heavy showers will also be possible Friday afternoon across parts of Scotland, perhaps weakly-electrified given strong shear but marginal instability - hence an eastwards extension of the LOW threat level. In the post-frontal environment, cooling aloft will steepen lapse rates and bring a few scattered showers late in the day and into the night across the Republic of Ireland, Northern Ireland and into western Scotland - some perhaps weakly electrified.
- Much longer!
