VALID 06:00 UTC Wed 28 Apr 2021 - 05:59 UTC Thu 29 Apr 2021
ISSUED 06:44 UTC Wed 28 Apr 2021
ISSUED BY: Dan
An active cluster of thunderstorms are ongoing over Lyme Bay, in close proximity to the low centre / convergence and on the leading edge of a marked dry intrusion. This feature could potentially persist for a few hours before eventually weakening. Either way, an upper low covers southern Britain during Wednesday with forcing aloft encouraging areas of showery rain to develop, some perhaps capable of producing isolated lightning strikes.
Depending on sufficient cloud breaks, surface heating over southern counties of England (West Sussex across to Devon) could yield in excess of 500 J/kg CAPE by the afternoon hours in an environment with weak shear but steep mid-level lapse rates. Low-level convergence may aid the development of heavy showers and a few weak thunderstorms in this area, although given the saturated profiles and weak shear, pulse-type mode is most likely (lightning risk 15-20%). Some small hail may be possible, and locally high rainfall totals / localised flooding given the slow movement. Reluctant to issue a SLGT at this stage due to uncertainty over extent of cloud breaks/surface heating - but an expansion to the SLGT may be required if confidence improves.