VALID 06:00 UTC Tue 07 Aug 2018 - 05:59 UTC Wed 08 Aug 2018
ISSUED 08:30 UTC Tue 07 Aug 2018
ISSUED BY: Dan
On the forward side of the sharpening Atlantic upper trough, a warm and humid airmass will be drawn northwards from Iberia towards SE England on Tuesday. Initially profiles look rather dry and capped (to surface-based convection), but an approaching shortwave from Biscay should gradually moisten profiles by Tuesday afternoon, providing a lifting mechanism.
A gradual increase in elevated showers / thunderstorms seems likely during the late afternoon and through evening hours, beginning over the English Channel and spreading northeastwards - generally clearing the coast to the North Sea soon after 00z.
Given the magnitude of instability, steep mid-level lapse rates, mid-level dry intrusion and sheared profiles, early cells for the first few hours could become sufficiently organised to produce frequent lightning and perhaps hail up to 2.5cm in diameter. As the evening progresses, the main threats will become mainly heavy rain, with lightning frequency probably also reducing. Therefore areas with the best overlap of ingredients for hail / frequent lightning will be Kent / Sussex and perhaps east Essex - a low-end MDT has been issued to better highlight this, though there is still some uncertainty as to how widespread activity will be.
A few isolated lightning strikes may also be possible over the Celtic Sea late in the night.