As the low pressure across the Celtic Sea moves into western parts of the Republic of Ireland, an upper trough associated with a cold front will bring showers across most parts of England. Daytime temperature will widely exceed 25C° with dew point between 18C° and 21C° suggesting a sufficient amount of moisture across most parts of England, hence allowing severe thunderstorms to initiate.
Sporadic showers will first develop across Wales/ East Midlands due to the shallow amount of CAPE, perhaps producing a few sporadic lightning before sunset. The main focus is across central southern England as the trough moves in, sufficient daytime heating allowing thunderstorms to initiate across parts of Isle of Wight, then rapidly develop across most of southeast England, East Midlands and East Anglia between 10z and 17z. This cluster of thunderstorms might develop into an organised line of thunderstorms in the late morning, perhaps bringing some gusty wind (35-45 mph locally), localised flooding and small hail in places.
Although the main line of thunderstorms will turn increasingly fragile and weaken in the early afternoon, some severe thunderstorms might linger across the eastern corner of southeast England and eastern parts of East Anglia over the afternoon. These might bring some locally heavy downpours, perhaps falling 30-50 mm within 3 hours, hence bringing an enhanced risk of flooding. Strong CAPE across this region (800-1200 J/kg) with strong low-level and mid-level shear allows large hail to develop in the heaviest showers, perhaps exceeding 2 cm in diameter. This also means there is also an enhanced chance of supercell development along Essex, Sussex, London and Kent.
Even though the cold front will clear from Western England and Wales in the early afternoon, an occluded front will move in at around 14z, which allows a short period of warmer air to return across parts of West Midland and Northern England. Few showers will initiate across West Midland, then rapidly develop into strong cells of thunderstorms, with a risk of forming into a supercell. The timing of this particulate event is uncertain, which can cause the amount of lightning to vary, so an update might be needed later to reduce/expand the risk. Given the lapse rate and moderate wind shear, there is a risk of marginally severe size hail (1-2 cm in diameter).
An update will be issued on Saturday early morning, perhaps introducing a high risk across the east.
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