On Friday, a continental low-pressure system over the Bay of Biscay will push several complex weather fronts into southern England and Wales. These fronts will bring warm, moist Theta-E air along with them. This type of setup is unusual for the UK due to the rare advancement from this direction.
As an occluded front moves westward toward the Celtic Sea on Thursday night, the atmosphere will become less saturated. By early evening, a shortwave from Belgium will trigger the development of a wave along the frontal boundary, increasing the likelihood of dynamic rainfall. The northeastern side of the frontal wave will experience isentropic upglide, leading to scattered elevated thunderstorms over southeast England during the early morning. Thunderstorms were already reported in Belgium by 7 PM local time. Forecast models show relatively high mixed-layer CAPE values (1000-1500 J/kg) late on Thursday, an uncommon occurrence in the UK. These storms will become elevated as they move over the English Channel.
Strong low-level and deep directional and speed shear will help organize the thunderstorms, while warm, moist inflow from easterly winds will enable the storms to persist. There is potential for these thunderstorms to grow into a Mesoscale Convective System (MCS) as they drift northwest across central southern England.
High precipitable water values (over 40 mm) will increase the risk of localized flooding.
Additional surface-based thunderstorms could develop along the M4 corridor during the afternoon. The remaining CAPE may allow for a few active cells, possibly with locally frequent lightning. However, limited deep-level shear may reduce the lifespan of these storms.
Training effects are likely in the SEVERE risk areas, with up to three rounds of thunderstorms (starting Thursday afternoon) impacting the same regions within 24 hours. There is strong model agreement that 30-50 mm of rain could fall within that time, with some areas potentially seeing 70-100 mm.
You can download the kml file of the convective outlook here:
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