On Wednesday, a continental low pressure is expected to form over Germany and Benelux, ushering in a relatively warm Theta-E airmass along the northern and eastern parts of the warm front. This setup is rare in the UK due to the unusual advancing direction.
The atmosphere will become less saturated as the occluded front clears westward toward Wales and southwest England during Wednesday night, especially toward N France/ SE England. By late afternoon/early evening, a shortwave from the southeast will trigger the development of a wave along the frontal boundary, increasing the potential for dynamic rainfall. Isentropic upglide on the northeastern flank of the frontal wave will lead to scattered elevated thunderstorms over southeast England during the evening hours. This will allow thunderstorms to initiate near Paris and become increasingly frequent across the English Channel. Forecast models indicate relatively high CAPE values (700 - 1000 J/kg) late on Wednesday, a rarity for May according to climatology. Strong deep directional and speed shear will facilitate organized thunderstorms, fueled by warm, moist inflow from easterly low-level winds will allow the thunderstorms to be long-lasting. The potential for upscale growth into a Mesoscale Convective System (MCS) exists as the thunderstorm complex drifts northwest across the English Channel. A supercell is also possible across the warm front given the strong support from both LLS and DLS. The majority of high-resolution models are supporting the area of thunderstorms to move from southeast England into central southern England, perhaps some of the remaining will affect Wales during the early morning.
Moderate PWAT (25-30mm) will increase the risk of localised flooding. The potential of 1.5-2 cm hail is also possible in the southeast due to the moderate lapse rate.
Given the strong background signal, there is potential for an enhanced/moderate risk to be issued, though pinpointing the exact area is challenging at this time. Therefore an update will be issued on Tuesday night/ Wednesday morning.
You can download the kml file of the convective outlook here:
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