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 early evening, a shortwave from Northeastern France 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 00-02Z. This will allow thunderstorms to initiate near Paris and become increasingly frequent across the English Channel. Forecast models indicate relatively high MLCAPE values (1000 - 1500 J/kg) late on Wednesday, a rarity for May according to climatology. Thunderstorms will turn elevated when they reach the English Channel, where a new area of thunderstorms will develop to the north of the elevated thunderstorms, due to the less saturated profile and very high MLCAPE.
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 central southern England. A supercell is also possible across the north of the trough given the strong support from both LLS and DLS. Some of the high-resolution models are supporting the new area of thunderstorms to move from Brighton into central southern England, perhaps some of the remaining will affect Wales during the morning.
Moderate PWAT (25-30mm) will increase the risk of localised flooding. The potential of 1.5-2.5 cm hail is also possible along the M4 and southeast England due to the moderate lapse rate.
Further surface based thunderstorms are possible to develop across southern England /Wales/ Midlands during the afternoon. The remaining CAPE will be able to allow a couples of active cells to develop. Some lightning, locally frequent, are possible within these cells. However, the poor cross model agreement in regard of these isolated thunderstorms prevented for another enhanced risk to be issued.
Given the strong background signal, there is potential for a moderate risk to be issued, though pinpointing the exact area is challenging at this time, especially how north and east will it affect. Therefore an update will be issued on Wednesday afternoon.
You can download the kml file of the convective outlook here:
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