UPDATE (14:17 BST): An enhanced risk has been issued for parts of Southwest England. Thunderstorms are expected to break out during the evening as the weakening occluded front will allow dinural heating to take place across Devon and Somerset. The enhanced risk across east England has also been expanded northwestward as well.
It should be stressed that destabilising plume events are always fraught with uncertainties given the highly complex situation at play, and therefore there is always an element of uncertainty in these situations. A complex, elongated low-pressure system will move northward from the Celtic Sea into Wales during the afternoon. An occluded front associated with this system will bring heavy downpours to western parts of England and Wales from late morning through early afternoon. Scattered showers will develop in parts of Scotland due to localized convergence zones, with CAPE values around 400 - 600 J/kg sufficient to produce scattered thunderstorms in these areas. These showers will be slow-moving, posing a risk of localized flooding during the afternoon.
The occluded front will weaken in the afternoon as it encounters a tongue of dry air, leading to a less saturated environment behind it. Additionally, due to diurnal heating, scattered thunderstorms could develop across northern parts of France, with the potential for some to form locally in Devon during the latter part of the afternoon.
Another warm front associated with the low-pressure system will develop across northern France and the English Channel during the afternoon, accompanied by a moist, warm airmass. This will result in a line of scattered thunderstorms developing along the warm front and moving northeastward into southeastern England. As this line of thunderstorms enters a strongly sheared environment in the evening, there is potential for small hail, isolated tornadoes, and strong gusts embedded within the line. High PWAT (30mm +) could also post a risk of flash flooding. The thunderstorms are expected to become organized as they move into eastern parts of the English Channel, potentially bringing frequent lightning strikes to parts of southeast England, mainly Sussex and Kent.
There remains some uncertainty about the behavior of the thunderstorms over the Channel. If the thunderstorms remain discrete and detached from the main band of showers (occluded front), convection could remain active as it moves into East Anglia. Otherwise, the increasingly saturated profile in eastern parts of England overnight will suppress lightning activity and reduce the chance of convection.
Nonetheless, moisture convergence over the Midlands could increase as the front and embedded storms move northward. This could create favorable conditions for MUCAPE values exceeding 400 J/kg and lingering deep-layer shear over 30 knots, allowing for the formation of a few more storms.
You can download the kml file of the convective outlook here:
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