Discussion: 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.
Wrote by @/Handry_Outlook
In-Depth Discussion: During Monday evening and potentially overnight a Theta-E advection could cause storms to erupt over northern France and the channel. Elevated convection could form with the SE and southern England on the edge of the advection, perhaps 500-1000+ J/KG of MUCAPE forming. However 50+ knots of deep-layer shear will help allow convection to last a long time despite the potentially weak MUCAPE. Along with a well angled lobe of vorticity with a NE moving direction from northern France into the SE/north sea. However, the timing is very awkward and might be too late and convection could occur once the vorticity lobe is too far west and the convection moves NE into the north sea rather than onto the south coast. The initiation is therefore very important for whether storms hit the UK or not. There's also a chance of isolated storms embedded in the frontal rain in the SW during late morning towards early afternoon with a little bit of moisture advection emerging from the channel. In the main threat, there's a weak chance of a Supercell with the deep-layer shear along with some small hail production potential given the energy and buoyancy post-boundary layer. The fairly strong low-level shear could rip that potential up though. Low-level shear is potentially strong enough for a few funnel clouds and a small chance of and isolated tornado. High saturation keeps lightning risk perhaps a little lower but the shearing and strong enough energy does mean that lightning could still organise. Especially with the elevated nature, that tends to be where the saturation is slightly lower. Furthermore, the almost tropical amounts of moisture tends to negotiate the high saturation in similar past events. There is a minimal chance of organisation going right across the eastern UK moving more northwards according to a few models. Could end up in thundery rain for eastern and Midlands parts. Later into the night, there could be some return of moisture convergence over the midlands as the front the storms embed into moves more northwards now. There's a chance that it could allow favourable conditions for 400+ J/KG of MUCAPE and just enough lingering 30+ knots of deep-layer shear for a few more storms to form.