UPDATE (09:00 UTC+1):
Reduce the coverage of the enhanced risk due to a strongly saturated profile. A severe risk for strong gusts has been added as well.
A strong south-shifted Atlantic jet stream will usher a series of low pressures toward Great Britain, bringing outbreaks of heavy showers and potential thunderstorms in places.
An occluded front will sweep across many parts of the UK over the daytime, bringing heavy rain to many. Slight clearance behind the occluded front will allow shallow convection, with MUCAPE of 300-600 J/kg across Northern England. Due to the strongly saturated profile, this convection will be limited in activity, therefore only a few sporadic strikes are possible, mainly across northeastern England, hence a low thunderstorm risk has been issued for these areas.
Another surface low pressure will develop behind the milder southwesterly air mass during the early night. A sector of relatively high Theta-E air mass will be introduced by a warm front associated with the low pressure. Convective showers behind the warm front will develop due to an area of strong 0-3 km CAPE (>150 J/kg locally) and strong buoyancy. Surface-based thunderstorms will initialise across the Celtic Sea, advancing northeastward, parallel to the coast. A slightly less saturated profile across the north of Cornwall and western parts of the English Channel might allow slightly more long-tracking thunderstorms to develop. A strong lapse rate could bring some hail, but strong low-level shear could limit it to a marginally severe size (only). The combination of moderate shearing across far SW England could also allow a waterspout / weak tornado.
The highly saturated profile will limit the lightning activity, but some high-resolution models suggest a slightly less saturated behind the band of thundery showers, allowing another area of thunderstorms to initialise across western parts of the English Channel. Moving northeastward into Bournemouth/ Isle of Wight during 03-05z. Slightly strong deep-level shear across the central and eastern parts of the English Channel could also benefit this area of thunderstorms to be slightly more long-tracked compared to others across the Celtic Sea. Strong gusts (55 - 65 mph) are also possible across the English Channel as well. A supercell is possible, tracking northeastward into Bournemouth from the western parts of the English channel but overall confidence is low due to the saturated profile.
Further weak elevated thunderstorms could continue to develop along eastern parts of the English Channel / southeast coasts, but confidence is low.
You can download the KML file of the convective outlook here:
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