The cold air aloft that triggered severe thunderstorms across parts of France and England on Friday will allow a shallow low-pressure system to develop over the Bay of Biscay. Further moist and warm theta-E advection into England and Wales will increase instability on Saturday afternoon, leading to widespread showers, with potential for thunderstorms, some of which could be locally severe.
An occluded front associated with the shallow low triggered scattered elevated thunderstorms over the English Channel on Friday night, with one storm west of the Channel Islands and the main one near Doudeville in northern France. Pockets of shallow CAPE (around 500 J/kg) should support some lightning activity from these elevated storms, though insufficient deep-level shear will limit their lifespan. However, a sea convergence zone may enhance wind shear, possibly leading to an increase in lightning activity as the storms move into Hampshire around 04z. An enhanced risk has been issued for parts of Wiltshire, Somerset, and Gloucestershire, where frequent lightning may occur within the elevated thunderstorms. Another area of elevated thunderstorms west of the Channel Islands is also expected to redevelop south of Cornwall, prompting another enhanced risk due to the threat of frequent lightning and, to a lesser extent, isolated tornadoes or waterspouts.
These elevated thunderstorms are expected to weaken and move westward into Wales by early afternoon. As daytime heating intensifies, a surge in strong CAPE, potentially reaching 1500 J/kg, will create significant instability. Any lingering showers from the earlier elevated thunderstorms could redevelop into stronger thunderstorms as mid-level saturation decreases. The first thunderstorms of the afternoon are likely to initiate near St. Albans and Milton Keynes, expanding northeastward through the rest of the afternoon. A moderate risk has been issued for this region, as significant downpours may lead to localized flooding, and steep lapse rates could produce hail up to 2.5 cm in diameter within the strongest cells. There is a slight chance that some thunderstorms may take on supercellular characteristics if low-level shear is sufficient. Simultaneously, scattered thunderstorms will develop over Gloucestershire and move northwest into Wales throughout the afternoon, also carrying the potential for large hail and heavy downpours. There is a risk of tornadoes, waterspouts, or funnel clouds forming along the Bristol Channel, where low-level shear is moderate. Both thunderstorm areas will track northeastward into Wales and eventually weaken into rain by nighttime.
Scattered elevated thunderstorms may develop over the Channel again on Saturday night along another occluded front, but the oversaturated profiles suggest the overall risk will be low.
You can download the kml file of the convective outlook here:
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