A low-pressure system across the Northern Atlantic will move eastward into Iceland over the weekend. This will introduce southerly winds into France and the British Isles. These southerly winds will converge with the outflow of the high pressure across Scandinavia, producing multiple troughs over the weekend.
One of the disrupting upper trough will move northward into southwest England during early Sunday, potentially bringing some non-convective showery rain locally. MUCAPE of 1500-2000 J/kg will not be utilised during the early morning due to a shallow capping which requires 22-23°C to break, creating a loaded gun scenario.
As temperatures are expected to reach 22-23°C during the late morning/early afternoon, heavy and thundery showers will break out across Devon and Somerset (potentially Cotswold area aswell), moving northward during the first parts of the afternoon, then northeastward during the later afternoon/early evening. Frequent or very frequent lightning is possible due to the low saturated profile and a sufficient amount of CAPE. One or two supercells could develop in places. Latest models are suggesting cells which is exposed in slightly stronger deep-level shear could develop into a supercell, which potentially create large hail and frequent lightning, this is most likely to develop across North Devon/ NW Wales.
Moderate low-level shear across parts of southwest England and south/mid Wales can enhance updraft rotation, which increases the lifespan and intensity of the thunderstorms. This can also allow temperature and moisture gradients to increase, further boosting updrafts and allowing sustained hail growth. However, slightly weaker deep-level shear may limit the potential for significant hail growth, reducing the size of hailstones. Nonetheless, hail of 1.5-3 cm is possible during the afternoon.
Elsewhere, weak low-level shearing could result in outflow-dominant pulse storms that collapse and create substantial cold pools; daughter cells then form nearby, and this whole process distorts the shape and location of the convergence zone (CZ), which may shift and wriggle north/east at times through the afternoon and evening, dictating where new cells develop. The greatest concern is flash flooding from slow-moving torrential downpours, which could occur in several places; indeed, some areas could locally receive accumulations exceeding 50mm.
These thunderstorms are expected to develop in an area with high PWAT (>30 mm) and topographic lift. This could enhance instability as moist air is forced to rise over mountainous areas in Wales, then cools and condenses, releasing latent heat and further destabilizing the atmosphere. Topographic lift can also create localized areas of enhanced upward motion and convergence, especially on south-facing slopes. These areas could become focal points for thunderstorm initiation and may experience more intense and persistent thunderstorm activity compared to surrounding areas. Coupling with high PWAT, localized flooding is likely. Isolated tornadoes are also possible due to topographically lifted environments. A moderate risk has been issued to highlight the area with the high chance of very frequent lightning due to topographic lift.
During the early evening, thunderstorms might merge into a longer spell of heavy rain when they move into Northwest England, but the remaining CAPE and stronger deep level shear across northern England should be able to support embedded thunderstorms within the heavy rain as the profile is not too saturated. Frequent lightning is possible locally, especially across parts of northwestern England.
Isolated thunderstorms could also develop across parts of Scotland, Ireland, and Northern Ireland, bringing localised flooding in places. Some frequent lightning is possible across west of Dublin but the likelihood is too low to issue an enhanced risk.
You can download the kml file of the convective outlook here:
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