Update (11/05/2024 22:00): Latest high-resolution models indicate the presence of increased deep-level shear, prompting the addition of two high-risk areas and expansion of the moderate-risk zone.
A low-pressure system across the Northern Atlantic will move northeastward 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 1700-2500 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 along the Bristol Channel surrounding area, 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 and N 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. Latest CAM models are also suggesting an upgrade in terms of deep-level shear comparing to previous runs. Therefore hail of 1.5-3 cm in diameter 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.
Thunderstorms are forecasted to form in regions with elevated PWAT levels (>30 mm) and topographic lift. This combination may intensify instability as moist air ascends over Welsh mountains as well as Exmoor, cooling and condensing, releasing latent heat and further destabilizing the atmosphere. Topographic lift can additionally generate localized zones of increased upward motion and convergence, particularly on south-facing slopes, serving as potential thunderstorm initiation points. These areas might witness more vigorous and sustained thunderstorm activity compared to their surroundings. With the coupling of high PWAT, localized flooding is probable, and isolated tornadoes could occur due to topographically induced environments. Two distinct high-risk zones have been designated to pinpoint areas with a heightened likelihood of frequent lightning, primarily due to topographic lift. Strong gusts (up to 55 mph) and isolated tornadoes are possible should any supercells develop.
During the early evening, strong thunderstorms across N Wales might merge into a MCS when they move into Northwest England, remaining CAPE and stronger deep level shear across northern England should be able to support frequent lightning thunderstorms within the potential MCS as the profile is not too saturated. Heavy and torrential downpours are expected across NW England, bringing a risk of flooding,
Isolated thunderstorms could also develop across parts of Scotland, Ireland, and Northern Ireland, bringing localised flooding in places. Some frequent lightning is possible across Ireland and Northern Ireland 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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