A negatively tilted upper trough will allow a very warm theta-e plume to move well into England and Wales late Sunday into early Monday. This warm air mass will interact with an active cold front associated with a deep low-pressure system in the Northern Atlantic. As moisture from the low-pressure system advances, complex storms are likely to develop within the highly sufficient CAPE pool behind the warm front. A line of active thunderstorms may form, potentially evolving into a squall line as it nears Scotland.
Earlier today, showers and scattered thunderstorms began forming over the Bay of Biscay, signaling a high-energy, high-moisture scenario. Some of these storms are expected to move northward into Ireland, possibly reaching the St. George Channel by early evening. As these thunderstorms interact with the cold front, they will intensify, bringing torrential downpours and frequent lightning to parts of Ireland and Northern Ireland. The main severe threat in these areas will be localized flooding due to the relatively high PWAT values, with the possibility of strong gusts if a squall line develops.
As the cluster of thunderstorms moves into western Scotland, it will become more defined, aided by increased low-level shear. These cells are expected to be long-lasting due to strong deep-layer and low-level shear. As the cold front advances northeastward overnight, another trough will develop across NW Wales and the Isle of Man, triggering a new round of thunderstorms overnight and into the early morning hours. These storms will form in an even more favorable environment, with high CAPE values and a strong shear profile, leading to the rapid development of thunderstorms capable of producing heavy rainfall and possibly hail up to 2 cm in diameter.
Early morning thunderstorms will gradually weaken and move into the North Sea by early afternoon, but there remains a risk of strong and severe thunderstorms developing across parts of NE England and East Anglia. Unusually high CAPE values and a surface low-pressure system will create a conducive environment for severe storms. However, a warm, dry air layer at 700 hPa may act as a cap, preventing significant storm development. If temperatures reach 34°C, this cap could erode, leading to the development of one or two thunderstorms later in the afternoon, potentially producing very large hail (up to 5 cm in diameter) and even evolving into a supercell. This "high risk, high reward" scenario, often described as a "loaded-gun" environment, makes it challenging to issue a risk higher than SLIGHT unless the forecast changes tomorrow.
Further thunderstorms are expected to develop over the English Channel early Monday night, with the potential for one or two storms affecting Kent and nearby areas.
You can download the kml file of the convective outlook here:
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