A warm front has already brought warm theta-E air mass to most of England and Wales over the night, and a trough to usher in heavy downpours and elevated thunderstorms during late morning and early afternoon across portions of southwestern England and the southern regions of Wales. Significant horizontal wind shear at lower altitudes will cause these thunderstorms to move sluggishly and have a relatively brief lifespan. The gradual storm movement will lead to the intersection of the downdraft with the updraft, resulting in a slight reduction in storm intensity. Within the moisture-rich environment where these storms originate, rainfall may become intense, albeit for a short duration.
In the late afternoon, as a trough approaches, there's a chance of surface-based thunderstorms forming in parts of central southern England, specifically Somerset, Gloucestershire, Wiltshire, and potentially Oxfordshire. These storms may sporadically produce lightning, with the possibility of one or two cells exhibiting supercell characteristics. This could result in locally heavy rain and marginally-severe hail, reaching sizes of up to 2 cm in diameter. There is also a threat of locally high rainfall exceeding 40 mm within 3 hours, or perhaps exceeding 60 mm within 6 hours across North Somerset, South Gloucestershire, and Gwent, bringing an elevated chance of flooding and disruption.
In the early night, another trough will approach southeastern England, generating some potent elevated thunderstorms in places. The strong shearing conditions increase the likelihood of isolated supercells characterized by solitary rotating updrafts. These conditions are optimal for nurturing such rotating updrafts. The presence of significant speed shear facilitates swift storm movement, effectively maintaining a separation between the updraft and downdraft. At the same time, directional shear plays a pivotal role in inducing rotation within the updraft, endowing these cells with the potential to produce marginally severe size hail (up to 2.5 cm in diameter), intense rainfall (up to 50 mm within 6 hours), and strong gusts (up to 55 mph).
At the same time, the potential for additional surface-based thunderstorms emerges. These storms initially develop in Dorset and progress northward into Somerset, Wiltshire, and sections of the West Midlands as a trough moves northeastward. There's a possibility that this trough may even extend into northern areas of Lincolnshire and Yorkshire overnight. However, the absence of significant wind shear will confine these thunderstorms to localized and short-lived events.
Thunderstorms in South Wales, Somerset, and Gloucestershire may exhibit greater activity due to higher levels of 3CAPE (170-190 J/KG). This heightened instability could potentially result in localized heavy rainfall (up to 40 mm within 6 hours), raising the risk of flooding in these areas.
As a cold front approaches Wales overnight, a sharp temperature contrast will allow sporadic thunderstorms to form in parts of Wales and southwestern England. Elevated levels of 3CAPE (>150 J/Kg) may foster isolated lightning within thunderstorm cells.
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