A shortwave associated with a cold air mass aloft will move westward into England and Wales on Friday. As the cold air at mid-levels interacts with the environment, the lapse rates will steepen throughout the day, leading to cooler temperatures aloft. At the same time, a warm and humid theta-E plume will be introduced at lower levels, creating an increasingly energized atmosphere. Convective Available Potential Energy (CAPE) is expected to build up, peaking at over 1000 J/kg across Central-Southern England and parts of the Midlands. Strong 0-3 km CAPE (around 250 J/kg) suggests that any thunderstorms that develop will be accompanied by frequent lightning.
The upper trough across western Germany will shift westward overnight, possibly triggering elevated thunderstorms in the early morning over parts of East Anglia and the East Midlands. As these morning showers dissipate by early afternoon, convection will develop across southern England, especially along a convergence zone. High-resolution models indicate the main convergence zone will form along the southern coasts and the M4 corridor, driven by a combination of sea breezes and northeasterly winds.
Due to the unsaturated profile, isolated cells are likely to pop up easily, leading to a risk of localized flooding from training storms. Strong low-level shear (20-30 knots) could support rotating updrafts, and combined with the convergence zone, there is a possibility of a supercell or even a tornado, particularly in areas under a moderate (MDT) risk. The steep lapse rates and strong CAPE will also allow hail to form, potentially reaching 2.5 cm in diameter. Strong gusts of wind (up to 55 mph) are possible, especially if a supercell forms. The main limiting factor is the lack of deep-layer shear, which could shorten the lifespan of the storms.
Most high-resolution models predict a cluster of thunderstorms will form over Dorset, moving westward, and possibly northwestward if a right-splitting supercell develops, into Somerset and Devon by late afternoon, prompting the issuance of an MDT risk. Some models, like AROME and DMI, also suggest the potential for another supercell forming from Swindon to Chepstow, which could also become a right-splitting supercell due to strong low-level shear. While an MDT risk is being considered for this second cell, the lack of agreement between models has prevented its issuance.
You can download the kml file of the convective outlook here:
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