During the early afternoon, a warm front is set to form over western regions of Ireland, resulting in the sporadic development of showers across the northwestern parts of the country. Locally, CAPE levels are anticipated to exceed 400 J/kg, providing just enough instability to generate isolated surface-based thunderstorms in the evening. However, the limited presence of deep-level shear will curtail the longevity of these thunderstorms.
Simultaneously, an upper trough is poised to transport a substantial Theta-E airmass, ushering in warm moisture along the Celtic Sea and portions of the English Channel. MLCAPE values are forecasted to locally surpass 1000 J/kg, prompting the development of isolated showers along the upper trough. These showers are expected to progress northward at a gradual pace overnight.
Soundings indicate the possibility of a shallow cap in the early night, with most models suggesting its eventual erosion. This cap breaking could lead to the rapid transformation of isolated showers into an organized system, with the potential for frequent lightning, robust gusts, and heavy downpours. Some high-skill models even hint at the possibility of these showers coalescing into a weak but expansive Mesoscale Convective System (MCS).
The presence of substantial deep-level shear combined with low-level shear has the potential to prolong the duration and intensify the severity of thunderstorms. Additionally, the upper-level trough enhances the atmospheric conditions conducive to thunderstorm formation. Furthermore, the existence of strong updraft helicity, exceeding 20 m^2/s^2, heightens the likelihood of isolated supercell development overnight, possibly resulting in hailstones reaching diameters of up to 4.5 cm.
Localized flooding is a concern, with some models suggesting the possibility of up to 60 mm of rainfall within a 6-hour period across Cornwall and potentially western parts of Wales. However, due to the elevated nature of the thunderstorms, the overall amount of rainfall reaching lower levels may be somewhat reduced due to evaporation. Nevertheless, significant rainfall amounts are expected, with more than 40 mm of rain anticipated in some areas overnight.
You can download the kml file of the convective outlook here
(You can upload the file to windy.com to have the graphic overlay)
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