Convective Outlook
VALID 06:00 UTC Mon 11 Apr 2022 - 05:59 UTC Tue 12 Apr 2022
ISSUED 06:50 UTC Mon 11 Apr 2022
ISSUED BY: Dan
Upper ridge axis over eastern Britain on Monday morning will shift eastwards to the eastern North Sea by 06z Tuesday. A gradually filling longwave upper trough over the Atlantic will continue to approach from the west, and the net result is a strengthening southerly flow aloft allowing a tongue of relatively high Theta-W, emanating from Iberia, to progress northwards across southern and central portions of Britain and Ireland during Monday daytime. This first tongue appears to become engaged by some subtle forcing aloft to allow a line of elevated showers to develop and drift northeastwards through the day. Given cloud bases from this convection at or above 6,500ft with fairly dry near-surface layers suggests much of any precipitation, at least initially, will likely evaporate before reaching the ground. A few elevated thunderstorms may be possible, more especially early in the morning close to the Scilly Isles and Cornwall, and while the risk is considered too low to warrant a SLGT elsewhere the odd isolated thunderstorm cannot be ruled out with this line of showers as it continues to track northeastwards, reaching SW Scotland by late afternoon/early evening.
A renewed pulse of relatively high Theta-W air will advect northwards from northern Spain around noon, reaching the Brest peninsula by 21z and southern England by 00z. This will mark the leading edge of a more substantial change in low-level airmass, as 2m dewpoints rise through the night, in some places from near-freezing values during Monday daytime to close to 10C by 06z Tuesday. Forecast cross-sections reveal elements of isentropic upglide, with elevated showers and a few thunderstorms possibly forming on the leading edge of a low-level jet coupled with increasing PVA migrating northwards on the forward side of the upper trough. This would likely place the Channel Islands at risk 21z-01z, and the south coast of England from around 23-00z onwards. Forecast profiles reveal potential for deep, elevated convection with cloud-layer shear of ~25kts, sufficient for organisation into multicell clusters. Given slight backing of winds in the lower troposphere, there may even be scope for an elevated supercell. This does not necessarily mean severe weather - shallow/low-topped supercells with no lightning or severe weather have been documented in the UK before, merely an individual cell may exhibit some characteristics of a supercell. But any cells that can become somewhat organised could produced quite a few lightning strikes for a time.
In general, limited/somewhat skinny CAPE and increasingly moist profiles with time suggests lightning may not be too widespread, but a few active elevated thunderstorms could be possible. Greatest risk is likely near the Channel Islands late evening initially, and then portions of Cen S / SE England overnight. Any hail threat is probably limited by both modest/skinny CAPE and melting below high cloud bases. It is possible a few thunderstorms could develop further north and west, towards the West Country, SE Wales and Midlands - although based on past experience have refrained from expanding the SLGT at this stage, and it is certainly conceivable lightning activity could be primarily in the eastern half of the SLGT area. Nonetheless, some pulses of very heavy rain could still affect these areas during the night.
