Convective Outlook: Tue 16 Aug 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Tue 16 Aug 2022 - 05:59 UTC Wed 17 Aug 2022

ISSUED 06:35 UTC Tue 16 Aug 2022

ISSUED BY: Dan

Longwave upper trough over western Europe will disrupt, forming a cut-off upper low over the Bay of Biscay by 06z Wednesday. The net result is the cold pool over the British Isles, atop a warm, moist low-level airmass (15-17C dewpoints), will begin to warm aloft as heights rise across the neck between the cut-off low to the south, and the ejected trough to the northeast - perhaps limiting the depth of convection in places. This, combined with rather extensive cloud and fairly moist profiles casts some concern over the quality of deep convection that can develop. Nonetheless, most model guidance is rather keen on showers/thunderstorms developing, so we entertain this possibility for now. 

Showers/thunderstorms/outbreaks of rain may be ongoing across southern England and the English Channel first thing on Tuesday morning, and this may slowly lift north through the morning, turning increasingly convective in nature towards midday. Peninsula convergence is expected to develop fairly quickly through the morning across SW England, and by late morning a similar evolution to Monday is possible across Devon and Cornwall (but perhaps extended even further southwest towards Penzance etc) with slow-moving torrential thunderstorms developing. Additional heavy downpours and thunderstorms are likely to develop along marked low-level convergence zone(s) running from SW England northeastwards across the south Midlands / M4 corridor to western East Anglia, aided also by a small surface low developing in the vicinity by the afternoon. In this area shear will be rather weak, resulting in outflow-dominant pulse storms that collapse and create substantial cold pools; daughter cells then form nearby and this whole process distorts the shape and location of the CZ such that it may shift and wriggle north/south at times through the afternoon and evening, dictating where new cells develop. The greatest concern is flash flooding from slow-moving torrential downpours which could occur in a number of places, indeed some areas could very locally receive accumulations exceeding 100mm. Some hail and gusty winds may accompany the most intense cells, but in general below severe threshold. Vorticity stretching by updrafts may allow a couple of funnel clouds to occur. 

Showers/storms may persist well into the evening and night, eventually morphing into large areas of showery rain and becoming an effective cold front. Meanwhile, attention turns to the English Channel and Cen S / SE England where clusters of elevated showers/thunderstorms could drift northwards, perhaps eventually into SE East Anglia - associated with PV lobes rounding the base of the Biscay low and ejecting northwards.