Convective Outlook
VALID 06:00 UTC Fri 21 Oct 2022 - 05:59 UTC Sat 22 Oct 2022
ISSUED 21:21 UTC Thu 20 Oct 2022
ISSUED BY: Dan
A vertically-stacked low, having lingered west of Biscay for much of the week, will drift slowly northwards across Ireland on Friday and Friday night. Its closer proximity to the British Isles, and associated cold pool aloft, will generate a widely unstable environment in response to SSTs and diurnal heating inland, yielding generally 300-600 J/kg CAPE. The high CAPE values (800+ J/kg) simulated in parts of East Anglia/SE England during the morning hours primarily are generated by capped surface parcels which would require surface temperatures (including SSTs) of >20C to generate surface-based convection. That said, a few isolated elevated cells could develop, more especially offshore.
Otherwise, the main focus will be with numerous clusters/bands of showers spreading northeastwards through the day, initially focussed across Wales and SW England but expanding across more of England as the day goes on - aided by lobes of PVA on the forward side of the upper low. Fairly skinny CAPE will tend to limit the lightning potential somewhat, but there is scope for 20-30kts shear (hints of backing of the surface winds) which may allow for some organised cells to bring the potential for sporadic lightning. Where showers line up parallel to the steering flow (such as near the Severn Estuary northeastwards into the Midlands) surface water flooding may occur from locally high rainfall totals.
The presence of a LLJ moving into southern Britain during the afternoon and evening hours suggests potential for some rather gusty winds (45-55mph) around the most intense cells. Some CAM guidance suggests the potential for rotating updrafts in parts of southern England, although the limited magnitude of CAPE should tend to restrict any severe weather (large hail etc). That said, vortex shedding to the lee of geographic features, such as the Isle of Wight into West Sussex, can aid the development of isolated tornadoes when stretched/ingested by strong updrafts, and this is one potential worth monitoring for southern coastal counties in particular.
