VALID 06:00 UTC Wed 14 Aug 2019 - 05:59 UTC Thu 15 Aug 2019
ISSUED 20:32 UTC Tue 13 Aug 2019
ISSUED BY: Dan
A broad westerly flow covers the British Isles on Wednesday, with a rather complex surface pattern. A split frontal system will slide eastwards across England and Wales, with a period of dynamic rain followed by extensive low cloud / drizzle characteristic of a shallow moist zone (SMZ). If sufficient cloud breaks can develop, heating of this moist low-level airmass (dewpoints 15-17C) could yield a few hundred J/kg CAPE. A marked dry intrusion will overspread this SMZ, effectively acting as a weak cap to surface-based convection.
Nonetheless, the SMZ will be reasonably deep with high moisture content, and carries the risk at least of a few heavy showers to develop over the Midlands / southern parts of northern England during the afternoon and evening hours. If convection can grow deep enough into the dry layers aloft then some sporadic lightning may occur - otherwise lightning may be rather limited, despite the presence of heavy showers. A low-end SLGT has been introduced to highlight the main area of interest, although confidence of how much lightning may occur is rather low. Localised surface water flooding will be the main threat from very heavy bursts of rain.
Ahead of the main surface trough / low, backed low-level winds and strong shear could allow a low-topped supercell to occur (if convection can grow deep enough). The low cloud bases combined with strong low-level shear / vorticity may allow an isolated weak tornado to occur.
The left exit of an approaching mid/upper-level jet streak will overspread the SMZ during the evening hours, and given near-saturated low-levels this will encourage additional showers to develop even through to late evening across East Midlands / East Anglia / Yorkshire - though this convection may be somewhat elevated (with bases around 900-950mb). Hence the risk of some sporadic lightning may persist well into the evening hours, and may actually develop further offshore over the North Sea on Wednesday night.
Elsewhere, an occluded front will slide eastwards across Ireland and western Scotland during Wednesday daytime, with sufficient insolation allowing reasonable instability to build in advance of this front. A messy mixture of dynamic and convective precipitation seems likely along this frontal boundary, with scope for some embedded sporadic lightning - though the depth of convection is questionable, and so confidence for much in the way of lightning activity is rather low. A low-end SLGT has also been issued here.