VALID 06:00 UTC Sun 09 Aug 2020 - 05:59 UTC Mon 10 Aug 2020
ISSUED 07:08 UTC Sun 09 Aug 2020
ISSUED BY: Dan
As has been the case for the past couple of days, an elevated mixed layer will cover much of England and Wales on Sunday, migrating slowly northwestwards through this forecast period. This will serve as a cap to surface-based convection, while creating very steep mid-level lapse rates. The leading edge of the EML will be marked by a notable 850mb Theta-E gradient, and isentropic lift along this boundary may enable scattered elevated showers or the odd weak thunderstorm to occur on Sunday morning, primarily over northern England (especially Peak District eastwards to Yorkshire). This boundary will lift northwards through the day, and so the area at risk of elevated showers will move accordingly, probably reaching the Scottish border by the end of the afternoon - however, by this point the Theta-E gradient is likely to become a little more diffuse, and so convection will probably become weaker with the risk of lightning becoming rather low. Nonetheless, this boundary will continue to advance northwards reaching the Moray coast by 06z Monday.
Elsewhere across England and Wales, any minor impulse aloft may be sufficient to generate isolated elevated showers or thunderstorms above the EML - most areas will remain void of lightning activity with the risk very low in any one location. However, during Sunday evening and night, an increasing 850mb Theta-E gradient is expected to develop over Wales and NW England, while a developing shortwave / PV filament approaches from the southwest as the Atlantic largescale trough approaches.
The net result is likely to be increasing mid-level instability and consequently scattered elevated showers and thunderstorms developing over Wales / NW England and Irish Sea region during the overnight period, these then expanding in coverage as they drift northwestwards across the Irish Sea. Forecast profiles suggest these are likely to root around the 800mb level, with cloud bases 6-7,000ft and consequently can utilise the very steep mid-level lapse rates and significant CAPE available. As a result, any thunderstorms that do develop could produce quite frequent lightning.
However, there is some uncertainty as to how quickly destabilisation may occur, and it is possible activity could remain fairly isolated until after this forecast period (i.e. beyond 06z Monday). It is also worth noting other additional scattered elevated thunderstorms will be possible elsewhere, such as across the Midlands and perhaps even southern England and / or East Anglia - but confidence here is lower.