VALID 06:00 UTC Thu 08 Aug 2019 - 05:59 UTC Fri 09 Aug 2019
ISSUED 20:31 UTC Wed 07 Aug 2019
ISSUED BY: Dan
An upper ridge will build across the British Isles on Thursday, largely suppressing deep convection. However, isolated showers are likely across parts of Scotland in particular, but generally weak instability and limited cloud depth suggests the risk of lightning is rather low.
The main interest during this forecast period will be during Thursday night. An upper trough over the mid North Atlantic will swing northeastwards, encouraging northward advection of subtropical moisture towards the British Isles. A warm front will mark the boundary of this high Theta-W air, which will gradually spread northwards across the English Channel / Celtic Sea, then southern and central Britain and Ireland through the overnight period.
Pockets of elevated convection will be possible embedded within the main frontal rain, which will arrive over SW England during Thursday afternoon - but the window of best potential for deep moist convection (and hence lightning activity) will be within the warm sector / behind the main warm frontal rain as low-levels continue to warm (and moisten) while subtle cooling aloft occurs, ultimately steepening mid-level lapse rates and increasing instability.
On the forward side of the upper trough, large-scale forced ascent (particularly associated with the cold front) combined with elements of isentropic upglide will likely result in scattered elevated thunderstorms developing over parts of the Celtic Sea and western England Channel during the second half of Thursday evening, with other elevated thunderstorms sporadically developing as they drift NNE-wards through the night hours.
The exact coverage of lightning activity is questionable due to a combination of fairly saturated profiles (which will serve to reduce lightning potential) and also due to the bifurcating upper flow, which will tend to steer cells near SW England / S Wales towards the NNE, while cells near Ireland will drift more to the NNW, potentially leaving a relative minima of activity in-between. The areas with the greatest lightning potential are highlighted by the low-end MDT, but sporadic lightning will be possible elsewhere within the SLGT during the early hours of Friday from more localised pockets of mid-level instability release. There is often more uncertainty with elevated thunderstorms (such as in this case) than surface-based thunderstorms, and hence the risk areas tend to be a lot broader - and it is expected quite a few places even within the SLGT/MDT will remain void of lightning.