Convective Outlook
VALID 06:00 UTC Sat 19 Jun 2021 - 05:59 UTC Sun 20 Jun 2021
ISSUED 07:06 UTC Sat 19 Jun 2021
ISSUED BY: Dan
Disrupting upper trough will result in an upper low sliding southeastwards on Saturday from west of Ireland towards Biscay. At the same time, the remaining disturbance over Biscay/Iberia will lift northeastwards across France during the day, by the evening and night hours this will become increasingly negatively-tilted as it tracks across southern Britain, the associated PVA/divergence on the forward side (also in the left-exit region of a pronounced mid-level jet) providing broad ascent across the area and as a result surface pressure will lower and outbreaks of rain will tend to develop later on Saturday and especially overnight, with some embedded convection.
The speed at which the upper pattern evolves and flow backs varies between model guidance, and this will have some impact on how much the high Theta-W air over the nearby Continent can advect northwestwards into southern England (especially SE England) on Saturday evening and night. The vast majority of guidance suggests a mere graze with SE England, probably not helped by numerous thunderstorms that will develop over France on Saturday daytime which will tend to modify the environment to the north (i.e. closer to the English Channel). As such, confidence is low on much in the way of elevated thunderstorm / lightning activity over southern Britain, most likely a Kent Clipper but realistically lightning activity may be confined to mainland Europe.
That said, a marked dry intrusion is expected to push northwards across the English Channel, and this could induce some forced ascent and increase potential instability - and so would not entirely discount a few random elevated showers/weak thunderstorms developing almost anywhere over the English Channel into southern England and perhaps even the south Midlands / East Anglia late in the night - but exactly where these occur (if indeed any lightning does) is too uncertain to highlight any specific areas.
A low-end SLGT has been introduced to cater for the risk that some lightning may pass close to Kent mid-late evening (but more likely remaining in France), and then for a separate risk area affecting portions of the Channel Islands and English Channel towards Cen S England during the late evening and overnight hours - but confidence is rather low on much in the way of lightning.
