Convective/Thunderstorm Discussion : Summer/Autumn 2021

Day 1 Convective Outlook

VALID 06:00 UTC Sat 02 Oct 2021 - 05:59 UTC Sun 03 Oct 2021

ISSUED 07:26 UTC Sat 02 Oct 2021

ISSUED BY: Dan

Large, initially positively-tilted, upper trough covers the UK and Ireland on Saturday, its axis to the west over the Atlantic. Through the forecast period the trough will become increasingly negatively-tilted, while its centre remains anchored broadly to the northwest of Scotland. On its forward side, a strong jet will drive a deepening surface low northeastwards from the English Channel across England and to the North Sea, to become centred near Shetland by 06z Sunday. A notably high Theta-W airmass will be associated with this low, as subtropical air is advected northwards on its eastern flank. Widespread rain will spread northeastwards across many parts of Britain - however, forecast profiles reveal a mid-level warm bulge (around 700-750mb) that may encourage embedded elevated convection to occur. As such, some sporadic lightning and very heavy rain will be possible in a SW-NE zone from the Channel Islands and English Channel across Cen S / SE England by late afternoon and towards East Anglia during the evening. The convective potential is highly sensitive to the exact thermodynamic profile, which varies amongst model guidance, and not least complicated by rather saturated profiles. Therefore confidence is low as to whether much lightning will occur, despite some very heavy rain in places. Dry air wrapping around the deep low over the North Sea could result in an uptick in lightning activity close to Orkney/Shetland late in the night.

Elsewhere, the focus for lightning will be primarily related to sea-driven convection, fuelled by warm SSTs as showers move inland. Exposed western coasts are most prone to a few isolated lightning strikes, although exactly where these may occur is very uncertain and the risk in any one location is considered rather low. Perhaps a slightly better environment in western Ireland during the night hours (where a low-end SLGT has been issued), and towards dawn over Cumbria.