Detailed analysis of thunderstorm risk across parts of southern and western England, Wales and Ireland between Sunday and Wednesday,
An upper-level low located over northwest Spain and surface low over northern Spain will drift north over the Bay of Biscay over the weekend, the upper and collocated surface low then pivots off northwest France on Monday, before drifting southwest over the Bay of Biscay.
As the low drifts north to the west of France, resultant strengthening southeasterly flow will increase warm air advection over northwestern Europe, with a plume of higher theta-w air (12C+ at 850 hPa) pulled northwest across England and Wales from Sunday and through Monday - ahead of the low to the southwest.
Increased cyclonic vorticity and cooling mid-level temperatures with approach of upper low across NW France while warm air advection increases from the southeast will generate increasing instability, initially across SW England early Sunday, then further north across Wales, western England and east across southern England later in the day and through Monday. Tuesday will see thunderstorm risk become more confined to SW England and south Wales, as low pressure pulls away to the southwest. Thunderstorms will be driven by a combination of mid-level distabilisation, as shortwave troughs move northwest in the strengthening flow and also by surface breeze convergence combined with surface heating of warm and moist air by day.
Arrival of upper low towards NW France on Sunday will strengthen low to mid-level warm air advection while, at the same time, temperatures cool aloft - this will be manifest in an increasing risk of elevated thunderstorms either moving up from NW France or developing from the western English Channel moving across SW England early on Sunday, perhaps spreading into south Wales too. As daytime heating of increasingly moist airmas into the low to mid 20s Celsius occurs across S England and S Wales and breeze convergence develops from SE England west/northwest into Wales, some isolated to scattered heavy showers and thunderstorms may develop. Any storms may produce heavy rain leading to localised flooding, hail and CG lightning. Further elevated thunderstorms may also cross the English Channel and move in across parts of S England later in the day / Sunday night.
Moisture pushing in from the southeast will be more widespread on Monday, so surface heating into the low to mid-20s and surface breeze convergence zones developing again will likely trigger isolated to scattered thunderstorms across southern England, Midlands and Wales. Deep layer shear will be generally weak away from SW England / S Wales - where 30-40 knots of DL shear may be sufficient for one or two supercell structures to develop - with an isolated risk of large hail and strong wind gusts. Otherwise, thunderstorms may produce localised flooding, hail and CG lightning.
Thunderstorms once again developing from surface heating and breeze convergence, but more confined to central south and SW England along with south Wales and southern Ireland. Heavy rain leading to localised flooding, hail and CG lightning possible.
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