VALID 06:00 UTC Mon 12 Jul 2021 - 05:59 UTC Tue 13 Jul 2021

ISSUED 06:49 UTC Mon 12 Jul 2021

ISSUED BY: Dan

A rather complex and therefore highly uncertain pattern is expected on Monday, as an upper low slowly drifts eastwards across southern Britain. Areas of rain will affect parts of England and Wales during the morning, and the extent of such rain (and associated cloud) will have a big impact on potential thunderstorm developments later in the day. That said, some elevated thunderstorms may be possible towards S/SE England in particular, but lightning activity is uncertain - in either case, some very heavy rain may be possible. If sufficient surface heating can occur, then some fairly substantial CAPE (600-1,000 J/kg) may be possible given warm, moist low-level air (dewpoints of 15-17C) beneath cool mid-levels. Low-level convergence would likely provide the main focus for initiation, this most plausible along the south coast with the potential for slow-moving cells to initiate here and then mature as they move offshore to the English Channel. A second zone may also be from the London area/northern Home Counties into western parts of East Anglia. Aside from lightning, flash flooding is the main concern with model guidance suggesting potentially as much as 90mm - but the location varies between and within model runs, and is highly sensitive to the exact evolution of events through the day. There could be additional heavy showers that develop over SE England and eastern parts of East Anglia during the overnight hours, but lightning activity with this is uncertain.