Convective Outlook: Mon 17 Aug 2020
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Convective Outlook

VALID 06:00 UTC Mon 17 Aug 2020 - 05:59 UTC Tue 18 Aug 2020

ISSUED 06:43 UTC Mon 17 Aug 2020


Upper trough will migrate northwards across Britain and Ireland on Monday, the cold pool aloft serving to increase instability and steepen mid-level lapse rates. Remnants of convective rainfall from Sunday are likely to be affecting parts of northern England, southern Scotland, Ireland and perhaps SW Wales / SW England on Monday morning, and while a few isolated lightning strikes are possible the risk is considered rather low. Through the day, diurnal heating of the residual warm, moist low-level airmass with temperatures in the low 20s Celsius and dewpoints in the mid-high teens Celsius will yield 500-1,000 J/kg CAPE. Scattered showers and a few thunderstorms are likely to develop across many parts of England and Wales, particularly aided by orographic forcing and low-level convergence zones (such as peninsula convergence). Steering flow will move showers/storms towards the N or NNE initially, but this will veer to the NE by evening.

PWAT is lower than previous days, but still 25-30mm, and hence the risk of localised flooding is possible where showers train over similar areas, but overall the impressively high local rainfall totals of the weekend are not expected to be repeated. Shear will be rather weak and so showers/storms will tend to pulse, with daughter cells developing from outflow - the strongest cells could produce gusts 35-40mph and perhaps hail up to 1.5cm in diameter locally. Low cloud bases, enhanced low-level vorticity near convergence boundaries and this magnitude of CAPE suggests that a few funnel clouds or a weak tornado may be possible. Showers/storms are expected to slowly weaken through the evening hours, although may persist close to some southern and western coasts overnight.