Convective Outlook
VALID 06:00 UTC Mon 15 Jun 2020 - 05:59 UTC Tue 16 Jun 2020
ISSUED 06:44 UTC Mon 15 Jun 2020
ISSUED BY: Dan
Upper low over the Southwest Approaches will meander slowly towards the Brest peninsula during Monday, but overall the upper pattern remains relatively unchanged through the forecast period with broad troughing dominating. The cold pool aloft atop warm, moist low-level air will lead to fairly widespread convection in response to diurnal heating. Given the lack of any noteworthy forcing aloft, the primary trigger will be from low-level convergence and orographic forcing, and so showers/thunderstorms will be primarily located over high ground or along convergence zones. Some mixing of the boundary layer is likely to occur once again across the south Midlands and southern England, resulting in a slight decrease in dewpoint temperatures. This area may also be a little cloudier due to an area of cloud and showery rain that may move inland from the English Channel during the morning hours, and this may also reduce surface temperatures a little - although the cloud should gradually break.
Surface dewpoints of 13-15C from East Anglia and the north Midlands northwards, with temperatures likely to peak around 20-23C, will yield 300-700 J/kg CAPE. Scattered showers and thunderstorms are therefore most likely from north Wales - north Midlands - northern East Anglia northwards, and also scattered across northern and western Ireland. Shear is rather weak in the low-levels but does increase slightly nearer the cloud tops, but in general showers/storms will tend to pulse up, produce a few sporadic lightning strikes and then probably weaken, the outflow then potentially triggering daughter cells nearby. Slow storm motion of 15-20mph to the NW over England and Wales, or to the WSW over Ireland, and perhaps elements of shower-training will bring the risk of localised surface water flooding given PWAT 20-25mm. Low-level vorticity along convergence zones may also be ingested into updrafts to bring the risk of a weak tornado.
The SLGTs issued highlight the main focus for sporadic lightning through the day, but a few isolated strikes may occur elsewhere, such as the western Highlands, SW England and the Isle of Man. Showers and thunderstorms will slowly weaken during the evening hours, although there may be a relative uptick in activity in the London area / M4 corridor across the south Midlands into south/mid Wales during the evening and early part of the night, drifting to the northwest as a minor strip of mid-level positive vorticity approaches from the south. However, this will be occurring at a time when CAPE is gradually reducing as nocturnal cooling of the boundary layer commences, and so while there may be an increase in showery precipitation the coverage of lightning is somewhat questionable.
