VALID 06:00 UTC Sat 08 Aug 2020 - 05:59 UTC Sun 09 Aug 2020
ISSUED 06:55 UTC Sat 08 Aug 2020
ISSUED BY: Dan
A pronounced elevated mixed layer (EML) will reside over much of central / southern / eastern England throughout Saturday, creating an environment with very steep mid-level lapse rates. An upper ridge will extend across much of the nearby Continent, placing a relatively light southwest flow aloft across the British Isles. The EML will act as a cap to surface-based convection, despite building CAPE due to diurnal heating. However, any minor impulses aloft (and NWP guidance does suggest some weak PVA at times) could trigger some elevated convection with bases typically around 10-12,000ft at almost any time during this forecast period, and almost anywhere within the LOW threat level. However, most areas will remain void of lightning activity.
The lack of substantial forcing or shear aloft suggests that any cells that do manage to develop will probably not last particularly long, but may be deep enough to produce a few lightning strikes. Much of any rain will probably evaporate before reaching the ground, such is the depth of hot, dry air below. Highlighting specific areas with a higher risk of lightning is difficult given no obvious forcing mechanisms. However, as the flow at ~850mb backs and strengthens during the evening and night hours, increasing warm advection will encourage lift / istentropic upglide along two main zones of interest - one from east Wales across the north Midlands into northern England, and then another over the eastern English Channel into SE England. Consequently, there may be an uptick in rather scattered lightning activity over these areas during the overnight period, but confidence is not particularly high and it is still possible other random isolated cells could develop elsewhere too. A low-end SLGT has been issued to highlight the area most at risk (especially towards the eastern side), but another SLGT may be considered for SE England if confidence increases.