VALID 06:00 UTC Sat 04 Jul 2020 - 05:59 UTC Sun 05 Jul 2020
ISSUED 08:08 UTC Sat 04 Jul 2020
ISSUED BY: Dan
Much of this forecast period is dominated by a warm and very moist airmass across the British Isles (PWAT close to 40mm over southern England), strong upslope flow aiding significant rain totals on western hills through Saturday, while sheltered eastern areas will be markedly drier with Fohn effect also likely to lift temperatures locally. However, little deep convection is anticipated through Saturday due to deep warm/most profiles and meagre instability. A few showers may affect northern and western Scotland during Saturday daytime, but these are likely to be restricted in depth and so the lightning risk is considered very low.
The main interest begins close to midnight Saturday night, in association with a rapidly deepening surface low that will be approaching the Outer Hebrides at this time. Under the influence of the left exit of the mid-level jet and tropopause fold, very dry mid/upper-level air will overspread a very moist low-level airmass, creating substantial potential instability. Given the very strong forcing aloft, some embedded convection could develop in the vicinity of the southern Outer Hebrides, this would then drift quickly eastwards across the Inner Hebrides and then northeast across the mainland from western to northeast Scotland during the early hours of Sunday. Forecast profiles suggest such convection would be elevated above the 750-800mb layer, perhaps aided by a strong low-level jet. The exact west-east corridor, and also the extent of lightning (if any) is a little uncertain - primarily due to the exact track of the surface low - but should confidence improve then a SLGT may be considered.