VALID 06:00 UTC Mon 26 Jul 2021 - 05:59 UTC Tue 27 Jul 2021
ISSUED 07:58 UTC Mon 26 Jul 2021
ISSUED BY: Dan
Upper low centred over the North Sea early on Monday will continue to extend northwards and merge with an existing large upper low near Iceland. Between these two features subtle upper ridging covers southern Britain tending to restrict the coverage of showers here during the day, before the Iceland upper low extends a trough southwards later in the day and overnight. Overall, the pattern is rather complex with several areas of interest that will tend to change with time during the forecast period, and so realistically there could be a few lightning strikes almost anywhere at some point today.
Firstly, showery outbreaks of rain will affect the Northern Isles during the morning and afternoon, and could produce a few lightning strikes. Then, diurnal heating across England, Wales and Scotland will yield 500-1,000 J/kg CAPE in places with a few scattered showers and thunderstorms developing. The greatest risk appears to be in Scotland, NE England, S Wales and SW England, where steeper mid-level lapse rates will combine with low-level convergence and orographic forcing. That said, a few isolated heavy showers and thunderstorms will also be possible elsewhere, such as East Anglia and SE England, but coverage is likely to be fairly isolated. Towards late afternoon and through the evening hours a shortwave trough will swing across SW England and S Wales, and this will likely encourage additional scattered showers or thunderstorms which may persist well into the evening across the south Midlands and Cen S England (rather than the usual nocturnal decay) - in fact there may be sufficient forcing and residual instability to maintain them or even provide an uptick through the night as they track into the East Midlands, East Anglia and SE England with some sporadic lightning in places. Some marginally-severe hail may be possible with cells during the afternoon/early-evening in SW England and S Wales.
Across Ireland, showery outbreaks of rain will track eastwards through Monday - the leading edge could see a few scattered thunderstorms developing, but there is some uncertainty as to how active these may be (from a lightning perspective). This feature will then swing into Wales and SW England on Monday night, with an ongoing risk of a few sporadic lightning strikes, but confidence on location of these is rather low.