Convective Outlook: Sat 13 Aug 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 13 Aug 2022 - 05:59 UTC Sun 14 Aug 2022

ISSUED 06:44 UTC Sat 13 Aug 2022

ISSUED BY: Dan

Mid-level heights will slowly fall on Saturday as an upper low to the west of Iberia drifts northeastwards to become located broadly over the western English Channel by 06z Sunday. Cooling aloft will occur progressively northeastwards above a hot, dry and deeply mixed low-level airmass in place across much of the British Isles. The net result is steepening mid-level lapse rates, the leading edge near the Scilly Isles at 06z Saturday and then generally expanding erratically northwards across Ireland and western Britain through Saturday and Saturday night. As such, pockets of AcCas with bases typically around 11,000-14,000ft will be possible associated with this environment, with the potential for elevated showers from very high cloud bases to semi-randomly develop at times depending on subtle forcing/moistening aloft. Most model guidance suggests many areas will remain dry/void of showers, but the thermodynamic environment would support the potential for a few. The greatest mid-level instability will often be located near the Scilly Isles and broader western English Channel area closest to the approaching upper trough, and so perhaps the best chance of a few high-based showers/thunderstorms will be in this area. Given the magnitude of instability present above the EML, lightning could be quite active for a time with any cells that do manage to develop. A marginal SLGT has been introduced, primarily for the overnight period.

Meanwhile, across Ireland in particular there is scope for surface-based development if temperatures can reach ~30C. Moisture pooling along low-level convergence zones and/or upslope flow will be crucial, and a few CAMS suggest one or two isolated heavy showers may develop for a couple of hours in the evening before collapsing and decaying. The magnitude of CAPE is fairly significant and would promote rapid convective cloud development and fairly active lightning for a time, however the rather dry profiles and weak speed shear, combined with cooling of the surface during the evening, will likely result in a relatively short lifespan. A similar scenario is also possible in NW Wales and southern Scotland. A marginal SLGT has been introduced to cover the potential for both evening isolated surface-based developments, and for possible overnight elevated activity farther north.

Later in the night, an uptick in showery outbreaks of rain is possible across parts of Ireland / Northern Ireland / Scotland drifting to the northeast, as PVA on the forward side of the approaching upper trough impinges on the northwestern periphery of the EML. There could be some elevated shower/thunderstorm activity as a result (most likely northern half of Ireland into Northern Ireland), but confidence is low as to whether much develops during this forecast period or occurs primarily deeper into Sunday morning.