Convective Outlook
VALID 06:00 UTC Mon 19 Jun 2023 - 05:59 UTC Tue 20 Jun 2023
ISSUED 06:25 UTC Mon 19 Jun 2023
ISSUED BY: Dan
Heavy rainfall over Scotland on Monday morning, associated with a pronounced shortwave, will gradually clear northwards trough the day - although there is scope for embedded elevated convection in eastern Scotland, feeding inland from the North Sea. Elsewhere, a bright and breezy day is anticipated underneath a weak, transient upper ridge. That said, diurnal heating will yield 500-900 J/kg MLCAPE over Ireland/Northern Ireland beneath the eastern flank of a cold pool aloft associated with the broad upper low/trough to the west over the Atlantic. As such, scattered showers and a few thunderstorms are likely here, especially during the afternoon and early evening hours. Showers/storms may become particularly focussed along a couple of marked SW-NE convergence zones, especially from Munster to mid-Leinster and perhaps another from Connacht into Ulster. Modest southwest flow aloft with a unidirectional wind profile will result in 15-20kts effective shear, allowing some cells to become somewhat organised and capable of producing hail potentially 1.0-1.5cm in diameter (sub-severe). The risk of flooding will be augmented by stronger steering flow than recent days, however showers running along any marked CZ may tend to train over similar areas, resulting in locally large rainfall totals. A few showers/thunderstorms may also develop in parts of Scotland during the afternoon and early evening hours as well.
For England and Wales, while diurnal heating will lead to increasing instability, forecast profiles reveal substantial subsidence aloft and mid-level capping. It is still plausible a few heavy showers may develop, especially from north Wales and across northern England where a tongue of higher surface dewpoints may exist, although a few odd showers may be possible elsewhere - perhaps also from south Wales and across the Midlands to Lincolnshire. One or two cells may produce a few lightning strikes, but the risk overall is higher in Ireland. If any convection is able to punch higher than the mid-level capping then it may benefit from substantially stronger directional shear given the presence of a jet streak aloft.
While showers/storms will tend to decay during the mid-late evening hours, attention then turns to northern France as a shortwave rounding the base of the main upper trough to the west of Biscay engages with the western flank of a relatively high Theta-W airmass over France. The net result is a developing area of showery rain with embedded elevated thunderstorms over the Bay of Biscay during the afternoon hours, moving across the Brest peninsular through the evening hours, this then likely to grow upscale into an area of heavy rainfall across the English Channel and then moving into SW/Cen S England overnight. Exactly how much lightning activity occurs with this feature is uncertain, but there may still be pockets of activity embedded within the broader precipitation shield (perhaps most likely on the eastern flank of precipitation with unimpeded inflow). Additional, more localised elevated showers/thunderstorms will be possible further east along the south coast. The main threat will be the risk of locally high rainfall totals in some southern coastal counties of England, and perhaps also the Channel Islands.
