Convective Outlook
VALID 06:00 UTC Mon 08 May 2023 - 05:59 UTC Tue 09 May 2023
ISSUED 07:01 UTC Mon 08 May 2023
ISSUED BY: Dan
Upper ridge over western Europe will slowly topple southeastwards on Monday, as an upper trough advances from the Atlantic (the trough axis not crossing Ireland until Tuesday morning). A series of fronts will cross the British Isles, bringing extensive cloud and outbreaks of rain across Britain for a time, but leaving Ireland/Northern Ireland with increasing amounts of insolation through the day as convective overturning breaks up the cloud. As such, heating of the moist low-level airmass (dewpoints 13-15C) coupled with cooling aloft as the upper trough approaches, is likely to yield an increasingly unstable environment with scope for 500-1,000 J/kg MLCAPE (and up to 1,500 J/kg SBCAPE). Low-level convergence will also increase as surface troughing develops over Leinster during the afternoon hours. Forecast soundings (and confirmed by the 00z Valentia radiosonde) reveal a very dry and somewhat warm 500-700hPa layer, acting as a cap to deep convection initially, but this should erode by around noon as cooling aloft occurs. As such, heavy showers and thunderstorms should develop fairly readily once surface temperatures reach the mid-high teens Celsius - generally from 12-14z onwards. Thunderstorms will mature in size and intensity as they drift gradually to the east/northeast at 15-25mph, therefore the most widespread coverage of lightning is anticipated in eastern areas where a MDT was introduced. CAM guidance has been relatively consistent at two main areas of more widespread thunderstorm activity - across Donegal and northern and western portions of Northern Ireland, and a separate area across central, southern and eastern Leinster, with perhaps a relative minima in activity in NE Leinster and SE Ulster.
Forecast soundings suggest convective cloud tops could reach -50C / 30,000ft. Shear is somewhat limited due to the main jet activity remaining to the south of Ireland, but that said 20-30kts should be sufficient, especially in central and southern Leinster, to help separate updrafts/downdrafts and aid some organisation of cells, perhaps with some transient supercellular characteristics. Some very active and long-lasting thunderstorm clusters are possible, perhaps back-building such that some areas receive multiple thunderstorms and therefore increasing the risk of local surface water flooding. Hail and gusty winds will likely accompany the strongest cells, although both aspects probably below severe limits (1.0-1.5cm diameter and 35-45mph respectively). Thunderstorms may well persist until mid-late evening in eastern areas, but a gradual decay is anticipated through the evening hours as they drift across the Irish Sea towards the Isle of Man, W/NW Wales and NW England.
Elsewhere, thunderstorms emanating from Northern Ireland may spread into S/SW Scotland, especially during the late afternoon and evening hours, although how long they continue to produce lightning is questionable given only modest instability here. In addition, surface heating in N/NE Scotland and northern England, coupled with orographic forcing and some low-level convergence, may encourage a few heavy showers/weak thunderstorms to develop here during the late afternoon and early evening hours - but confidence on much in the way of lightning is too low to warrant a SLGT at this stage. A few lightning strikes may also be possible in the morning/midday period near/over SW England and Wales on the rear of the frontal rain, but forecast profiles do not appear to be fairly conducive to sustain deep convection and so have refrained from introducing a SLGT here. Towards the end of Monday night (and this forecast period), a shortwave trough approaching SW Ireland may produce a few sporadic lightning strikes, but much (if any) activity may hold off until 06z Tuesday before making landfall. Similarly, NE Scotland may see a few isolated lightning strikes towards the end of the night.
