Convective Outlook
VALID 06:00 UTC Thu 23 Mar 2023 - 05:59 UTC Fri 24 Mar 2023
ISSUED 07:27 UTC Thu 23 Mar 2023
ISSUED BY: Dan
Upper vortex continues to sit to the west of Ireland during Thursday, but will gradually creep eastwards as a pronounced shortwave rounding its base becomes increasingly negatively-tilted on approach to western Ireland during the second half of the afternoon, sweeping northeastwards during the late afternoon and evening hours. This will likely provide the primary focus for deep convection during this forecast period. Initially, showers and areas of showery rain, partly driven/enhanced by sea convection, will pose the threat of a few sporadic lightning strikes along some Atlantic and Irish/Celtic Sea coasts exposed to the southwesterly steering winds. With time, diurnal heating will lead to an increasingly unstable environment inland across Ireland and portions of northern England (typically yielding 400-700 J/kg CAPE) both helping to sustain showers moving inland and also fostering additional shower development through the day.
Forecast profiles suggest convection could reach tops around 25,000ft over Ireland, with modest 20-30kts 0-6km shear perhaps helping a little with cell organisation/longevity. As was the case on Wednesday, some concern exists about the potential for convection to morph into larger areas/clusters of showery rain at times, and this could limit lightning activity - however, it does seem plausible there could be more sporadic lightning on Thursday than compared with Wednesday, primarily near southern/western coasts initially but perhaps increasingly so inland through the afternoon and early evening. For northern England, despite a strong anticyclonic jet streak in the vicinity, the depth of convection may be a little more restricted. That said, tops may still reach/exceed ~15,000ft in an environment with potentially >40kts cloud-layer shear (really strong flow just above the cloud tops). This may still be sufficient to allow a few semi-discrete cells to become organised, although the severe risk may be limited somewhat by modest instability. Tentatively issued a low-end SLGT. Depending on the extent of the frontal wave precipitation across central/southern Britain, the risk zone may need extending further south into more of the Midlands.
Some embedded line convection is possible in parts of SE England and the English Channel during the evening hours, while simultaneously an uptick in deep convection/lightning risk is possible in Wales/SW England for a time during the evening as an organised band of squally showers spreads eastwards.
