Convective Outlook: Sun 25 Jun 2023
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sun 25 Jun 2023 - 05:59 UTC Mon 26 Jun 2023

ISSUED 07:06 UTC Sun 25 Jun 2023

ISSUED BY: Dan

An upper ridge will persist across much of Europe on Sunday, while a longwave upper trough slides eastwards across central/northern Britain. In the low-levels, a high Theta-W plume (>18C at 850hPa) will reside across Britain ahead of an active Atlantic cold front located N-S over Ireland at 06z Sunday. The exact evolution through the day carries some uncertainty and will depend on a number of factors phasing together at the right time. Assuming a degree of cloud-free skies across much of England on Sunday morning, strong surface heating with temperatures reaching the high 20s in eastern England (in excess of 30C over East Anglia and SE England), combined with cooling of the mid/upper troposphere as the upper trough approaches, could potentially yield >1,000 J/kg MLCAPE. Strengthening S/SW flow aloft (55kts at 500hPa, 85kts at 300hPa) will overspread Britain progressively through the day, creating an environment with effective shear of 30-50kts. Lee/pre-frontal troughing is expected to develop over northern England and across central/northern Scotland, aiding low-level convergence in the area and, in conjunction with orographic lift, providing the focus for deep convection to occur. This appears mostly likely just inland from the North Sea coast, with thunderstorms developing here and maturing as they drift close to the coast, and then offshore. Exactly how close to the coast initiation occurs is a little uncertain, with some CAM guidance suggesting that the vast majority of thunderstorms develop even just offshore. For now, catering for the idea of initiation further inland and then expanding in coverage near coastal areas. In addition, thunderstorms may also develop on the leading edge of the cold front across northern England aided also by PVA on the forward side of the upper trough, maturing as they drift northeastwards - and so it is possible some areas close to the North Sea coast could potentially have a couple of rounds of thunderstorms. 

Fairly unidirectional wind profiles near-parallel to surface boundaries suggests thunderstorms may develop into linear clusters, but also scope for splitting supercells if cells can remain a little more discrete. The environment would be supportive of hail 2-3cm in diameter in the most organised cells (perhaps larger in any discrete supercells), and wind gusts of 40-60mph. Weaker forcing with southward extent, both at the surface and aloft, suggests thunderstorms may be more isolated towards the Midlands, Lincolnshire and Norfolk, but any that do occur here still have the potential to be severe. The lack of more substantial backing of the low-level winds, coupled with relatively high cloud-bases (at least initially) due to the deep, well-mixed boundary layer will tend to limit the tornado risk. However, close to the North Sea coasts where an onshore wind is likely for a time, and more discrete cells with longer residence time over land (allowing a better chance to become more organised, more especially over Yorkshire/Lincolnshire/NE Midlands), may pose the risk of an isolated tornado.

In the post-frontal environment, scattered showers and a few thunderstorms are also possible in Ireland/Northern Ireland beneath the upper trough axis, aided by a shortwave drifting eastwards, and these too could exhibit some organisation due to 20-30kts effective shear with perhaps some marginally-severe hail. A more organised cluster may evolve over Ulster and spread into SW Scotland. While a general weakening trend is anticipated through the evening hours, a few showers/thunderstorms may drift across the Irish Sea into SW Scotland/NW Wales/NW England during the evening and night.