Skimming thunderstorms - Timelapse - Herne Bay, Kent - 9th May 2023

A day of skimming convection with a decent multicellular storm's structure skimming overhead just after 6 pm with loud rumbles of thunder heard with some rainfall although not a lot! The convective outlook from the day by Dan Holley will be below.
Convective Outlook
VALID 06:00 UTC Tue 09 May 2023 - 05:59 UTC Wed 10 May 2023
ISSUED 06:31 UTC Tue 09 May 2023
ISSUED BY: Dan
It is worth stressing from the outset this is a potentially very messy forecast evolution that carries a lot of uncertainty and is subject to updates/adjustments. In a broad sense, an upper trough will migrate eastwards across the British Isles on Tuesday, with a cut-off upper low within its base that will tend to drift eastwards through the English Channel. This will be associated with a fairly significant cold pool aloft (T500 around -26C, for example) with a cyclonically curved jet streak rounding the base of this upper low and placing the left exit region in the vicinity of southern England. The exact shape/evolution of this mid-level feature will be, at least partly, instrumental in how things develop through the day, and various NWP output varies in its handling of this feature. Therefore, the forecast evolution is highly sensitive to subtle changes in forcing aloft and/or the thermodynamic/kinematic profile which may not be fully resolved in NWP. At the surface, a moist low-level airmass will be present across England and Wales (having originated over eastern Ireland on Monday afternoon), with dewpoints typically around 10-13C. A rather diffuse cold front will migrate eastwards through the day, with a slight reduction in dewpoints likely accompanied by a veering of the low-level winds. Despite the fairly large spread in possible evolution over central and southern England in particular, key themes are outlined using a multi-model blend approach.
Fairly extensive cloud is likely during the morning across much of England and Wales, with some hill and coastal fog. Through the day this should thin and break, with increasing insolation then resulting in better surface heating. A selection of forecast soundings in central/southern Britain, while fairly moist through much of the troposphere, do appear to become unstable with only modest surface heating and as such a few heavy showers may develop around 11z-13z (in addition to an area of showery rain drifting across SW England with the potential for a few odd lightning strikes with this feature). Exactly how unstable profiles become will depend on both the amount of surface heating that can be achieved, and also the exact thermodynamic profile. Most model guidance suggests 500-800 J/kg MLCAPE may be achievable (with SBCAPE potentially in excess of 1,000 J/kg). Showers may initially be fairly scattered and particularly tied to sea breeze convergence zones, such as near the Dorset/Hampshire coast for a time before lifting further north, eventually evolving into thunderstorms with time. PV lobes in the vicinity of the upper low aiding synoptic-scale ascent may encourage thunderstorms to grow upscale into a larger multicell complexes across parts of the Midlands and central southern England, drifting into East Anglia and SE England later in the afternoon and into the evening. This may also coincide with a modest increase in effective bulk shear as the aforementioned jet streak rounds the base of the upper trough and shifts northeastwards into SE England (12z GFS more keen/further north than 12z ECMWF, for example). Outflow from thunderstorms will significantly impact where new cells develop, and this will undoubtedly lead to a lot of uncertainty in locations with the greatest lightning coverage. Fairly moist profiles casts some concern over exactly how much lightning activity will occur. The relatively slow movement may lead to local surface water flooding, potentially very disruptive when coinciding with the evening commute for the likes of London and the Home Counties. This may be exacerbated by local strong downburst winds, gusts typically 40-50mph (and therefore below SVR criteria). Hail may accompany the strongest cells, but probably no larger than 1.0-1.5cm in diameter. In addition, a small surface low may linger close to or over East Anglia, maximising low-level convergence and forced ascent and potentially resulting in quasi-stationary heavy downpours/thunderstorms here that may also lead to local flooding away from the larger complex of thunderstorms further to the west.
Elsewhere, an arguably simpler evolution is anticipated with scattered heavy showers and a few thunderstorms developing in Ulster and Leinster through the day, and also along the east coast of Britain (perhaps also NE Scotland) in response to sea breeze convergence and some orographic forcing. Reasonably strong updrafts may aid vorticity stretching along low-level CZs to produce a few funnel clouds.

[@Iceman2606Wx]

That structure towards the end!🤩 Lovely time-lapse👌