Convective Outlook
VALID 06:00 UTC Sun 23 Oct 2022 - 05:59 UTC Mon 24 Oct 2022
ISSUED 06:58 UTC Sun 23 Oct 2022
ISSUED BY: Dan
An upper ridge over central Europe will drift eastwards on Sunday, allowing an upper low west of Biscay to swing northeastwards into SW Britain, before being absorbed by a negatively-tilted Atlantic longwave trough during the evening and overnight hours over the British Isles. A lead impulse will be driving an area of showery rain, perhaps with embedded thunderstorms initially, along a cold front gradually northwards and eastwards during the morning hours, but weakening in intensity with time. This front will generally shunt the earlier 14C 850hPa Theta-W plume eastwards, largely confined to East Anglia and SE England by the afternoon where the front is expected to stall and bend back westwards as an effective warm front through the English Channel to NW France by noon. This frontal boundary, and the humid airmass in the vicinity, will provide the focus for perhaps several waves of showers/thunderstorms across southern and eastern England during Sunday afternoon and evening, as waves develop along the frontal boundary due to increased forcing aloft.
The next shortwave is expected over N France / English Channel by 12z, and will continue northwards into S Britain during the afternoon hours. The exact evolution hereafter becomes complicated and messy, as each convective cluster impacts the overall environment and model guidance will likely struggle to handle the influence on the thermodynamic profile from earlier convection. Nonetheless, one or two main clusters of showers/thunderstorms may spread across the English Channel into S / SE England and later East Anglia. Temperatures across northern France are expected to peak in the low 20s Celsius and this warm air will advect across the English Channel, supporting 500-1,000 J/kg MLCAPE in the vicinity. TMax of 16-19C over inland parts of SE Britain will still yield 500-800 J/kg MLCAPE. With a strong southwesterly jet aloft, DLS of 30-40kts is likely, enhanced where surface winds become backed ahead of convective clusters. The net result is an environment supportive of vigorous convection, organised at times with scope for both supercell and bowing structures. If a cell can remain fairly discrete and long-lasting, hail perhaps greater than 2.0cm is possible, however the tall cloud depth with tops potentially above 10km / 33,000ft (and therefore CAPE stretched over a greater height) coupled with fairly moist profiles may tend to limit the severe hail potential overall. Perhaps of greater concern, and the main basis for the introduction of a SVR, is for damaging winds with scope for 60-70mph gusts locally - especially offshore/near windward coasts, but not exclusively. Given potentially backed surface winds near some convective clusters enhancing LLS, there may also be scope for one or two tornadoes. Some concerns do exist over the quality of convection - with fairly moist profiles and mass ascent, convection could grow upscale quickly into larger areas of heavy convective rainfall, which will tend to restrict the lightning potential and reduce the risk of severe winds/tornadoes. So realistically, convection will need to remain semi-discrete if damaging winds/frequent lightning are to occur. Another thing worth mentioning is as a mid-level jet streak arrives later in the afternoon, the forward motion of individual cells could be around 50mph - and perhaps even faster where bowing segments accelerate forwards.
While the focus is on SE Britain for severe convection, showers and a few weak thunderstorms are also possible elsewhere - especially C + S Ireland in response to diurnal heating. Numerous showers will continue overnight, especially across much of southern and eastern Britain, perhaps merging to give some longer spells of rain.
