Convective Outlook
VALID 06:00 UTC Sun 12 Apr 2020 - 05:59 UTC Mon 13 Apr 2020
ISSUED 06:51 UTC Sun 12 Apr 2020
ISSUED BY: Dan
A potent upper low will slide southwards over the Republic of Ireland on Sunday, to reach western Biscay by late Sunday night. This will place England and Wales on the forward side, with some divergence aloft encouraging broad lift. At the surface, pressure will fall and a low centre will likely develop and gradually deepen, centred somewhere near SW England (exact location unclear depending on how the overall pattern evolves). Regardless, the pattern will be fairly slack initially, with surface winds becoming increasingly backed as this surface low develops, combined with the onset of sea breeze development in response to inland surface heating (yielding 500-900 J/kg CAPE). Consequently several areas of confluence and convergence are expected to develop, especially (but not exclusively) in a west-east line from Wales across the Midlands to East Anglia, associated with an elongating surface trough. These will provide the main focus for deep convection to develop, especially by late afternoon and into the evening hours.
Isolated to widely-scattered heavy showers and a few thunderstorms are expected to develop during the afternoon and evening hours, the most intense cells will be capable of producing hail 2-3cm in diameter, gusts of wind 40-45mph and localised flooding given their slow motion and hence prolonged nature. Shear is rather weak, and so pulse-type storms are most likely (where an individual storm may develop, peak and collapse within 1 hour, but the outflow can help trigger daughter cells nearby). However, shear will be somewhat stronger over Wales and northern England, and so any cells that can develop here may become more organised and longer-lasting. While winds aloft are not particularly strong, a nicely backed low-level SE-ly flow veering to a SSW-ly flow aloft may encourage some element of rotation in the more organised cells, and as such a supercell is not completely ruled out. Showers/storms will eventually merge into a more general area of showery rain through Sunday night as the cold airmass over northern Britain continues to dig southwards.
Concerns exist over just how many storms may develop, and whether they can become self-sustaining. It is possible that only a few showers/storms may have developed by Sunday evening, leaving many areas of the SLGT void of any lightning. During the late afternoon and evening hours, showers/thunderstorms over northern France will likely drift northwestwards across the Channel Islands towards Dorset/Devon but are likely to lose much of their lightning activity due to weakening instability.
