VALID 06:00 UTC Fri 07 Jun 2019 - 05:59 UTC Sat 08 Jun 2019
ISSUED 21:19 UTC Thu 06 Jun 2019
ISSUED BY: Dan
An elongated upper trough, with axis over Ireland, will become negatively tilted during Friday as an upper low over Biscay swings northwards towards southern Britain. At the surface, an area of low pressure will lift north across the English Channel towards southern England, while gradually filling - preceded by a period of frontal rain in many areas (with perhaps the exception of northern Scotland).
Behind the frontal rain, closer to the low centre, cold air aloft combined with heating of a warm, moist low-level airmass (dewpoints 11-13C) will yield 300-600 J/kg CAPE. Forcing aloft, post-frontal troughing and areas of low-level convergence will provide the lifting mechanism for scattered showers and a few thunderstorms to develop, initially over southern England around midday / early afternoon, but the risk then extending north across the Midlands / Wales later in the day, perhaps even into southern parts of northern England during the evening.
Concerns exist over how much residual cloud cover will linger following early rain, which may hinder adequate surface heating - much of which will depend on the exact shape and position of the surface low. There is also the risk of an area of showery rain moving into SE England later in the day which may or may not contain some embedded thunderstorm activity. As such, confidence is not particularly high in the placement of areas with the greatest lightning potential. Likewise, rather saturated profiles will tend to limit buoyancy of air parcels somewhat, restricting the lightning potential. Any showers or storms that develop to the north of the surface low will benefit from a better-sheared environment, and hence some organisation - capable of producing hail up to 1.5cm in diameter and some localised surface water flooding. Low cloud bases (LCLs) and maximised low-level convergence suggests the potential for a few funnel clouds.
For now, a SLGT has been issued where there is better multi-model agreement, but it is certainly plausible that this may need reshaping. Showers/storms that do develop may persist well into the evening, particularly Midlands / southern parts of northern England, as forcing aloft and upper low continue to drift northwards.