VALID 06:00 UTC Sun 02 Dec 2018 - 05:59 UTC Mon 03 Dec 2018
ISSUED 21:22 UTC Sat 01 Dec 2018
ISSUED BY: Dan
A rather messy picture is expected on Sunday with a broad WSW-ly flow across the British Isles allowing advection of a relatively warm, moist low-level airmass (2m dewpoints of 10-13C typically). However, a series of shortwaves, and associated cooling aloft, will help to deepen the convective layer at times, generating a few hundred J/kg CAPE. Clusters of heavy showers will likely develop and migrate ENE-wards at various times through this forecast period across Ireland and central and southern Britain - hence a rather broad LOW threat level.
Particular attention is given to central/southern Ireland during the late morning and into the afternoon where a favourable overlap of stronger upper forcing, (marginal) CAPE and reasonable shear is likely to occur. Some cells here could become reasonably well-organised, with sporadic lightning possible, hail up to 1.5cm in diameter and gusty winds. Despite fairly saturated profiles and weak CAPE, the shear component may be enough to compensate for some lightning activity.
Given relatively low cloud bases and strong shear in the low levels (even subtle hints of slightly backed low-level winds) a tornado cannot be ruled out. This favourable environment then transfers eastwards to Wales by the second half of the afternoon and early evening.
Heavy showers are also expected across Wales and SW England on Sunday afternoon, which will continue to move eastwards through the evening and night in a rather messy fashion. A few lightning strikes and small hail will be possible from the strongest cells. Detail is a little unclear as to the exact distribution of such showers given differences amongst NWP guidance in the handling of various shortwave features.