VALID 06:00 UTC Thu 15 Mar 2018 - 05:59 UTC Fri 16 Mar 2018
ISSUED 21:39 UTC Wed 14 Mar 2018
ISSUED BY: Dan
Upper trough migrating slowly NE-wards across southern Britain and Ireland on Thursday. At the surface, an occluded front and associated frontal rain will shift northeastwards across northern and eastern areas, leaving somewhat of a clearance in central and southern parts.
Cold air aloft and steep mid-level lapse rates, associated with the upper trough, atop diurnal heating will generate 400-700 J/kg CAPE with a few scattered showers or thunderstorms developing from midday through to the early evening in association with a convergence zone (marked as an occlusion on the latest T+24 FSXX). Mean steering flow will result in showers/storms moving SSE to NNW, and while the main focus will be on surface-based convection over England and Wales, it is possible later in the afternoon and first part of the evening for some showers/thunderstorms over N France to approach S/SE England, though probably weakening by mid-evening.
Main threats are localised surface water issues from relatively slow-moving heavy downpours, and perhaps some hail 1.0-1.5cm in diameter. Some NWP guidance suggests showers/storms could be quite isolated, meaning many places even within the SLGT may not experience any lightning nearby - and there is some uncertainty as to the extent of the cloud cover left behind the main occluded front, which could serve to inhibit deep convection by reducing insolation. Such convergence zones can sometimes produce brief funnel clouds or a weak tornado.