Convective Outlook
VALID 06:00 UTC Sat 15 May 2021 - 05:59 UTC Sun 16 May 2021
ISSUED 06:53 UTC Sat 15 May 2021
ISSUED BY: Dan
Cold air in the mid-levels will spread eastwards across the UK/Ireland on Saturday, following the passage of a weakening occlusion. The net result is an environment with modestly steep mid-level lapse rates and unstable to SSTs and surface heating inland. As such, 300-600 J/kg skinny CAPE is likely with numerous showers developing quickly in response to diurnal heating, tending to become organised along distinct zones of low-level convergence - such as along portions of southern England, a zone from Wales to the south Midlands and across portions of Ireland. Even the occlusion across parts of northern and eastern England is likely to turn increasingly convective/showery in nature through the day. Sporadic lightning will be possible from virtually any shower that develops, making it difficult to highlight specific areas - so for now have introduced two SLGTs where shower coverage should be more widespread, but lightning is certainly possible outside of these areas too. Overall, Ireland is most likely to see more widespread/active lightning activity.
For the most part, deep layer shear will be relatively weak leading to pulse-type storm mode much of the time - however, a notable wind-shift line with a light south/southeasterly flow ahead vs a breezier westerly flow behind may be the focus for locally enhanced directional shear, especially in the low-levels and may encourage some weak updraft rotation. Guidance varies considerably over the exact shape and location of this boundary at a given time, but generally located from west Wales to central southern England at 12z and shifting northeast to be somewhere around Liverpool-Colchester by 18z. A similar situation is also true for the Irish Midlands, with a light easterly surface flow on the leading edge of the main shower activity likely enhancing low-level shear. A small surface low from Surrey through to north Kent/the Thames Estuary during the afternoon may further aid the potential for low-level rotation. However the exact strength and location of such small-scale features is subject to some uncertainty.
Main hazards will be small hail and localised surface water flooding from showers training over similar areas. Enhanced low-level vorticity along convergence zones may encourage a couple of funnel clouds to develop. Showers will tend to weaken in intensity through the evening hours, but some will persist right through the night - especially near southern and western coastal areas, but not exclusively so. Therefore there will be an ongoing low risk of isolated lightning overnight.
