Convective Outlook
VALID 06:00 UTC Wed 03 Jun 2020 - 05:59 UTC Thu 04 Jun 2020
ISSUED 20:20 UTC Tue 02 Jun 2020
ISSUED BY: Dan
Upper trough will continue to extend southwards across western Europe during Wednesday, its axis over the British Isles. The associated cooling and moistening of the mid-levels will in part aid in showery outbreaks of rain developing fairly widely across England and Wales during the day. This will be accompanied by extensive medium- and low-level cloud. Pockets of weak elevated convection may exist, embedded within areas of showery rain, and could produce the odd isolated lightning strike. However, the main focus will be linked to any sufficient cloud breaks that can develop to aid in surface heating across S / SW England and S Wales and yield 200-500 J/kg CAPE. Given the slack surface pattern, areas of low-level convergence may develop and this will provide the primary forced ascent to generate deep convection.
Big concerns exist over saturated profiles and extensive cloud cover. Shear is also likely to be fairly weak given the location within the trough axis. Hence, even if deep convection does indeed develop, showers may struggle to maintain themselves for any length of time with pulse-type showers likely (although outflow may aid additional development nearby). This, combined with hints of a slight warm nose in the 500-600mb layer may also serve to restrict the depth of convection too. All in all, this does not bode well for much in the way of lightning activity, unless sufficient sunshine can significantly lift surface temperatures and be able to force parcels higher than the 500-600mb layer. Current thinking is a few scattered heavy, slow-moving showers may develop during the day - which poses a risk of localised surface water flooding - but the risk of lightning is rather limited. Given the low cloud bases and low-level convergence present, a few funnel clouds may be possible.
Model guidance also varies in location of the most favoured areas for heavy showers to break out too. As a general rule, a zone from S/SE Wales and across southern England / southern parts of the Midlands is at risk, perhaps moreso close to coasts where low-level convergence will be maximised, but pinpointing specific areas where heavy showers are most likely, and then on top the aforementioned concerns over lightning potential, negates the need for a SLGT unless confidence in either aspect increases.
