Convective Outlook: Sun 16 May 2021
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sun 16 May 2021 - 05:59 UTC Mon 17 May 2021

ISSUED 07:13 UTC Sun 16 May 2021

ISSUED BY: Dan

An upper low will migrate slowly eastwards across the UK/Ireland on Sunday, with slightly colder mid-levels than on Saturday. Overall, the environment is fairly similar - weak deep layer shear, resulting in mostly pulse-type convection, fairly most profiles which tends to reduce the buoyancy of a rising air parcel, and tall, skinny CAPE (300-600 J/kg) which typically results in slower parcel accelerations. Around the periphery of the upper low, a PVA lobe will swing eastwards across southern England during the day providing broad ascent and areas of showery rain for a time. This may contain some pockets of stronger convection and isolated lightning, but will ultimately serve to reduce surface temperatures, especially in the southeast coupled with a brisk onshore flow - and so some uncertainty exists about how much strong convection may develop south and east of the A303/M3 for example. Arguably here the best chance of lightning (albeit reducing) is probably in the mid-late evening period when the flow turns more W/WNW-ly. Elsewhere, in the absence of any noteworthy forcing aloft convective cloud will quickly build through the day in response to diurnal heating and forced ascent from low-level convergence and orographic forcing. Showers will tend to form quite widely in many areas, and the strongest cells will be capable of producing hail (locally >1cm in diameter) and some sporadic lightning. Cold pools/outflow will also likely aid daughter cells nearby, creating a rather complex and messy distribution of showers and weak thunderstorms.

A flabby surface low will slowly drift eastwards across SE Ireland - Wales - Midlands - East Anglia during this forecast period, and so the steering flow for the showers will tend to shift accordingly with time. On the north side of this low, an easterly component to the low-level winds could result in an increase in (modest) shear during the afternoon/early evening across East Anglia, Lincolnshire and perhaps the East Midlands, with directional shear in the low-levels perhaps aiding some rotation of any strong updrafts. The strongest flow aloft will be over southern England, on the south side of the upper low, and this could result in 15-20kts bulk shear, and so ultimately any deep convection in these two areas of interest could become a little better organised/longer-lasting at times. This aside, the generally slack surface pattern and resultant low-level convergence/outflow could encourage a few funnel clouds or weak tornado to develop. Slow storm motion will bring the risk of localised surface water flooding. Showers will slowly weaken during the evening hours, but will likely persist in some areas through the night.