Convective Outlook: Sat 14 May 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 14 May 2022 - 05:59 UTC Sun 15 May 2022

ISSUED 06:32 UTC Sat 14 May 2022

ISSUED BY: Dan

An elongated upper vortex will track northeastwards across England and Wales on Saturday daytime, but the main upper low centre will hang back to the southwest and run across the Celtic Sea during the evening hours. This will likely help generate the first wave of showery rain across SW England. Meanwhile, a warm, moist airmass originating from France and Iberia over the preceding days and characterised by 850 hPa Theta-W of 14-16C will advect northwards and reach the Channel Islands during the evening, continuing to move towards southern England during Saturday night. At the same time, falling pressure over the Bay of Biscay whilst the surface high over the North Sea slowly strengthens will result in a tightening surface pressure gradient and a strengthening east/northeasterly low-level wind through the English Channel. The net result, as confirmed by forecast cross-sections, is a shallow, sloping cool layer near the surface (cold undercut) forcing the relatively high Theta-W airmass upwards to ride over the top as it journeys northwards across the English Channel due to isentropic upglide. This, in tandem with a low-level jet, is likely to result in elevated convection developing during the late evening and overnight hours, bringing an increasing risk of showery rain falling from high cloud bases (probably around 4,000-5,000ft) - some of this initially evaporating in the drier near-surface layer.

The biggest issue from a lightning perspective is the limited amount of instability; the upper vortex appears too displaced to the northwest to induce much in the way of cooling aloft atop the Theta-W plume, and if anything T500 increases during the second half of the night from the south as the vortex drifts away up the Irish Sea. This suggests there is a window of a few hours where conditions may be favourable for some sporadic lightning from elevated convection, most likely tied closest to the Theta-W plume ejecting from France, and so areas around the Channel Islands and portions of the English Channel towards Dorset would typically be most favoured before the overall risk wanes later in the night. If the upper vortex slows/takes a slightly more easterly track and/or the Theta-W plume advects slightly further west/earlier, then this would encourage a more favourable overlap of ingredients for lightning. That said, strong wind shear through the cloud-bearing layer would support organisation of any deep convection that can develop and pose the threat of some gusty winds (40-45mph).