Convective Outlook: Fri 18 Jun 2021
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Fri 18 Jun 2021 - 05:59 UTC Sat 19 Jun 2021

ISSUED 06:33 UTC Fri 18 Jun 2021

ISSUED BY: Dan

Longwave upper trough near western Europe will partially disrupt on Friday, with one portion lifting northeastwards across France. On the forward side, PVA/divergence aloft will encourage broad ascent reinvigorating an old frontal boundary straddling East Anglia and Cen S / SE England and resulting in rain become more extensive through the day, wrapping around a surface low over northern France. On the northern flank of the low, a southeasterly flow aloft will encourage a bulge/tongue of high Theta-W air to advect westwards into SE England and East Anglia, with PWAT also once again close to 40mm. All-in-all, this setup has the potential to produce a lot of heavy rain in parts of southern and eastern England, and the associated risk of flooding - and not just from short-duration individual cells, but also from more widespread heavy rain with embedded convective elements.

The exact location and shape of the overall pattern varies slightly amongst model guidance, and this casts some uncertainty over the site-specific detail (such as how active / far west the wrap-around rainfall will be). Either way, forecast profiles reveal a marked cold undercut, and so convection will be largely elevated and rooted from the 800-900mb layer warm bulge (surface temperatures of at least mid 20s Celsius, ideally higher, would be required for surface-based convection). While convective elements will be possible within the main rain band, here profiles look fairly saturated which will tend to limit the buoyancy of air parcels and naturally reduce the lightning risk. Meanwhile the degree of mid-level instability increases quite sharply with eastward extent - and so the position of various features will be key as to how much of this unstable environment can feed into SE England and East Anglia. There is substantial directional shear in the low-levels (northeasterly round to a southerly in the lowest 1km) but this will be below the cloud base, but nonetheless some workable shear is available within the cloud layer (both speed and directional). Rather moist profiles will also tend to limit the potential somewhat, but in either case there is the chance of some elevated thunderstorms developing over NE France and adjacent portions of the English Channel and southern North Sea that may get close to SE England and East Anglia during the afternoon and evening hours - the focus generally shifting northwards with time as the upper and surface lows advance slowly northwards. Eventually most activity should clear to the North Sea during the evening hours.