Convective Outlook: Thu 17 Jun 2021
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Thu 17 Jun 2021 - 05:59 UTC Fri 18 Jun 2021

ISSUED 08:45 UTC Thu 17 Jun 2021

ISSUED BY: Dan

This whole forecast period carries low confidence. A longwave upper trough will affect western Europe on Thursday, with a plume of high Theta-W covering much of the nearby Continent, occasional tongues extending into East Anglia / SE England at times. Model guidance does not have a great handle of clusters of elevated convection that are running northeastwards along the western edge of this plume, primarily across SE England and East Anglia, and this casts a lot of uncertainty over how the remainder of the day will evolve. It is likely pulses of showery rain will continue to run northeastwards at times, perhaps with a few lightning strikes but confidence is low on this aspect.

Fairly extensive cloud will tend to suppress surface temperatures and therefore surface-based convection. However, if sufficient insolation can lift surface temperatures close to the mid 20s Celsius, given dewpoints near 20C, this may aid the development of surface-based thunderstorm(s) during the afternoon/early evening hours. The main focus at present is SE England and the extreme east of East Anglia, especially aided by sea breeze convergence and perhaps some orographic forcing (e.g. North Downs). Some fairly substantial CAPE could evolve here, and given significant cloud-layer shear this could result in a severe thunderstorm capable of producing large hail. Backed low-level winds, provided strong updrafts, also suggests the threat of an isolated tornado. However, given the uncertainties this is considered a low risk for now, and no higher threat levels have been introduced.

Overnight, a portion of the upper trough will begin to lift northeastwards and become increasingly negatively-tilted - the net result is the flow aloft will back and allow the Theta-W plume over France to advect back northwestwards with time. PVA/forced ascent will likely lead to areas of rain developing once again over parts of southern and eastern England, and towards the end of the night there may be a small chance of elevated thunderstorm activity to creep close to east Kent. However, it seems more likely lightning activity will remain offshore/over France/Belgium etc.