Convective Outlook: Wed 24 Aug 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 24 Aug 2016 - 05:59 UTC Thu 25 Aug 2016

ISSUED 08:03 UTC Wed 24 Aug 2016

ISSUED BY: Dan

Most plume events carry an element of uncertainty, but this particular event appears to have greater than average uncertainty. Even at this stage there is a large spread of possible outcomes amongst various models. There is good agreement amongst NWP to keep the warm, moist plume across England capped during Wednesday daytime, despite a build-up of SBCAPE in response to strong heating - particularly across E Anglia / Home Counties / SE England / CS England. Whilst currently unexpected, should the cap be eroded locally, then a strong to perhaps severe thunderstorm would be possible capable of large hail, given strong deep layer shear and high values of storm relatively helicity.

The most likely evolution, however, is an increase in elevated convection to occur as the warm plume begins to destabilise, beginning over the English Channel and environs (Dorset/Devon etc) early-mid afternoon, with activity generally migrating NNE-wards through the evening and night hours. Initially, given hot, dry surface air, a lot of virga with not much precipitation reaching the ground is expected, but still scope for some (at least sporadic) lightning from elevated thunderstorms. That said, upper forcing is rather weak much of the time, and a low-end SLGT has been issued to highlight this risk, but with some uncertainty over the exact areas that may be affected, and hence a shift to the W or E may be possible in any later updates.

Later in the night slightly stronger forcing arrives from the southwest, but by this stage the most favourable conditions for thunderstorms will have shifted east - and so a broad LOW threat level is issued for an uptick in precipitation coverage later in the night over the Midlands for example, but questionable as to how much lightning activity will be associated with this.

The true extent of lightning activity (in terms of frequency and coverage) is a little unclear at this time, and at this stage does not look to be as frequent/widespread as the 19th-20th July 2016 events.  Any thunderstorm activity that does develop on Wednesday night will continue to track NNE-wards beyond this forecast period into Thursday morning.

Scattered showers and a few weakly-electrified thunderstorms will also be possible over W Scotland and western fringes of Ireland on Wednesday night.