Convective Outlook: Tue 22 Aug 2017
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Tue 22 Aug 2017 - 05:59 UTC Wed 23 Aug 2017

ISSUED 14:25 UTC Tue 22 Aug 2017

ISSUED BY: Dan

UPDATE 14:25 UTC Adjustments made to existing SLGT and LOW threat levels to expand the elevated thunderstorm risk farther south overnight, while a MDT has been introduced to highlight areas where elevated thunderstorms seem most likely (while acknowledging greater instability is found farther south). In general, high-res guidance continues to suggest increasing covering of thunderstorms from mid-evening over Northern Ireland into western fringes of Scotland and the Hebrides, the Theta-E plume then destabilising farther and farther south through the early hours as the cold front and upper forcing approaches from the west. Hints in several models of a few elevated thunderstorms developing ahead of this within the Theta-E ridge over the likes of Derbyshire / Nottinghamshire / Yorkshire / Lincolnshire during the early hours. Some elevated showers (of questionable depth) possible this evening over parts of East Anglia too...

Plume of high WBPT will be drawn northwards across the British Isles on Tuesday, ahead of an approaching Atlantic upper trough. Shortwaves running ahead of the main trough will provide the focus for mostly mid-level instability release, resulting in elevated showers and perhaps a few thunderstorms - the greatest risk of deeper, more widespread convection seems most likely on Tuesday evening / night as better forcing arrives from the southwest, destabilising the plume.

Some uncertainty exists as to the extent of this destabilisation (especially considering the messy mix of stratiform vs convective precipitation), and hence how widespread lightning activity may be - a SLGT has been issued for now, though this could perhaps be upgraded locally to a MDT should confidence improve. In general, better forcing and shear will be found towards the NW, whilst instability will be more significant farther southeast. Should any convection become rooted in the boundary layer, more likely late afternoon/early evening, then there would be scope for an isolated tornado - particularly across northern parts of the Republic of Ireland into Northern Ireland. Some hail is also possible, but the greatest threat will probably be from localised surface water issues given embedded convective elements.