Convective Outlook: Fri 25 Aug 2017
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Fri 25 Aug 2017 - 05:59 UTC Sat 26 Aug 2017

ISSUED 12:00 UTC Thu 24 Aug 2017

ISSUED BY: Dan

... SCOTLAND / NORTHERN IRELAND ...
Upper vortex will be located to the west of Scotland for much of Friday, with modestly-steep mid-level lapse rates covering Scotland and Northern Ireland. The forecast is complicated somewhat by cloud and showery outbreaks of rain along an occlusion, but ahead of and behind this feature there will be scope for some deep convection to develop in response to both diurnal heating and SSTs. Low-level convergence may play a role in N + NW Scotland, and this combined with a few hundred Jkg-1 CAPE should allow some slow-moving heavy showers or thunderstorms to develop - and hence a low-end SLGT has been issued. Some localised surface water issues may evolve given the slow-moving nature of the showers. A few weak funnel clouds or a break tornado may also be possible from this activity.

As the upper vortex and associated steeper lapse rates moves eastwards overnight, so the risk of some isolated lightning continues, though limited somewhat by nocturnal cooling of surface temperatures and areas of more stratiform rain associated with old occlusions.

... SE ENGLAND ...
As an Atlantic upper trough digs south to the west of Iberia, becoming a cut-off low, the flow over western Europe will back during Friday, allowing advection of a higher WBPT airmass across the English Channel and into SE England. Still some uncertainty, but there is potential for some elevated showers or thunderstorms to develop through isentropic lift over the eastern half of the English Channel late Friday afternoon into the evening, which may get close to both SE England and the Channel Islands. Too much uncertainty at present for any upgrades to SLGT, with the vast majority of NWP output keeping any activity over mainland France.

... N / NE ENGLAND ...
As the upper vortex over Scotland drifts eastwards on Friday night, the upper flow will strengthen on the forward side, with a shortwave perhaps engaging with the moisture axis over eastern England. Some showery outbreaks of rain may develop, largely from the mid-levels, which could produce some isolated lightning - moreso as it exits to the North Sea. In fact this area of development then becomes the focus for some potentially quite active lightning activity over the North Sea during Saturday.