Convective Outlook: Sat 04 Jun 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 04 Jun 2016 - 05:59 UTC Sun 05 Jun 2016

ISSUED 14:05 UTC Sat 04 Jun 2016

ISSUED BY: Dan

UPDATE 14:05 UTC Have removed SLGT from W Scotland, and shrunk SLGT over SE England - slower clearance of low cloud / drizzle raises some uncertainty about whether trigger temperatures will actually be reached to generate isolated thunderstorm development later this afternoon across SE Eng. Have retained a smaller SLGT for now, but trends continue to suggest the chance of thunderstorms here is almost sub-SLGT threshold now. 
Area with most likely thunderstorm initiation continues to be signalled across W Midlands / NW England / N Wales. Isolated heavy showers are also likely over W Northern Ireland and parts of C - W Scotland, particularly over high ground, but some uncertainty as to their likelihood to produce lightning, hence sub-SLGT for now.

High WBPT airmass will continue to advect westwards from the nearby Continent through Saturday, with hints perhaps of elements of elevated convection possible first thing across C to SE England (low probability). Profiles initially exhibit a notable warm nose inversion around 850-900mb, acting as a cap to any surface-based convection until sufficient diurnal heating combined with low-level convergence / orographic forcing enables air parcels to rise to form deep convection - particularly so mid/late afternoon and through the evening hours, assuming adequate clearance of existing stratus and fog trapped beneath inversion.

Given initial cap and CAPE values of 700-1,000 Jkg-1, convective development could be fairly rapid. However, relatively dry profiles and a lack of any notable shear will probably result in isolated to well-scattered marginally-organised showers and thunderstorms, particularly focussed over areas where low-level forcing is most prominent (sea breeze through Thames Estuary environs, orographic forcing over Wales / W Midlands / NW England / C + W Scotland etc). Instability is greatest towards the SE, and so here cells may be quite electrically active. Have highlighted areas at greatest risk with SLGTs, but it is plausible isolated cells may develop elsewhere within the LOW.

Overall evolution sees instability axis shifting slowly southwestward through the evening hours, and hence the zone with thunderstorm potential will be shifting SW-ward towards southern and western parts of Britain - some uncertainty as to how long through the evening hours convection can be sustained given nocturnal cooling of the boundary layer, but it certainly seems possible that some (isolated) thunderstorms could perhaps survive until 00z. There may also be elements of elevated convection approaching SE England later in the night.

Although some hail is possible in the strongest cells, main risk will be local flash flooding given slow storm motion in an environment characterised by PWAT values of 27-30mm - potential for 16-18mm to fall in just 1 hour.