Convective Outlook: Sat 29 Jun 2019
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 29 Jun 2019 - 05:59 UTC Sun 30 Jun 2019

ISSUED 11:51 UTC Sat 29 Jun 2019


UPDATE 11:51 UTC While confidence is still not particularly high, elevated thunderstorms are likely to increase in coverage later this afternoon and through the evening hours over northern England, S + E Scotland. As such, a MDT has been introduced, with a westward-expansion of the SVR area. Thunderstorms will be primarily elevated, and hence the main threat will be large hail.
Guidance varies as to how far south thunderstorms may develop over northern England, potentially as far south as Liverpool - Bridlington, but the best multi-model consensus exists over the MDT area.

UPDATE 07:16 UTC Elevated thunderstorms are ongoing over eastern Ireland (outline in text forecast below) and are likely to continue northeastwards across parts of Scotland. SLGT has been expanded to cover this risk. Confidence on developments later today over northern England / E Scotland is still not high enough to upgrade to MDT, but will monitor trends through the day

Upper ridge over western Europe will gradually pivot and shrink southwards on Saturday, allowing an upper trough west of Ireland to relax away to the northeast. An elevated mixed layer (EML) will be located along a north-south corridor over western Britain, slowly shifting northeastwards with time. This zone will be the focus for very steep mid-level lapse rates and 1,500 - 2,000 J/kg CAPE (from elevated parcels) - a rather impressive environment for the British Isles.

A second, narrow corridor of (weaker) elevated CAPE will be advected northwards from Biscay towards the Irish Sea on Saturday morning, as a subtle impulse migrates north in the strong southerly flow aloft. This may provide the focus for a few scattered elevated showers and/or thunderstorms on Saturday morning, primarily Celtic Sea / Irish Sea and perhaps clipping eastern Ireland - however, confidence in lightning activity is low given limited NWP support and so have refrained from issuing a SLGT for now.

Stronger forcing will arrive on Saturday evening, as the upper trough advances and overlaps more favourably with the existing instability axis, by this stage over eastern Scotland / northern England. An increase in elevated convection is likely, though it could be well into the evening hours before significant thunderstorm activity occurs. Nonetheless, the risk increases as the evening progresses and the instability axis shifts eastwards, which would place the far east of Scotland (more specifically offshore) and NE England / E Borders at greatest risk.

Forecast profiles suggest any convection will be largely elevated, though the environment would be conducive to perhaps elevated supercells. Regardless, the magnitude of CAPE/shear and steep lapse rates suggests severe thunderstorms capable of producing very frequent lightning and hail 3-4cm in diameter will be possible. Should the plume destabilise earlier (as per some model guidance) then the risk would extend farther west to Cumbria / central southern Scotland etc, but current thinking is a slower destabilisation would favour later initialisation over North Sea coastal counties, before activity ultimately moves offshore and continues to grow upscale. Have refrained from introducing a MDT for now due to low confidence on timing of initialisation. Isolated elevated showers will be possible across parts of the Midlands / Wales / SW England, though probably limited in depth to produce much in the way of lightning.

Scattered showers, some weakly-electrified, are also likely in the post-frontal environment across western Ireland (and later western Scotland) as the main upper trough axis swings through, the cool mid-levels helping to yield a few hundred J/kg CAPE. Limited cloud depth precludes the introduction of a SLGT for now.