Convective Outlook: Sat 29 Jul 2017
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 29 Jul 2017 - 05:59 UTC Sun 30 Jul 2017

ISSUED 12:31 UTC Sat 29 Jul 2017

ISSUED BY: Dan

UPDATE 12:31 UTC Latest guidance has trended a little farther west, and hence a westwards extension of the SLGT. Broad theme remains the same though with (a) some mid-level instability release possible this afternoon over SE England with this initial pulse of precipitation, (b) then behind the warm front still some potential for deep convection to occur during the evening hours along the Theta-E ridge from Kent and north along the eastern fringe of East Anglia, and (c) finally the cold front surging eastwards during the early hours could produce some linear convective elements (perhaps void of lightning depending on true convective depth) capable of some very strong, gusty winds

Upper vortex to the NW of Scotland on Saturday will maintain a feed of cool mid-levels and hence marginally-steep mid-level lapse rates. Once more, numerous showers are expected across Scotland and Northern Ireland, some weakly-electrified - this more likely over N + NE mainland Scotland, and perhaps also E Northern Ireland into SW Scotland, though in general instability a little weaker than on Friday and complicated somewhat by some dynamic precipitation in western Scotland along an old occlusion. CAA overnight will steepen mid-level lapse rates from the west, with an increasing risk of some isolated/sporadic lightning spreading eastwards across Ireland and Scotland. Some small hail will be possible with the strongest cells.

Farther south, a developing wave along the quasi-stationary frontal boundary straddling the English Channel will begin to back the flow, allowing advection of a higher WBPT airmass into SE England as the warm front lifts back north. This will bring outbreaks of rain with some embedded convective elements across southern England during Saturday daytime, though little lightning at this stage given rather saturated profiles, limited convective depth and unimpressive mid-level lapse rates. Nonetheless, some heavy convective rain will be possible embedded within the returning warm front.

During the evening hours, falling heights and dry intrusion above the warm sector, and some instability advecting from France, may allow a few showers/thunderstorms (probably elevated, though perhaps not exclusively so) to develop over Kent/Surrey and/or Essex/Suffolk/Norfolk, before moving NE-wards out to the North Sea. Instability parameters are not particularly impressive in current NWP guidance, but given strong DLS any deep convection that does occur could become fairly well-organised, capable of gusty winds and perhaps some hail.

Stronger forcing arrives over the western Channel by midnight, which may then be responsible for some deep convection to develop during the early hours of Sunday, running NE-wards over CS/SE England, Home Counties and East Anglia, and largely clearing to the North Sea by the end of this forecast period. Lightning activity is uncertain but some will be possible, especially given steepening mid-level lapse rates within a strongly-sheared environment. Of more interest may be the scope for some linear elements, particularly near or along the cold front which could be capable of some very strong, gusty winds - if any convection can become rooted within the boundary layer, particularly as the cold front surges east, then there would be potential for a tornado also - especially from the Isle of Wight eastwards.

Worth noting that the forecast evolution associated with the frontal wave (turn closed low) in southern Britain currently carries a lot of uncertainty, and will need monitoring - further updates may be issued as confidence increases. It is possible that no lightning may occur at all with these developments.