Convective Outlook: Tue 18 Jul 2017
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Tue 18 Jul 2017 - 05:59 UTC Wed 19 Jul 2017

ISSUED 19:58 UTC Tue 18 Jul 2017

ISSUED BY: Dan

UPDATE 19:58 UTC MDT redrawn based on latest guidance, minor adjustments locally to SLGT. In general, clusters of primarily elevated thunderstorms will continue to develop through the remainder of the forecast period, and head generally N or NE, such that some areas could see some locally disruptive storms with very frequent lightning, while others stay storm-free. It is impossible to say exactly which locations will be impacted, but the MDT offers a reasonable guide as to those areas expected to have a higher chance if nothing else. Potential for portions of the Republic of Ireland to also be upgraded to MDT, but have refrained for now. Risk of hail and flash flooding continues overnight

Upper ridge over the British Isles on Tuesday will slowly drift east, as a negatively-tilted upper trough approaches from the Atlantic. On its forward side, poleward advection of a high WBPT airmass will occur from western Europe into southern then central Britain. Falling heights as upper trough approaches, combined with subtle shortwaves, will provide the focus for destabilisation of the mid-levels, with significant CAPE available from approx 850mb upwards by Tuesday evening/night.

Heavy showers / isolated thunderstorms could be possible as early as Tuesday morning close to Cornwall/Devon/Channel Islands from AcCas, though in general the risk of elevated thunderstorms here increases moreso during the afternoon hours, perhaps extending towards Dorset and Somerset by late afternoon. Bifurcating flow could also see an elevated thunderstorm complex pass close to Kent/Sussex during the first half of the evening, but more likely to stay a little to the south and hence no MDT issued for now.

Elsewhere, likely still dry at this stage, but through the evening hours increasing isentropic lift will allow numerous scattered elevated thunderstorms to develop on the leading edge of the Theta-E ridge, particularly focussed over SW England then extending east across Cen-S England, though the exact phasing of upper dynamics interacting with the plume is still uncertain - earlier destabilisation would then require the MDT to be extended farther south towards the south coast. A broken line of scattered elevated thunderstorms is then expected to drift north through the night into the Midlands, parts of Wales, and perhaps extending towards London and East Anglia - some question marks over how far east these may extend, and so have refrained from stretching the MDT too far east for now.

Some elevated thunderstorms could also form over the Republic of Ireland on Tuesday night, through a combination of advection of existing cells from SW England / SW Wales and new cells developing in-situ, generally moving towards the NW.

The magnitude of CAPE and steep mid-level lapse rates is such that lightning is likely to be very frequent with some thunderstorms, particularly those over England and Wales, and hail up to 1.5-2.0cm in diameter would be possible with the most potent cells. The risk of flash flooding is mitigated somewhat by notably dry surface layers, at least initially, but these will tend to moisten with time and bring a greater threat of some locally high rainfall totals should multiple storms move over the same area. Given the lead time, there is undoubtedly still some uncertainty over the exact forecast evolution, and further updates may be issued as new guidance becomes available.