Convective Outlook: Tue 30 Jul 2019
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Tue 30 Jul 2019 - 05:59 UTC Wed 31 Jul 2019

ISSUED 21:00 UTC Mon 29 Jul 2019

ISSUED BY: Dan

A near-vertically-stacked low (slightly tilted to the NW with height) will drift slowly northeastwards from Devon to Yorkshire through Tuesday and Tuesday night. The associated upper cold pool will overspread warm SSTs and diurnally-heated land to bring a day of widespread deep convection. 


Showers and perhaps a few weakly-electrified thunderstorms will already be affecting parts of SW England / S Wales early on Tuesday morning, and are expected to expand in coverage across much of Britain through the day - organising into distinct circular bands rotating around the main surface low centre. Instability will increase through the day in response to diurnal heating, with 300-700 J/kg CAPE likely fairly widely, and up to 1,000 J/kg CAPE in parts of northern England and SW Scotland.

Almost any shower could produce a few sporadic lightning strikes, although fairly saturated profiles / excessive cloud cover and fairly weak shear will tend to limit the lightning potential. That said, a zone of steep mid-level lapse rates will exist on the northern flank of the upper low, in an environment with some reasonable shear and good forcing aloft, after a morning of relatively cloud-free skies - hence conditions will be favourable for a greater coverage of lightning activity over N / NW England, adjacent Irish Sea and perhaps SW Scotland, especially enhanced by both orographic forcing and low-level convergence during the afternoon and evening hours.
As such, a MDT has been issued for the risk of lightning - and here hail up to 2.0cm in diameter will be possible from the strongest cells. An isolated supercell cannot be ruled out if organised convection can develop relatively early before shear reduces through the afternoon.

The main threats will be flash flooding from both slow storm-motion (so prolonged downpours), but also shower training over similar areas - especially given already saturated ground from recent heavy rain over the NW Midlands / NW England etc. There could also be quite a few funnel clouds / weak tornadoes (or waterspout) close to the low centre (so primarily in a zone from the West Country to the W Midlands, and later NW England). Regardless, some strong gusts of wind are likely around many of the showers.