Convective Outlook: Wed 04 Aug 2021
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 04 Aug 2021 - 05:59 UTC Thu 05 Aug 2021

ISSUED 06:54 UTC Wed 04 Aug 2021

ISSUED BY: Dan

A slack surface pressure pattern dominates once again across much of the UK/Ireland, with subtle mid-level troughing. As with previous days, cool air in the mid-levels combined with surface heating through the day will yield a few hundred J/kg CAPE in an environment with modestly-steep mid-level lapse rates. Scattered showers are likely to develop in places, tied to low-level convergence (such as sea breeze) and orographic forcing. Lower surface dewpoints in a zone from Cambridgeshire - Northamptonshire - Humber and across parts of southern Scotland could lead to a relative minima in shower coverage in these areas. For much of England and Wales, convection will tend to be limited by a capping inversion around ~650mb (~13,000ft) as warming aloft occurs in advance of the Atlantic frontal system arriving overnight; therefore areas that retain colder air in the mid-levels for longest will have the greatest potential for deeper convection and some sporadic lightning strikes, i.e. north Midlands / northern England. That said, even here convection could still be a little restricted in depth. Flow will be weak through much of the cloud-bearing layer resulting in slow storm movement which could lead to prolonged heavy downpours on a local scale, bringing the risk of localised surface water flooding. Therefore the main hazard will be locally high rainfall totals, even in the absence of much in the way of lightning. Some evidence of backed low-level winds, coupled with vorticity stretching along convergence zones, could lead to a couple of funnel clouds. 

Across Scotland and Northern Ireland, stronger southwesterly flow aloft should allow individual cells to move slightly faster, but back-building or multiple cells running parallel in the flow could still lead to locally high rainfall totals - especially along northern coastal parts of Ulster and Caithness/Sutherland. Again, the main hazard will be locally high rainfall totals, regardless if there is much in the way of lightning or not.

During the overnight period, strong shear will develop across southern, central and western Ireland, with backed low-level winds and perhaps overlapping with a couple hundred J/kg CAPE. Frontal rain will spread northeastwards during the night, but it is possible later that should any surface-based deep convection manage to develop then this could produce some sporadic lightning and pose the risk of an isolated tornado - but confidence is rather low.