Convective Outlook
VALID 06:00 UTC Sun 04 Jul 2021 - 05:59 UTC Mon 05 Jul 2021
ISSUED 07:42 UTC Sun 04 Jul 2021
ISSUED BY: Dan
A messy setup is expected on Sunday, with several concerns over lightning potential. Broad upper troughing continues to cover the UK/Ireland from the Atlantic, to the north of a strong jet which will stretch from Biscay and across the Mediterranean. As was the case on Saturday, areas of showery rain will be affecting southern Britain and Ireland, and possibly NE Scotland, early on Sunday morning with some pockets of embedded convection in places - although weak and skinny CAPE suggests the risk of lightning is rather low. Through the day, as this showery rain lifts north, convective overturning will probably result in this breaking up into heavy showers by the afternoon, with additional heavy showers also developing in response to diurnal heating, low-level convergence and orographic forcing over portions of northern England, Scotland and N + E Ireland.
So, firstly cloud cover limiting surface heating creates our first concern over the potential for lightning, which may overall limit the resultant depth of convection. Secondly, generally weak shear with little change in speed with height is expected, some slightly stronger flow nearer 300mb but this will only be available for convection able to grow tall enough to utilise this, and even then probably not that significant. Locally backed low-level winds, especially closer to North Sea coasts, will enhance the directional shear. Thirdly, forecast profiles are quite moist throughout the vertical and despite MLCAPE in the range 300-800 J/kg, this may be stretched over a great depth (cloud tops in the strongest cells could well be in excess of 26,000ft) therefore resulting in skinny CAPE profiles. All of these factors suggest that, despite unstable profiles with potential for heavy showers, lightning may not be that widespread.
Later in the afternoon drier mid-level air will feed northwards into the Midlands and eastern England, and so this may improve buoyancy across these areas to bring a better potential for some lightning (although confidence on sufficient cloud breaks/surface heating is not particularly high so have refrained from introducing a SLGT here for now). Equally, drier mid-level air will already be present across much of Scotland during the day providing a more encouraging environment for lightning production. Existing heavy showers may eventually merge into longer spells of rain through the evening and night hours.
