Convective Outlook
VALID 06:00 UTC Tue 06 Jul 2021 - 05:59 UTC Wed 07 Jul 2021
ISSUED 06:17 UTC Tue 06 Jul 2021
ISSUED BY: Dan
The environment with the best thunderstorm/lightning potential will be located near to the surface low that tracks northeastwards from Cen S England to East Anglia on Tuesday morning, a small secondary low may form to the north near the Yorkshire/Co Durham coast and become the dominant low by the afternoon. Initially, areas of showery rain - with an ongoing low risk of isolated lightning - will be wrapped around the surface low, but this will tend to break up through the day as it drifts northeastwards. The environment is generally characterised by moist low-levels (dewpoints of 13-14C) beneath steep mid-level lapse rates. Cloud cover may inhibit insolation, but if modest surface heating can occur this could yield 500-800 J/kg CAPE, locally higher.
There will be a reasonably small window of opportunity, from late morning to early afternoon, for deep convection to occur before the upper low exits to the North Sea and warming aloft follows. The region at greatest risk currently appears to be east of the Pennines into Yorkshire and Lincolnshire (surprise surprise), perhaps extending into parts of East Anglia depending on cloud clearance. Cloud-layer shear is modest, around 15kts (stronger in East Anglia but here convection may be capped around ~500mb), although backed surface winds near the east coast could enhance low-level shear and promote some rotation to any strong updrafts that cross the area. Confidence on anything occurring is quite low.
Additional showers will likely develop across central/southern England during the afternoon, running northeastwards into East Anglia/SE England. For the most part, convection will probably top at ~500mb and while this is certainly deep enough for a few lightning strikes (such as near M4 corridor), coverage is probably not great enough to warrant a SLGT at this stage. Fairly unidirectional winds through the profile, largely 25-30kts throughout, suggests generally weak cloud-layer shear. Slightly steeper mid-level lapse rates will likely exist over Ireland, and profiles suggest the potential for deeper convection albeit with skinny CAPE. Nonetheless, a few sporadic lightning strikes may be possible across C/E Ireland during the afternoon/early evening, with a SLGT introduced to cater for this risk. A few heavy showers will also be possible in W Scotland during the afternoon/early evening.
