Convective Outlook
VALID 06:00 UTC Fri 06 Aug 2021 - 05:59 UTC Sat 07 Aug 2021
ISSUED 06:33 UTC Fri 06 Aug 2021
ISSUED BY: Dan
Upper low becomes quasi-stationary across the UK/Ireland on Friday, while deepening. A sharp trough axis will rotate around its southern flank, arriving over the western English Channel during the overnight period. In general, cool mid-levels associated with the upper low will create an unstable environment in conjunction with diurnal heating inland and SSTs. Outbreaks of heavy rain from a slow-moving front will affect Scotland, especially during the morning where some lightning activity may be possible near the east coast. Other areas of showery rain will also be present across Ireland, Northern Ireland and northern England in particular, which casts some uncertainty as to the degree of surface heating that could develop due to fairly extensive cloud.
Convective overturning is likely to lead to initial areas of rain breaking up into heavy showers, with additional showers developing where sunny breaks occur. The risk is generally highest across northern and eastern England, southern and perhaps central Scotland and parts of Northern Ireland. Some sporadic lightning will be possible from the more intense cells - this perhaps most questionable in Northern Ireland where it may stay rather cloudy with rain for much of the day. Of greater concern is the potential for local surface water flooding given relatively weak steering flow in these areas, regardless of whether there is much lightning activity or not.
Across southern Britain, anticyclonic curvature will likely lead to descending air aloft which will tend to limit either the number of showers that develop, or at least the depth of convection for the most part. However, a low-level confluence zone is likely to become established from SW England through Cen S Eng/M4 corridor towards East Anglia, which may provide particular focus for scattered heavy showers and a few thunderstorms that may develop late afternoon and into the evening hours - this perhaps also aided by a PV anomaly arriving from the west. With both an incremental increase in wind speed and decrease in relative humidity with height, this environment may be favourable for some cell organisation with DLS around 20-30kts. Hail and gusty winds may feature from this most intense cells, and an isolated tornado is not ruled out given some slight backing of low-level winds and notable LLS. The hills just inland in S / SE Wales could also see a few lightning strikes, but confidence is not high enough to introduced a SLGT.
Most lightning activity will fade by mid-late evening, however the attention overnight may turn to the English Channel (including Channel Islands) with an increase in showers/showery outbreaks of rain as the marked shortwave approaches from the west - but confidence on much in the way of lightning with this is rather low.
