Convective Outlook
VALID 06:00 UTC Fri 26 Mar 2021 - 05:59 UTC Sat 27 Mar 2021
ISSUED 07:32 UTC Fri 26 Mar 2021
ISSUED BY: Dan
An upper trough will migrate eastwards across the British Isles during Friday and Friday night, the associated cold pool creating an environment with steep mid-level lapse rates and yielding 300-500 J/kg CAPE in response to SSTs and diurnal heating inland. A cold front and associated band of rain will track eastwards across Britain during daylight hours, clearing to the North Sea around 16z. Forcing and the strongly-sheared environment will allow embedded line convection to occur capable of producing hail and squally winds in portions of the front, and this may intensify somewhat as the front approaches the E Midlands / E Anglia / SE England during the afternoon hours given some modest surface heating prior to its arrival. An isolated tornado cannot be ruled out where breaks develop in any bowing segments.
Scattered showers will follow the cold front, especially numerous across Ireland during Friday afternoon as an occlusion swings through, this then tracking eastwards into western Britain during the evening hours. Showers will tend to become more isolated with time during the early hours of Saturday as ridging builds from the west, mainly confined to NW England and SW / W / NW Scotland by the end of the night. Some sporadic lightning could occur from any shower, hence the broad LOW threat level issued, with gusty winds and hail likely in many of the showers (and snow even on modest hills).
Shear weakens within the trough axis, but some notable backing of low-level winds is expected from Munster, N Leinster to Ulster during Friday daytime, and this could be the focus for some better organisation of cells with perhaps some slightly larger hail and an isolated tornado. Shear will increase across SW Ireland later in the afternoon as the trough axis shifts eastwards, although by this stage the depth of convection may be slightly shallower. A low-end SLGT has been introduced for some sporadic lightning, but confidence is rather low in this aspect due to a messy mixture of dynamic and convective modes. The focus for some sporadic lightning may shift to the Bristol Channel and adjacent parts of S Wales / SW England for a few hours during the evening period, along with W Scotland.
