Convective Outlook: Sat 25 Jul 2020
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 25 Jul 2020 - 05:59 UTC Sun 26 Jul 2020

ISSUED 07:16 UTC Sat 25 Jul 2020

ISSUED BY: Dan

A sharpening upper trough will swing northeastwards across the British Isles on Saturday, the trough axis located over Ireland at 15z, SW Scotland - SE England at 21z and clearing to the North Sea by 03z. The associated cold pool will create an environment with steep mid-level lapse rates, this most pronounced close to the trough axis. This ultimately means the most unstable environment will be over Ireland during the afternoon / early evening, migrating eastwards across England and Wales during the evening and night hours.

... IRELAND / NW + W WALES / NW ENGLAND / SW SCOTLAND ...
A shortwave will track eastwards during the afternoon hours, and this will provide the main focus for scattered showers and thunderstorms developing. The rather most airmass suggests cloud could be rather extensive, but there should be increasing breaks with each passing hour during the afternoon. Shear is a little meagre, generally 15-20kts and fairly unidirectional suggesting rather pulse-type mode most likely, and despite rather saturated profiles 400-900 J/kg CAPE should be sufficient for sporadic lightning, especially across southern and eastern Ireland. Hail 1.0-1.5cm in diameter and gusts of 35-45mph may be possible from the strongest cells.
Showers and thunderstorms will then drift eastwards across the Irish Sea during the evening hours into W / NW Wales, SW Scotland and NW England.

... SCOTLAND ...
Morning rain may contain some embedded weak elevated convection, and while a few isolated lightning strikes may be possible the risk is considered fairly low. Increasing insolation by the afternoon will likely yield 300-600 J/kg CAPE, primarily Aberdeenshire and to a lesser-extent Angus and Caithness. A few scattered heavy showers are possible here, with the risk of a few lightning strikes (especially Aberdeenshire). The slack surface pattern and associated low-level convergence vorticity may produce one or two funnel clouds.

... ENGLAND & WALES ...
On the forward side of the upper trough, broad divergence aloft will encourage fairly widespread lift and this in proximity to a frontal boundary is likely to result in a frontal wave developing in the vicinity of the English Channel into SE England, particularly during the afternoon hours. The extent of this frontal wave is naturally handled differently amongst various NWP guidance, with the 12z/Fri ECMWF perhaps the most bullish covering most areas from the Bristol Channel to the Humber southwards. Even if the precipitation is not this widespread, there will likely be a substantial high cloud shield to the north of the main area of precipitation, and this will ultimately impact the amount of surface heating of the rather most airmass (dewpoints 15-17C). Even if deep convection manages to develop in the afternoon, the warm bulge in the mid/upper levels will likely suppress the depth of convection somewhat until the evening hours when cooling aloft occurs as the upper trough approaches. 

That said, a small window of opportunity may exist around midday/early afternoon in the Cambs/Lincs/Norfolk vicinity before increasing cloud dilutes insolation and reduces surface temperatures.
Otherwise, it may be the evening hours that offer the best potential for deep convection and heavy showers / a few thunderstorms. One or two pronounced SW-NE convergence zones may develop, the lines of showers then migrating generally eastwards with time while individual cells track to the NE. Shear will be stronger here under the influence of a mid-level jet, and 30-40kts DLS may aid in cell organisation and longevity - especially across southern and eastern England. Hail up to 1.5cm in diameter may be possible, alongside gusts of 35-45mph. Low cloud bases and locally enhanced vorticity could produce an isolated tornado. Activity may persist / keep developing until the early hours, before finally clearing to the North Sea.

Confidence on lightning coverage over England is rather low, but have tentatively introduced a SLGT where some sporadic lightning is considered most likely.