SEVERE THUNDERSTORM WATCH - FRIDAY / SATURDAY

Forecast Details

Upper and collocated surface low to the south of Iceland and NW of the UK will drive a deeply cyclonic S to SWly flow across the UK. A cold front will move NE through Friday, bringing a band of heavy showery rain across many areas, some convection is likely embeded which may be deep enough to produce isolated lightning, more especially towards southern and western areas, where greater instability from onshore flow beneath cooling mid-levels will produce enough CAPE. 

Then attention is drawn to a risk of severe convective weather developing from the SW Friday night and Saturday morning, as a marked shortwave trough and left exit region of a strong SWly jet streak moving NE ahead of upper low creates increased forced ascent and strong deep layer shear as it approaches. The shortwave trough and occluded front will support an arcing band of heavy convective rainfall. Isolated lightning is possible with this activity, especially across Wales, SW and S England - where instability / CAPE will be greatest towards S and W coasts where instability will be generated over relatively warm seas beneath cooling air aloft. Broken line segments within this band, perhaps with transitory supercell structures, given deep layer shear of 50-70 knots, could produce isolated convective gusts in excess of 60mph, as strong winds aloft are brought down to the surface in strong convective downdrafts. There is also a risk of isolated tornadoes with stronger updrafts which rotate - enhanced by strong low level shear forecast as the upper trough and occluded front approach and back the surface wind flow against veering flow aloft in the lowest 0-1km. Intense rainfall leading to localised flooding will be an additional hazard.