TORRO SEVERE THUNDERSTORM WATCH 2015/004 and TORRO CONVECTIVE DISCUSSION 2015/007 A TORRO SEVERE THUNDERSTORM WATCH and CONVECTIVE DISCUSSION have been issued at 21:35GMT on Friday July 3rd 2015 Valid from/until: 21:35-06:00 (SVR WATCH) and 21:25-12:00 (CONV DISCUSSION) on Friday July 3rd/Saturday July 4th 2015 for the following regions of the United Kingdom & Eire: Much of England along with SE Wales (SEVERE THUNDERSTORM WATCH) Much of western England/Wales into Scotland, and eastern Ireland, and Northern Ireland (CONVECTIVE DISCUSSION) THREATS SVR WATCH (blue box): Hail to 20-50mm diameter; wind gusts to 60mph; frequent CG lightning; locally heavy rainfall. CONV DISCUSION (yellow box): Hail to 15mm diameter; wind gusts to 50mph; CG lightning; locally heavy rainfall. SYNOPSIS A plume of moisture and instability is moving quickly northwards across the British Isles this evening. Scattered thunderstorms have formed along the warm front in N Wales/N Midlands, and within the moisture plume close to Dorset. Other thunderstorms have formed close to southern Ireland. As a strong upper trough moves in overnight, thunderstorms are expected to increase in coverage across much of the British Isles, but the higher instability/shear combination will be reserved for the severe thunderstorm watch area. However, within this area, there is a stronger capping inversion, especially with south-eastward extent. Thus, storms may be more isolated towards SE England, and indeed may not develop here at all. The mixture of ~1000J/Kg of elevated CAPE and shear values of 30-40 knots across the severe thunderstorm watch area suggests that elevated supercells and well-organised multicells may develop, bringing the risk of large hail and locally strong winds. If storms tend to congeal into one or more clusters, or even an MCS, a more widespread wind threat could emerge. The tornado risk is considered low due to the slightly elevated nature of the storms. However, there may be a brief window of opportunity in the higher instability area in the south-east of the watch. For the convective discussion area, CAPE is somewhat more meagre, although forcing for ascent from the approaching upper trough will be stronger, and so thunderstorms are still likely. A few may become strong with moderately large hail and a few strong wind gusts. The activity will continue to shift northwards overnight with the risk decreasing quickly in the wake of it across southern and central areas later in the night. Forecaster: RPK.
