TORRO TORNADO WATCH 2012/004
A TORNADO WATCH has been issued at 0740GMT on Thursday 28th June 2012
Valid from/until: 0740 - 2000GMT on Thursday 28th June 2012, for the following regions of the United Kingdom & Eire:
Parts of (see map)
Southern and central Scotland
Tornadoes; hail to 40mm diameter; wind gusts to 65mph; Frequent CG lightning
Plume of high theta-w air has advected across much of Britain overnight with a large upper trough approaching from the west. At the surface, a cold front will move north-eastwards through the day.
Diffluent upper flow ahead of the trough is bringing forcing for ascent across portions of Wales already, and a number of thunderstorms have developed. At this stage they are likely marginally elevated above a coolish boundary layer, but strong cloud-layer shear suggests large hail to 20-30mm is possible with this early activity, with elevated supercells possible, with clusters of storms are likely. Gusty winds also possible.
This activity is expected to continue to move NNE and NE through this morning. Although there is a fair amount of mid-level cloud associated with the widespread large-scale ascent ahead of the trough, enough diurnal heating of the boundary layer is expected to allow storms to become surface based as the cluster of storms moves through the Midlands into N England, and later, Scotland. With 30-40 knots of deep layer shear, well-organised multicells and embedded supercell structures are likely. 20-30mm diameter hail and gusts to 60mph are possible. In addition, isolated tornadoes are possible.
In the wake of this activity, and across the more south-eastern parts of the WATCH area, further thunderstorms are expected to develop and the move north-east, although these will be more isolated with south-eastern extent. Modified ascents for afternoon temperatures suggest around 1200-1800 J/Kg of SBCAPE. WIth 0-6km shear remaining around 40 knots, and perhaps increasing a little, well-organised multicells and isolated supercells seem possible. With 0-1km SREH of ~150 J/Kg and fairly low LCLS, a few tornadoes may develop with this activity. Indeed, should an isolated supercell develop in an area with decent surface heating, a strong tornado cannot be ruled out. Hail to 40mm or so and wind gusts to 65mph possible with supercells too; otherwise, 20-30mm hail and 55mph gusts. If this activity can develop into northern parts of England too, it may tend more towards a squall line, with an associated wind threat.
The activity will end from the south-west through the afternoon and evening as the cold front moves in.