Summary: Active cluster of thunderstorms likely S/SE England Thursday morning, perhaps extending into S Midlands / East Anglia, clearing NE-wards. Showery precipitation with embedded lightning possible NE England for the first half of the morning, clearing into the North Sea. Elsewhere, scattered thunderstorms may develop during the afternoon hours from E Wales / W Midlands through NW/C N England, these could be severe as they drift east through the evening hours. As is often the case in these situations, uncertainty exists and is detailed below... 

Good agreement for a developing area of showery precipitation over N/NE England for the first half of Thursday morning at least, with perhaps some embedded lightning - this eventually moving out over the North Sea by late morning.

Elevated thunderstorms will likely be ongoing at the beginning of the forecast period over portions of S/SE England, these producing frequent lightning. General trend is for these to drift NE-wards with time, perhaps into parts of East Anglia and the south Midlands, though some uncertainty as to the northern and western extent, and whether they may start to weaken towards late morning and into the afternoon. There is a low risk that these storms could become rooted in the boundary layer if clearance is slow, and in which case they may begin to pose greater hazards from large hail and flash flooding.

Provided sufficient insolation occurs and cloud cover from early developments is not too extensive, then scattered thunderstorms may develop in a zone from E Wales / W Mids to NW Eng to NE Eng during the afternoon hours given dewpoints in the high teens coupled with forced ascent as a developing cold front begins to advance eastwards (and marked wind convergence).

Given 1,000-1,500 Jkg^-1 CAPE and some reasonable deep layer shear, large hail up to 3.0cm in diameter would be possible, along with strong gusts of wind and localised flash flooding. A tornado could not be ruled out, though the risk is generally low. In a broad sense, the greatest risk of severe hazards (hail, flooding and tornado) increases towards the southern end of the SVR threat area, but here storms are more likely to be isolated given stronger capping and perhaps a hang-back of cloud following morning elevated convection. Any storms that do develop will drift towards the east, perhaps a little more south of east by the evening hours, and could survive into the night as they exit across the North Sea.

Worth stressing storms will be scattered in nature and so inevitably some places will stay dry, despite a large geographical area potentially at risk.