Heavy showers and isolated thunderstorms are expected from early morning into early afternoon on Sunday.
Met Office weather maps show that the South of England will be hit particularly badly this weekend - though heavy showers and thunderstorms will move northwards tomorrow. Yesterday saw roads and railway lines flooded in the south of England.
Mr Box said Sunday was likely to bring large hail and gusty winds as well as rainfall of up to 90mm in places hit by multiple thunderstorms.
"We're going to see essentially some pulses of potentially thundery showers pushing their way up from the channel through the course of the morning affecting southern counties", he said.
There was travel chaos on the roads and train services in the area were disrupted due to the severe flooding.
The Met Office has issued yellow thunderstorm warnings until Monday, with the potential for flooding and damage to buildings from lightning and hail.
"With the continuing hot weather, large swathes of the United Kingdom continue to be at risk from thunderstorms, and potentially the impacts from heavy rain, into the early part of next week".
The Environment Agency has issued five flood alert warnings across the West Midlands for the weekend.
The Environment Agency has a warning for expected flooding at Winford Brook at Chew Magna, south of Bristol, along with six alerts for possible flooding around Birmingham.
Chief Meteorologist Frank Saunders added: "Up until Monday, thunderstorm warnings cover much of England and Wales, with parts of southern England and South Wales seeing the greatest likelihood of impacts".
Thunderstorms have brought flash flooding and power cuts to parts of Wales throughout the week, prompted by the spell of hot and humid air across parts of the UK.