The Met Office in Exeter has increased the severity of its weather warning for Saturday.
They say torrential rain which will caise localised flooding is likely in places.
Frequent lightning, large hail and locally strong gusts should also be expected and there could be "significant flooding".
The Met Office previously issued a yellow warning for heavy rain, meaning people should 'be aware'.
That's now increased to an amber status, meaning people should 'be prepared'.
A forecaster said: "Several areas of heavy, thundery showers will develop over England and Wales from Friday night before moving northwards during Saturday.
"Not everywhere within the warning area will see thunderstorms, and indeed some spells of warm sunshine are expected at times, but where thunderstorms do form, some torrential downpours are likely with frequent lightning, large hail and locally strong gusts.
"Significant flooding is possible where these do occur from surface water as well as from small, fast-responding watercourses."
The public should be prepared for the risk of disruption from any of these elements
