The Met Office has forecast thunderstorms, potential flooding and a chance of large hailstones for most of the UK today.
The capital is expected to be hit by storms on Tuesday evening.
Met Office spokesman Oli Claydon said some areas will experience between 20mm and 30mm of rainfall in an hour.
He said: “The whole of southern England and Wales will be affected by thunderstorms and potential heavy downpours.”
The Met Office said there was also a chance of large hailstones and localised flooding of homes, businesses and roads.
Frequent lightning may pose the risk of disruption to power networks on both days across all regions.
Mr Claydon said that on Wednesday “the storms will cover a much wider area, taking up all of Wales but stopping before Scotland", from the early hours through to 8pm.
He added: “There will be scattered storms and some areas could see 30mm of rainfall in an hour, while there could be rainfall of up to 50mm in two to three hours in some areas”
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