Severe thunderstorms could cause disruption across Wales and other parts of the UK over the next four days, according to the Met Office.
It said some areas could see large hailstones and heavy rain of up to 40mm (1.5in) in an hour, with some locations potentially receiving 80mm (3in).
The yellow weather warning is in place between Monday and Thursday.
It follows a weekend of hot weather which has seen large numbers enjoying the countryside and coast.
"There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes and large hail," said the Met Office.
"There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost."