Three weeks' rainfall 'in a day'
The rain is expected to last all day
As much as three-quarters of an average month's rain could fall across south Wales on Wednesday, weather forecasters have said.
The region is expected to bear the brunt of heavy rainfall across the country and parts of England.
Weather group Meteogroup UK said south Wales could see at least 30mm of rain, with up to 50mm falling on hills.
Some valley areas could be at risk of localised flooding by the end of the day with water run-off from the hills.
Forecaster Paul Mott said: "It will be a very wet day across south Wales tomorrow with some persistent rain, heavy at times.
"I think much of south Wales will see at least 30mm of rain through the day, probably up to about 50mm across hills and land.
"That's getting on for three-quarter's of the average month's rain fall."
The south west of England is expected to receive heavy rain fall too, although Mr Mott said it would probably not be as heavy as that in Wales.
The rain is expected to continue into the evening.
A spokesman for the Environment Agency said on Tuesday evening they were not issuing any flood warnings but urged people to check their website for updates.