Up to 60mm of rain could fall in parts of Wales on Saturday, with a yellow warning in place.
The Met Office said heavy rain could be severe in parts, with its "be prepared" advice in place from 06:00 GMT to 22:00 GMT.
All of Wales' 22 local authority areas apart from Anglesey and Monmouthshire are covered by the warning.
It is currently unclear which areas will experience the worst of the weather.
However, the warning said: "Some locations are likely to see 15 to 25 mm during the day with isolated totals of 40 to 60 mm possible over higher ground in particular."
The rain is expected to ease over the night,