
The Met Office said the heavy rain could cause local disruption to travel and outdoor bank holiday events.
The warning, valid until 20:00 BST on Saturday, says some areas may see up to 30mm of rain in a few hours.
The forecast says south west and south Wales are likely to experience most rain, where the showers are slow-moving.
On Thursday, flash flooding affected roads in Cardiff causing traffic problems.
An elderly couple had to be rescued from their home as storms and intense downpours caused flash flooding across south Wales.