The Met office has issued a yellow weather warning for rain from 5pm today until 11.45pm tomorrow in south Wales.
This afternoon heavy rain will move in from the west to southern regions and will persist throughout the night.
There may be difficult driving conditions, as well as fast-flowing or deep floodwater. The Met office warns spray and flooding on roads will probably make journey times longer.
Natural Resources Wales has issued five flood alerts across Wales. Three are in south east Wales, one is in south west Wales and one in the north.
The Met Office predicts 20-30 mm of rain is expected to fall within three to four hours.