The Met Office has issued a yellow for heavy rain on Thursday and Friday.
Up to 60mm of rainfall is possible in some places, with lightning and gusty winds according to forecasters, who have issued the warning from 5pm Thursday until 10am Friday.
The Met Office says: "Whilst some areas will miss the worst, areas of heavy, showery rain are expected to become widespread across a swathe of England and Wales during Thursday evening and on into Friday morning.
"There is some uncertainty in the details, but some locations may see up 20-30 mm of rain in 2-3 hours and perhaps 40-60 mm in 4-6 hours.
"Lightning and strong, gusty winds may be additional hazards, especially in southwest coastal areas.
"Given the recent very wet weather, this will bring the potential for further flooding and transport disruption."
The warning adds there is a slight chance of power loss along with a small chance of flooded homes and roads.
The warning area covers south Wales and goes as far north as Aberystwyth.