A yellow thunderstorm warning for Friday has been issued by the Met Office in Wales.
It warned there could be disruption caused by the rain, particularly to transport.
There could also be damage to buildings from lightning, as well as power cuts.
Showers and storms are expected to develop quickly on Friday morning before peaking in the afternoon. The warning covers from 11:00 BST to 20:00 BST.
By evening they will probably have become restricted to the south-east and southern coastal counties.
“Some places will miss the heavier rain, but other locations could see around 20mm within an hour, and perhaps 30mm to 40mm in around three hours, sufficient to cause issues on the roads,” a spokesman said.
A yellow rain warning is also in place on Thursday when there could be difficult driving conditions and transport disruption.
The Met Office said there is the potential for 30mm to 40mm of rain in five or six hours in some places.
Thursday’s warning applies until 19:00 BST. It covers Blaenau Gwent; Bridgend; Caerphilly; Cardiff; Carmarthenshire; Merthyr Tydfil; Monmouthshire; Neath Port Talbot; Newport; Powys; Rhondda Cynon Taf; Swansea; Torfaen and Vale of Glamorgan.