Heavy showers could see up to 15mm of rain fall in an hour on Saturday and into Sunday, the Met Office has warned.
It has issued a yellow warning of heavy rain across south and mid Wales.
Flooding of some homes and businesses is possible and localised surface water flooding could increase journey times, the warning said.
The alert is in place from 13:00 BST on Saturday to 06:00 BST on Sunday morning.
The counties affected are Bridgend, Caerphilly, Cardiff, Carmarthenshire, Ceredigion, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Newport, Pembrokeshire, Powys, Rhondda Cynon Taff, Swansea, Torfaen, Vale of Glamorgan and Blaenau Gwent.