Thunderstorms and lightning could cause flooding and other disruption across Wales from Thursday afternoon.
The Met Office alert comes as temperatures in some areas are set to reach 31C (88F) over the next two days.
It said "torrential downpours" could bring up to 40mm (1.5in) of rain in less than two hours in some areas.
Last week, 200 properties were hit by flash floods during a downpour in Rhondda Cynon Taf, forcing some out of their homes for a third time this year.
A yellow weather warning is in place from 16:00 BST on Thursday until 09:00 on Friday across Wales.