Heavy showers and thunderstorms could cause travel disruption and flooding across most of Wales.
A yellow warning has been issued by the Met Office for between 12:00 and 21:00 BST on Tuesday.
Forecasters said lightning strikes and flooding meant there was a “a chance of delays and some cancellations to train and bus services”.
“Spray and sudden flooding could lead to difficult driving conditions and some road closures,” they added.
There is also a “slight chance” that homes and businesses could be affected by power cuts.
The warning also mentioned a small risk of damage to buildings from floodwater, lightning strikes, hail or strong winds.
The yellow warning for thunderstorms covers all Welsh counties except Anglesey and Flintshire.
