Warning as one month's rain could fall in a day
Heavy rain is likely to affect many parts of Wales over the next two days.
The Met Office said that up to 70mm (2.8in) could fall in two to three hours in some places on Sunday, with 80-100mm (3-4in) over the course of 12 to 24 hours on Monday.
Average September rainfall across south Wales and southern England and is about 60-90mm (2-3in).
The National Grid said more than 400 homes had been left without power on Sunday morning around Chepstow, Monmouthshire.
Sunday's yellow weather warning affects every Welsh county except Anglesey, with Monday's alert also excluding Pembrokeshire.
Forecasters said driving conditions could be difficult and warned that some communities could be cut off.
The Met Office said: "There is still some uncertainty regarding which areas will be affected by the heaviest rain, but at this stage parts of the Midlands, northeast England and east Wales look most likely to see the greatest accumulations.
"However, anywhere within the warning area could have impactful rainfall."
