A yellow weather warning from the Met Office is in place until 4pm on Thursday, covering areas including Blaenau Gwent, Caerphilly, Monmouthshire, Newport, Torfaen, and the Vale of Glamorgan.
The Met Office has warned of a “medium likelihood” of disruption and a “low impact” in the affected areas.
A spokesman for the Met Office said: “Frequent showers and some longer spells of rain, heavy at times, are likely during Thursday.
“Given saturated ground, this may lead to some flooding and transport disruption.”
Between 5mm and 15mm of rain is expected to fall within a few hours in most areas, with some spots seeing up to 20mm.
The heaviest rainfall is expected to clear northeastwards by late afternoon.
Bus and train services may also face delays.
The Met Office has advised the public to check travel plans and consider alternative routes.
It also recommends looking up the latest weather updates throughout the day, as “Be prepared for weather warnings to change quickly.”
Advice has been issued to help people stay safe in heavy rain, including tips on safe driving, travelling during storms, and winter weather precautions.
The Met Office is reminding residents to review road conditions before setting out.
Anyone using public transport is encouraged to check for service updates and timetable changes before travelling.
