A Met Office spokesman said: "As Storm Callum passes to the northwest of the United Kingdom, a period of heavy rain will affect South Wales during Friday, with further spells of persistent and heavy rain at times during Saturday".
Met Office meteorologist Sarah Kent told MailOnline: 'There's an amber warning out for southern parts of Wales and the Welsh Valleys.
Arriva Trains Wales is planning to shut the line between 7am and 11am on Friday morning and in the evening.
Driving conditions will be hard, there is the risk of flooding.
Ferry services between Wales and Northern Ireland and Dublin have been suspended. Hundreds of homes and businesses are also without power, and 20,000 properties were without power in Ireland.
Rivers burst their banks, some homes were flooded and some lost power while travel was affected as Wales bore the brunt of torrential rain on Saturday.
NRW will issue flood alerts and warnings if rivers reach trigger levels, with teams monitoring river levels 24 hours a day.
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Friday's windy and rainy weather is set to continue this weekend. The high winds, combined with heavy rain, could see leaves and twigs blocking drains, making flooding more likely, Mr Burkhill said.
A yellow rain warning covers parts of Scotland, the West Midlands and the north west and is in place until midnight.
There has also been reports of tress down and flooding in parts with warnings of hazardous conditions through the day.
A combination of winds up to 60 miles per hour and heavy rainfall are set to make driving conditions hazardous - including along the coast at times of high tides.