People are being urged to be alert as a month's worth of rainfall is expected to fall across Wales.
Storm Callum is bringing strong winds and heavy and persistent rain, with the highest totals in south and southwest Wales, which could cause local rivers to rise.
An amber warning has been issued for south Wales from Friday morning until Saturday evening.
Natural Resources Wales said it is also monitoring weather conditions at sea as the storm could also cause flooding with large waves, strong winds and a tidal surge. It said it is checking flood defences, clearing gulleys and installing temporary defences to protect communities.