Flood warnings and alerts, external have also been issued by the Environment Agency.
They include the A361 East Lyng to Burrowbridge, River Parrett, Washford River, Salt Moor and North Moor, the River Fowey, Helford and Fal Rivers and North Cornwall Rivers.
The RNLI has also issued a warning, external to people living in coastal areas about sea swells.
The lifeboat service said potential waves of up to 15ft (4.5m) could cause sudden surges up beaches on Friday, and top over sea fronts and harbour walls, catching people unaware.
It advised people to stay a safe distance from the water as conditions could “knock you off your feet or wash you into the sea”.