The Met Office has issued a severe weather warning for heavy rain across Cornwall and west Devon.
It said rain will become heavier and more persistent in the afternoon, giving totals of up to an inch (25mm).
Drivers have been advised to take extra care and refer to the Highways Agency for advice on traffic disruption on motorways and trunk roads.
The Environment Agency currently has no flood warnings on rivers in the area.
The Met Office warning is valid until 1600 GMT.