The Met Office is warning of heavy downpours on Saturday, with a weather warning in place for West and Mid Cornwall.
The warning is in place for 24 hours starting just after midnight tonight, as an area of rain is expected to push north across the county, with the worst of the downpours in the early hours and the morning.
A spokesman said that the the rain will be persistent and sometimes heavy, especially across western areas at first.
There is the possibility that the warning area will be revised again, either later today or early on Saturday.
The rain should clear much of southwest England and then parts of south Wales during the day.
The public are advised to be aware of the risk of disruption to travel and outdoor activities due to localised surface water flooding.
The Met Office chief forecaster said: "A developing area of low pressure is expected to track north across the UK through the weekend. The heaviest rainfall is currently expected to develop on the western flank of the low, with west Wales, some western fringes of England and southwest Scotland seeming most at risk from widespread 20 to 40 mm of rain, and locally perhaps exceeding 50 mm in 12 hours.
"This is a complex situation, and the warning may well be updated again as the situation develops."