Gale force winds and up to 100mm of rain are expected to hit Cornwall over the next two days.
The Met Office has upgraded the warning level for Cornwall from yellow to amber.
The rain sweeping the county is expected to become heavy and prolonged on Thursday night, persisting through much of Friday.
A Met Office spokesman said the public should be prepared for localised flooding and disruption to travel.
The spokesmand said: "40 to 60mm is expected to fall widely over southern parts of both Cornwall and Devon by the end of Friday with the potential for in excess of 100mm to fall over exposed southern and southeastern upslopes.
Southeasterly winds will also increase to reach gale force around exposed coasts and also over high ground."
The Environment Agency also warned that heavy rain today and tomorrow could lead to flooding in South West England.It advised the public to stay away from swollen rivers and not to drive through floodwater.
Ben Johnstone for the Environment Agency said: "We strongly urge people to sign up to flood warnings on the Environment Agency website, keep a close eye on local weather forecasts and be prepared for possible flooding. We also ask that people stay safe and not try to wade or drive through any deep water."
The Agency is monitoring river levels and expected to issue flood alerts and possibly flood warnings. There is expected to be snow further north and this could lead to significant travel disruption.
It said the public can keep up to date with the latest forecasts and warnings for their area on the Environment Agency website and with forecasts on TV and radio.
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