Forecasters say flooding is likely as the ground is already saturated
Flood watches are in place across Devon and Cornwall with storms expected to hit the region, bringing high winds and heavy rain.
Forecasters said Cornwall was likely to bear the brunt of the weather first, before it sweeps through the rest of the country.
Winds of 50mph and up to two inches (50mm) rain have been predicted.
Flooding is likely because the ground is already "saturated" said forecaster Michael Dukes.
Mr Dukes, from Meteogroup, said: "There is an area of low pressure moving in from the Atlantic.
"Cornwall will be hit first tonight, and then the rain will move across the country tomorrow.
"After a soggy August the ground is already saturated and, with such large amounts of rain likely to fall in under 24 hours, there is a high risk of localised flooding."
The Environment Agency has flood watches in place in coastal areas and on rivers in north Devon and north Cornwall, on the south Devon coast and the upper river Tamar.