Two weeks of rain could fall in the some parts of the region in a single day today.
Forecasters say the South West could be deluged with up to 25mm in 24 hours.
The Met Office has issued a yellow warning across the South West, advising people to "be aware" of rain.
Met Office spokesman Helen Chivers said: "Today is going to be an extremely unpleasant day. We're expecting a lot of rain which could lead to flooding in some parts."
Isolated parts of the region may even see outbreaks of sleet and snow – but not enough to settle.
"It'll be an utterly miserable May day with all the rain and the cooler temperatures," Mrs Chivers added.
Matt Dobson, a forecaster with MeteoGroup, the weather division of the Press Association, said: "It's a lot for a day – a typical place in southern Britain would see around two inches of rain in a whole month.
"Places could see half a month's rain in a day.
"Towards the end of May we would see a return to more 'settled' weather."
He said: "There is some rain on the way. Tuesday is looking like the wettest day of the week for southern parts of the country.
"There will be lots of rain moving from Devon and Cornwall and the South West to Wales, with up to an inch or 25mm on Tuesday daytime.
But he said predictions of a summer as wet as last year were premature and added: "Time and time again it's been shown these kinds of long range forecasts don't work.
"Looking forward though there are signs of slightly more settled weather towards the end of May. It's still developmental work trying to predict a few months ahead."
Mixed weather is expected during the rest of the week with further outbreaks of rain tomorrow and on Thursday .
Towards the end of the week there will be spells of sunshine and showers, with temperatures climbing to 15C or 16C on Thursday and Friday.
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