The Met Office has warned that almost two weeks of rain could fall over a 30-hour period.
A rain warning has been issued for south-west England, with powercuts and flooding expected.
The worst of the weather will take place between noon today and 6pm tomorrow.
It comes after weeks of rain and flooding resulted in communities being cut off - and even a woman dying in Debyshire.
The latest Met Office weather warning states: "Showers on Friday morning will be replaced by more persistent rain during the afternoon and evening.
"The rain will be heavy at times, lasting through much of Friday night and Saturday morning before slowly dying out during Saturday afternoon.
"20 to 30 mm is likely to fall quite widely, and 40 to 50 mm may accumulate in a few places, most likely across the higher ground of south Devon."
The alert warns that a few homes and businesses will likely be flooded and some train and bus services will probably be delayed.
The average total rainfall in the UK for November is 121mm, meaning almost two weeks' worth could fall today and tomorrow.
The Environment Agency has issued six flood warnings and 45 less serious flood alerts.
The organisation's flooding forecast predicts: "Local surface water and river flooding is also possible in south-west England and south-west Wales... on Friday and more widely across parts of England and Wales on Saturday and into Sunday.
"Local surface water and river flooding is also possible across parts of the south of England and possible but not expected more widely across England and Wales on Monday.
"Land, roads and some properties may flood and there may be travel disruption."
A spokesman for Weather Outlook said: "(Today) rain spreads northeastwards and it turns drier in the south for a time.
"Later on more persistent spells of rain spread across much of southern England and Wales. Eastern counties should remain dry. Temperatures in the south will be a little higher than recently but in the north it remains cold.
"During Saturday heavy rain pushes northwards with drier conditions following into southern and central counties. Much of Scotland and Northern Ireland could stay dry until the evening.
"Temperatures should be close to the seasonal average. Brighter conditions are likely on Sunday in the southern half of the country."
Met Office five-day weather forecast
Today:
Bands of rain and showers moving northwards to affect many parts through the day. Heaviest and most persistent rain likely in the southwest. Brightest across northwest Scotland. Temperatures close to the late November average but feeling chilly in brisk winds.
Tonight:
Further rain moving northwards across many areas. Perhaps a touch of frost across northwest Scotland, otherwise milder than recent nights.
Saturday:
Rain in many places, heaviest in the southwest and first and then in the northeast later. Southern and western areas perhaps becoming drier later. Windy in the north.
Outlook for Sunday to Tuesday:
Further spells of rain or showers spreading from southwest to northeast across many areas. Driest in the northwest. Milder for many, especially by night.
