The Met Office said flooding could affect homes and businesses and cause transport disruption with communities cut off by waterlogged roads.
The heaviest rainfall is expected to be across the hills and mountains of northwest Wales and Cumbria, where six inches of rain could fall between Thursday morning and midnight Friday, when a yellow warning will be in place.
The alert covers parts of Wales and North West England as well as Shropshire and North and West Yorkshire.
A similar warning is in place on Friday, Saturday and Sunday, with parts of Scotland also expected to be battered by rain.
Chief Meteorologist Andy Page said the unsettled weather was being caused by ‘decaying tropical hurricanes’ which disrupt conditions when they enter the North Atlantic.
He said: ‘The remnants of former Hurricane Epsilon have been subsumed into another deep area of low pressure to the south-west of Iceland, and although the low centre is a long way from our shores and won’t cause disruption, the trailing weather fronts associated with this system will bring wind and rain to the UK, along with a potentially heavy swell.
‘However, it is rain rather than wind that will dominate our attention later in the week with a so-called ‘warm conveyor’ set up to bring a flow of very moist and warm air to the UK from further south in the Atlantic, triggering heavy rainfall for high ground in the west of the UK.’
He said that across the warning areas, accumulations of 30-40mm of rain can be expected, while isolated areas exposed to the strong south-westerly winds could be battered by 50-80mm of rain.
He added: ‘We expect the heaviest rainfall to occur in the mountains of Wales where some areas could witness 130mm of rainfall.’
The Met Office later said as much as 150mm – six inches – of rain could fall, and brought forward tomorrow’s warning from noon to 9am.
As well as Met Office weather warnings, the Environmental Agency has issued eight flood alerts mainly affecting coastal areas in the south.
The EA say flooding is possible on the North Cornwall and North Devon Coast, as well as around the River Ray in Wiltshire and River Derwent in Derbyshire.
October is usually the wettest month in the UK and rainfall this year has largely been concentrated in Greater London, the home counties in south-east England, and a smaller area of north-east Scotland.
But the Met Office said on Tuesday that London is having one of the wettest months for more than 150 years.
The capital has had 139mm of rain up to the 25th, compared with the full month’s average of 78mm. With rainfall for the final six days of the month yet to be recorded, October 2020 is already ranking ninth rainiest in London since 1862.
Met Office spokesperson Grahame Madge said with ‘considerable rainfall’ expected in the coming days it is ‘quite likely’ this month will break the October record, and potentially for any month.