Britain's third named storm of autumn will sweep in overnight, bringing seven weeks' worth of rain to parts of the country over the next two days along with 70mph gusts.
Storm Callum is expected to bring up to 6.3in (160mm) of rain to South Wales, with Met Office forecasters issuing an amber 'danger to life' warning amid concern over flooding causing chaos to roads, homes and businesses.
Forecasters said in the amber warning - running from 6am tomorrow until 6pm on Saturday - that there is a 'good chance' some communities could be cut off by flooded roads, while there could also be power cuts.
Yellow warnings for wind and rain have also been issued by the Met Office for nearly the whole of tomorrow in Northern Ireland and western England, Scotland and Wales – with the South West expected to be worst hit.
While eastern England should avoid the most severe weather, there is a 50 per cent chance of rain along with 16mph wind gusts and 63F (17C) temperatures in Windsor for Princess Eugenie and Jack Brooksbank's wedding.
Storm Callum is expected to bring up to 6.3in (160mm) of rain to South Wales - and also hit much of western Britain tomorrow
The dramatic turnaround in conditions comes hours after the UK's hottest October day in seven years yesterday, with West London reaching 76.3F (24.6C) and night-time temperatures only falling to 61F (16C) in the capital.
Rainfall of up to 6.3in (160mm) is expected in South Wales tomorrow, while up to 4in (100mm) could fall in the South West of England tomorrow. The UK average rainfall for the whole of October is normally 3.6in (92mm).
The highest wind gusts of 70mph are expected in the far North West of Scotland, including the Outer Hebrides, but gusts of 50mph to 60mph are also possible around exposed coasts and hills in other parts of western Britain.
Met Office chief meteorologist Will Lang said: 'The rain is expected to be accompanied by strong winds, which when combined with high tides may lead to some coastal impacts due to large waves.
Rain is expected across western parts of Britain today (left), before becoming heavier and more widespread tomorrow (right)
Temperatures could get up to about 70F (21C) today and tomorrow (pictured) - and possibly rise even further by Saturday
The Met Office has issued multiple weather warnings for parts of Britain tomorrow (left) and on Saturday (right)
'In addition, leaves and twigs, brought down by the wind, could block drains and culverts, increasing the likelihood of flooding.
And forecaster Greg Dewhurst said: 'Storm Callum will arrive on Friday, moving in from the west and hitting Wales and northern England. There will be strong winds and torrential downpours.
'It will be a big change from the sunshine of the last few days. Exposed areas of England and Wales could see winds of up to 60 or 70mph and it will be very blustery in the rest of mainland Britain.
'Because of the time of year those winds will blow leaves off trees, which combined with the rainfall could block drains and lead to flooding. We may see travel disruption, fallen trees and power cuts in parts of the UK.'
Princess Eugenie and Jack Brooksbank, pictured at Buckingham Palace in January, will be married in Windsor this Friday
However, he added that the South East of England will mostly avoid the bad weather - with temperatures up to 68F (20C tomorrow and even 72F (24C) on Saturday. The storm will have cleared by the end of the weekend.
Sioned Warrell, duty tactical manager for Natural Resources Wales, said: 'The current forecasted rain could cause flooding this week, especially in south and west Wales, and we are urging people to prepare themselves.
'Making sure you know what the situation is like where you live is really important. You can check our website or call Floodline on 0345 988 1188 for any Flood Warnings that may be in place and keep an eye on weather reports.
'Our teams are doing all they can to reduce the risk for communities, but if there is flooding we want to make sure people are doing all they can to keep themselves safe.'
Everything you need to know about Eugenie and Jack's royal wedding this Friday at Windsor Castle
THE WEDDING OF PRINCESS EUGENIE AND JACK BROOKSBANK
FRIDAY, OCTOBER 12, 2018 - 11AM - ST GEORGE'S CHAPEL, WINDSOR
Princess Eugenie poses with her long-term fiance Jack Brooksbank
Princess Eugenie is counting down the hours to her lavish royal wedding.
The Queen's grand-daughter has just one day to go until she says her marriage vows to tequila brand ambassador Jack Brooksbank in the gothic surrounds of St George's Chapel, Windsor Castle.
Some 850 guests will gather in the historic surroundings where the Duke and Duchess of Sussex wed five months ago.
Harry and Meghan and the Duke and Duchess of Cambridge are expected to be among the guests on Friday, along with the Queen and a host of royals.
Celebrity guests will include former Take That singer Robbie Williams and his wife and fellow X Factor judge Ayda Field.
Williams' six-year-old daughter Theodora is taking a starring role as a bridesmaid, alongside five-year-old Prince George and three-year-old Princess Charlotte.
Royal fans will be hoping for some fun-filled antics from the young helpers in the bridal party, which include Zara and Mike Tindall's spirited four-year-old Mia and Peter and Autumn Phillips' daughters Savannah and Isla.
Maud Windsor, who is Eugenie's goddaughter and the daughter of Lord Frederick Windsor and his actress wife Sophie Winkleman, is also a bridesmaid.
Future king George will be joined by fellow page boy Louis de Givenchy - the six-year-old son of JP Morgan banking executive Olivier de Givenchy and his wife Zoe.
The Queen's youngest grandchildren, 14-year-old Lady Louise Mountbatten-Windsor and 10-year-old Viscount Severn, who are the children of the Earl and Countess of Wessex, will take on the role of Special Attendants.
William and Kate's youngest child Prince Louis, who is only five and a half months old, is too young to attend and will be staying at home.
The Duchess of Cornwall will also be missing, having a long-standing engagement at a school in Scotland. Proud parents the Duke of York and Sarah, Duchess of York will be gearing up for the celebrations.
Andrew has remained on very friendly terms with his ex-wife the duchess amid speculation they might even remarry. However, her relationship with her former father-in-law the Duke of Edinburgh is said to be non-existent.
Philip is said to have never forgiven 'Fergie' for the scandal she caused when she was caught in the infamous 'toe-sucking' incident, having her feet kissed by financial adviser John Bryan in 1992.
The groom Jack has asked his brother Thomas to be his best man while Eugenie's older sister Princess Beatrice will be her maid of honour.
Stars predicted to attend the second royal wedding of the year include singer Ellie Goulding and fiance Caspar Jopling, supermodel Cindy Crawford, David and Victoria Beckham, Harry's exes Cressida Bonas and Chelsy Davy, model Cara Delevingne, singer James Blunt and wife Sofia Wellesley, and George and Amal Clooney.
Jack works as a drinks executive for Casamigos tequila, which was founded by Clooney and Crawford's husband Rande Gerber.
Italian singer-songwriter Andrea Bocelli will perform two pieces during the ceremony which begins at 11am, with members of the Royal Philharmonic Orchestra also featuring.
The wedding service will include a personal prayer written by Archbishop of York John Sentamu. Dean of Windsor David Conner will give the address and also officiate as the couple make their marriage vows.
The Queen is hosting an afternoon reception afterwards in the castle's St George's Hall, but the festivities are expected to continue into the early hours with an evening party at the York family home, the Royal Lodge in Windsor Great Park.
Royals fans hoping to catch a glimpse of the bride can wait for the newlyweds' carriage procession through Windsor town centre - but the couple will not be travelling up the Long Walk to the Castle like Harry and Meghan did.
About 1,200 members of the public have been given balloted invites to the castle's Lower Ward, as have charity representatives, children from Eugenie's old schools, and royal household staff.
Republic, a campaigning group working for an elected head of state, has criticised the security cost to the taxpayer, saying estimates have put the bill at £2 million. Eugenie is ninth in line to the throne.
The wedding is being screened in full on ITV as part of a special three-hour This Morning show live from Windsor, hosted by Eamonn Holmes and Ruth Langsford.
Broadcast footage will also be live streamed on The Royal Channel and The Duke of York's YouTube channels, and the royal family and Andrew's Facebook pages.