A "danger to life" warning has been issued as heavy rain is forecast across northern England and South Wales, while Scotland could see its first "dusting" of snow.
The Met Office said a low-pressure system over the Azores is expected to come across to the UK, bringing widespread rain with it.
It has issued an amber weather warning for rain in South Wales from 6pm on Friday until 11am on Saturday, with up to 80mm of rain expected widely across the area, and between 100-120mm falling on higher ground.
Across northern England up to 70mm of rainfall is expected – with "a little snow" possible in the Pennines.
It warned the heavy rainfall could bring flooding to areas and cause disruption to public transport and motorists.
Spokeswoman Nicola Maxey said: "The areas most at risk are the areas that are within the warning areas, so you've got parts of Cumbria and the Pennines.
"There's a warning over Wales and then there's another warning over parts of the South West.
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"We're looking at 30mm to 50mm of rain quite widely, with perhaps 120mm over higher ground for the warning in South Wales."
She also warned of a drop in temperatures heading into Saturday.
The northern half of the country should expect temperatures in the high single figures over the weekend, with southern parts experiencing milder conditions in the low double figures.
"We've also got some polar maritime air moving in across the country, so where this warmer air that's coming up from the south meets the colder air, you see the heaviest rain at that point.
"Because of the colder air, there's a possibility that some of the rain may fall as snow, but only over very high ground in the north.
"You might see a dusting on top of the Pennines, and you may see a centimetre or two over the mountains in Scotland."
Sky News weather producer Joanne Coles said the rain will be accompanied by strong winds and gales around the coasts.
She said: "On Saturday, the band of rain and strong winds will continue to plague parts of central and southern Britain, although will gradually clear southeastwards through the day.
"As the rain clears it will turn brighter but much colder as the winds turn more northerly. Scotland and Northern IrRead More – Source