Sky News meteorologist Chris England said it would turn “much colder” over the next few days, with a “raw wind from the Baltic bringing widespread snow showers”.
Snow has already started falling in the Grampian mountains in Scotland, he said.
The Met Office has issued yellow warnings for snow and ice, beginning at 3pm on Friday.
The 20cm (8in) of snow is forecast on higher ground and 5cm (2in) in other areas, which Met Office meteorologist Martin Bowles said was “enough to cause a fair amount of disruption to transport”.
“Significant snowfall” is expected on Sunday, when some parts of the country could have freezing temperatures all day.
“It is probably the coldest day of the lot,” Mr Bowles said.
He added that an upgraded amber warning on Sunday is “certainly a possibility”, but a red warning is “extremely unlikely”.
Snow will initially affect northeastern Scotland and northeastern England, before spreading to the rest of the UK.
“On Saturday that snow band in the east starts to move further south, including southeast England and London, (and) it will weaken as it does so,” Mr Bowles said.
“On Sunday we get that snow spreading more widely, particularly further west and into the south of England – Devon, Kent, the Midlands and parts of Wales.”
More from UK
Recap: Russia hits back at ‘harridan’ Defence Sec
Jeremy Corbyn again refuses to directly blame Russia for Salisbury poison attack
Salisbury attack: Defence Secretary tells Russia to ‘go away and shut up’
‘Space harpoon’ designed in UK could be the answer to space debris
Queen approves wedding of ‘Prince Henry and Rachel Markle’
Poppi Worthington’s mother ‘disappointed’ as prosecution ruled out
Low temperatures are likely to continue on Monday with temperatures rising “fairly rapidly” from Tuesday.
During the cold snap earlier this month, motorists in two parts of the UK reported getting stuck for 17 or 18 hours as snow and high winds brought treacherous driving conditions.