Two yellow Met Office warnings are in place covering parts of England, Scotland and Wales, with frequent sleet, hail, and snow showers forecast.
Motorists are facing hazardous driving conditions and schools have been closed as snow brings chaos to parts of the UK.
Two yellow Met Office ice warnings are in place for northwest England, southwest Scotland, central southern England and Wales until midnight, with frequent sleet, hail, and snow showers forecast.
They come with cautions about travel disruption, the chance of power cuts and icy patches on roads - making for tricky driving conditions which have left some cars stranded and forced schools to shut.
Sky News weather producer Joanna Robinson said: "Friday will bring more wintry showers, some heavy, and perhaps thundery.
"Snow will continue to accumulate on the hills, especially across Northern Ireland, western Scotland and northwest England - 2 to 5cm is possible above 100m and up to 15cm on the highest ground."
Grantown Grammar School and Kingussie High School are both closed in Scotland, with the former blaming "icy conditions and lack of grit on the roads".
Kingussie High said "dangerous" road conditions meant buses were unable to run.
Both schools said virtual classes would instead be held.
Several schools delayed opening until 10am in South Lanarkshire.
Prospect of thundersnow
Forecasters are warning snow showers and lightning strikes could disrupt travel and cause power outages.
The prospect of thundersnow - a relatively rare phenomenon - is driven by the same conditions which cause thunder in the summer, the difference in temperature between the ground and the air surrounding it.
Robinson said: "Thundersnow will remain a risk in the far North West, especially for northwest Scotland and Northern Ireland.
"Meanwhile, more general rain and hill snow will move into the windy South West."
National Highways warned of a "severe weather event" on its website and issued a "severe weather" alert for snow affecting the north of England and the West Midlands until 6pm on Friday.
The A66 in north England is closed with efforts to recover stranded vehicles, according to reports.
Traffic Scotland reported that heavy snow overnight was impacting many routes, including the M77 and M74, and urged drivers to take extra care.
It said multiple vehicles were stuck on the M8 E/B exit slip road at J16 due to difficult driving conditions, and that gritters were on their way to the scene.
Motorists tweeted images of vehicles stuck in the snow in Glasgow.
Bear North West Trunk Roads said there was snow on routes including the A82 in the Glencoe and Tyndrum area, the A9 and the A83.
Meanwhile, police in East Dunbartonshire tweeted: "There are a number of vehicles abandoned throughout East Dunbartonshire due to the weather. Stay at home if you can, and travel later once the roads are a bit clearer."
Coldest night of the winter so far
A temperature of -8C (17.6F) was recorded on Thursday morning in Topcliffe in North Yorkshire - the lowest in England this winter.
Blustery conditions made the temperatures feel colder for many throughout the day as air from the Canadian Arctic swept across the UK.
The Met Office has forecast milder conditions going into the weekend.
A spell of wet and windy weather is forecast across all parts of the UK on Saturday, with heavy rain likely in most areas.
Sunshine and scattered showers will replace the rain across western areas by the afternoon.
Sunday is expected to be mostly dry with spells of sunshine across most parts, with some showers forecast for northwest Scotland and Northern Ireland.
Robinson said: "It will be colder tonight than the one just gone. Expect a widespread frost across northern, eastern and central Britain, with icy stretches possible. Temperatures will rise in the West ahead of rain moving in.
"Saturday will be milder than recently, with temperatures above average for Wales and much of England. Expect an early high of 10-12C (50-53F) in the South West and around 6-9C (43-48F) in the North.
"Sunday will see temperatures around a degree or two down on Saturday, but it will probably feel warmer with more sunshine and lighter winds. Temperatures will range from 6 to 10C (43-50F)."