Up to 10cm of snow could accumulate in some places in less than three hours, while travel delays and power cuts are likely.
Much of Scotland is the subject of yellow warnings for snow and ice, including Shetland where there has been no power for thousands of homes since Monday. Warnings for Wales begin on Saturday and for England on Sunday.
The Scottish Government declared a major incident for Shetland on Tuesday but by Friday many on the island were still without power.
Snow has affected many major roads, Traffic Scotland said on Friday. It urged people to drive with care.
Glasgow airport tweeted: “Three hours of heavy snow, intermittent rainfall and ground temperatures well below freezing have created very challenging conditions on our airfield.
“Our winter teams are working extremely hard in these difficult conditions to resume operations, but only when it is safe to do so.
“Our runway was cleared for operation shortly after 11.30am and we are now working with our airline partners to help resume their flight schedules.”
Teams from Scottish and Southern Electricity Networks (SSEN) Distribution reconnected about 750 homes and businesses in parts of western mainland Shetland, Voe and Brae, on Thursday.
Mark Macdonald, regional head at SSEN Distribution, said: "I'd like to thank customers for their ongoing patience and reassure them that we're making every effort to restore supplies as soon as possible and provide support to those who need it.
"Armies of engineers have descended upon Shetland over the past 48 hours to restore power to homes and businesses across the islands, and more are on their way.
"We've made significant progress today in continued challenging conditions, reconnecting 750 homes across the islands."
Heavy snowfall in London in pictures
An amber weather warning was in force until midday on Friday, covering central Scotland as well as parts of Fife, Perth and Kinross, South Lanarkshire and Argyll and Bute.
Yellow warnings for snow and ice continue into Sunday. Similar warnings are in place for north-west England and west Wales.
On Sunday, the warnings will apply to large parts of England.