A second red weather warning has been issued by the Met Office.
The red alert, which means there is a danger to life, covers parts of south-west England and south Wales.
The warning comes into force at 3pm on Thursday and will remain until 2am the following day.
The area covered by the alert includes parts of Dartmoor, Exmoor and the Brecon Beacons national parks. Forecasters said heavy snow and blizzards are expected.
Strong easterly winds leading to severe drifting are likely to be encountered as Storm Emma sweeps the South of Britain. The Met Office said freezing rain later in the day could lead to significant ice build-up, with resultant long delays on public transport and blocked roads, with stranded vehicles.
Long power cuts are likely, with damage to trees and other structures due to the accumulation of heavy snow or ice.
The chief forecaster said: “Widespread snow is expected to develop through Thursday afternoon and evening.
“Around 10-20cm [4-8 inches] is likely to fall widely, with the potential for up to 30 to 50cm [12 to 20 inches] over parts of Dartmoor, Exmoor and parts of south-east Wales.
“Snowfall will be accompanied by strong to gale easterly winds, leading to severe drifting of lying snow especially in upland areas. Severe cold and wind chill will compound the dangerous conditions, with very poor visibility. Towards midnight, there is a chance of snow turning to freezing rain in places, mainly across the South of the area, with widespread icy stretches making driving conditions particularly dangerous.”
A red warning, issued for the first time in Scotland, expired at 10am on Thursday. Red alerts mean extreme weather is expected. The public should take action immediately to keep themselves safe from the impact of the weather. Widespread damage, travel and power disruption and risk to life is likely. You must avoid dangerous areas and follow the advice of the emergency services and local authorities.
Winds of more than 20mph are forecast over Brecon Beacons peaks, with windchill below -20C.
Dartmoor National Park Authority has cancelled planned meetings on Friday and its visitor centres at Princetown, Postbridge and Haytor are also closed.
The authority said: “Should snowfall be heavy and road closures are in place, do not drive beyond ‘road closed’ signs; the signs will be there for a reason. Additional traffic on roads that are closed hampers clearance operations as snow ploughs are unable to pass.
Further north, winter still grips the mountains, with a windchill of -27C forecast for the Cairngorms, with temperatures of -12C exacerbated by winds approaching 50mph.
Amber warnings for snow are in place across a wider area of south-west England and south and mid-Wales, along with central and eastern areas of northern England, along with much of Scotland, from the Borders to Sutherland. A yellow warning covers virtually all of the rest of Britain.
More details are on the Met Office website. Mountain weather forecasts can be accessed via grough’s links page.