© Met OfficeMet Office map showing, yellow, amber and red warning areas for heavy snowfall on Friday.
It applies to the Heads of the Valleys and Brecon Beacons in south Wales but snow is expected across much of Wales.
Red warning areas could see up to 30cm of snow and blizzards. People are warned to avoid non-essential journeys.
Around 30 schools have already decided to close while Rhigos, Bwlch and Maerdy mountain roads will close at 22:00 GMT on Thursday.
The schools which announced they would not open on Friday were mainly in Powys and Rhondda Cynon Taf (RCT), and no school transport will operate in either county.
A spokeswoman for RCT council said: "This snow is expected to be particularly severe between 3am and 9am and is likely to lead to widespread accumulations of four inches with six to eight inches over higher ground."
There is an amber warning in force covering most of the country, and a red warning for the south Wales Valleys and the Brecon Beacons
Schools are also closed in Merthyr Tydfil and Wrexham.
BBC Wales meteorologist Derek Brockway explained that a red warning is "rare and means take action".
The Met Office added that strong south-easterly winds could also cause severe drifting of lying snow "and severe disruption".
A band of snow is forecast to spread across much of Wales during Friday morning, lasting through the afternoon and evening.
An amber warning is covering other areas of the country, which means around 5 to 10cm (2 to 4 in) of snow is expected.
It started snowing in areas of Gwynedd and west Wales on Thursday.
And there was panic-buying at many supermarkets.
© Wales News ServiceBread shelves were empty at Tesco in Aberdare
There are more than 200,000 tonnes in stock after Wales began the winter with a record 243,000 tonnes and relatively little has been used since.
Cardiff council said just under half of its 600-mile network of roads will be gritted. Several cycle routes will also be treated.
Cardiff Airport said a snow plough will be in operation but to check with the airport for any schedule changes.
The Welsh Ambulance Service (WAS) asked for their service to be used "sensibly" and only for "genuine cases".
Gordon Roberts, from WAS said: "Vehicles operating in rural areas prone to snow and ice have been fitted with winter weather tyres as part of our winter plans.
"Partnership agreements are also in place with other agencies such as mountain rescue and the RAF to enable us to provide care safely and efficiently to patients in times of adverse weather.
Cardiff and Vale University Health Board said some outpatient clinics would be closed on Friday and patients affected had been contacted by telephone.
Temperatures have already dropped to -5C (23F) in areas of Wales this week.
Reader Comments
to our Newsletter