The big freeze disrupts mail deliveries across Wales
Snow and ice could hit mail deliveries and collections in four Welsh postcode areas, the Royal Mail says.
The warning comes as fresh snow is expected to fall in mid and eastern parts of Wales as well as the south Wales valleys during the day.
The travel disruption across the UK has even hit the Queen's plans to visit RAF Valley in Anglesey.
Meanwhile, councils in Wales are to have £7m extra for mending potholes and buying salt to grit roads this winter.
The Royal Mail has said the severe weather conditions in some areas is hampering its ability to operate normally.
It has warned that normal deliveries and collections may be affected in the LL, LD, SY and SA postcode areas.
The postcodes cover north Wales from Anglesey to Wrexham, parts of Powys and Ceredigion and south west Wales including Swansea, Neath Port Talbot, Carmarthenshire and Pembrokeshire.
It also said it was still offering a special delivery service but could not guarantee next-day delivery in all areas.
Cardiff Airport is open but flights to and from Edinburgh are suspended as that airport is shut until at least 1200 GMT due to snow.
BBC weather forecaster Derek Brockway said high pressure over Scandinavia meant north-easterly winds for Britain.
He said: "It will bring a dusting of snow in the south but heavier in Powys, Monmouthshire and the north east."
The Queen and Duke of Edinburgh plans to tour RAF Valley in Anglesey has fallen victim to the weather, Buckingham Palace has confirmed.
Heavy snow in England hit transport links for the royal visit but the event is to be re-arranged, a palace spokeswoman said.
Prince William, known as Flt Lt Wales, is based as Valley as a search and rescue helicopter pilot, but is currently in Zurich ahead of the 2018 World Cup announcement.
Wales' 22 councils have learnt they are to share in an extra £7m made available by the Welsh Assembly Government for road repair work and winter gritting.
More school closures are also expected as the big freeze continues its grip.