There are further warnings about hazardous driving conditions Temperatures are expected to plunge once again across Wales into Friday as fresh warnings are issued about snow and ice.
Parts of the country were expected to be as cold as -15C (5F) overnight and more snow has been forecast.
The Royal Mail said there had been disruption to services in some “deep rural” areas of Wales.
Drivers are again being warned to take extra care on the roads because of concerns about ice, snow and fog.
Councils across Wales have warned that grit supplies are “disappearing fast” in the wintry weather.
The big freeze is now starting to affect mail deliveries in some areas.
A Royal Mail spokesperson said: “All our postmen and women are working hard to keep the mail moving in Wales and the rest of the UK and we are making deliveries and collections where it is safe and possible to do so.
“However, given the adverse weather conditions, there has been some disruption to services – most recently in deep rural parts of Wrexham in north Wales and mid Wales.”
Councils gritting teams have been out across the country on Thursday night but supplies are being hit by the big freeze.
Three councils, Powys, Carmarthenshire and Caerphilly, have used at least a quarter of their stock, while across Wales 15% of the salt supply has gone.
Drivers are being urged to take extra care on the roads, particularly those that are untreated.
BBC Wales weather forecaster Derek Brockway said “a very hard frost” was expected on Thursday night.
“This will be followed on Friday by an awkward mix of rain, sleet and snow, with some freezing fog and more ice over the next couple of days,” he said.
“There will also be the odd snow shower on Thursday night and very low temperatures of -4C to -9C, with a widespread hard frost.”
He said some areas could see temperatures plunge to -15C (5F) and deep snow cover.
The lowest temperature recorded in December in Wales is -22.7C (-9F) at Corwen in Denbighshire on 13 December, 1981.
BBC Wales has the latest online weather updates.
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