The Met Office had warned of rain turning to snow as it moved east across the UK on Thursday, with snow confined to higher ground in Wales.
Snow and ice warnings, forecasting showers of sleet and hail, are also in place for the whole of Friday.
This may lead to "a risk of disruption" and "difficult driving conditions".
The chief forecaster said: "A cold north to north-westerly airstream will keep the risk of wintry weather going through Friday.
"However, amounts of snow are likely to be very variable with some places missing it altogether."
People were warned to expect longer journey times.
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On Thursday evening, police said the A470 Oerddrws Pass in Gwynedd was blocked in both directions, with 10 cars and a bus having become stuck in the road between Wyle Cop Street, in Dinas Mawddwy, and A487 at Cross Foxes.
The road was "completely impassable due to the snow", with the blockage affecting motorists travelling between Mallwyd and Dolgellau, police said.
Several cars were also involved in an accident in the snow on the A5104 at Treuddyn near Mold, in Flintshire, with no injuries reported.
Friday's warnings cover Bridgend, Carmarthenshire, Ceredigion, Conwy, Denbighshire, Flintshire, Gwynedd, Isle of Anglesey, Neath Port Talbot, Pembrokeshire, Powys, Rhondda Cynon Taff and Swansea.
Vale of Glamorgan, Wrexham, Caerphilly, Cardiff and Merthyr Tydfil are also included.
Meanwhile, Dyfed-Powys Police encouraged walkers to park sensibly around the Storey Arms-area of Brecon this weekend as a high volume of visitors is expected for the Fan Dance endurance event at Pen y Fan.
The force said snowfall over the Brecon Beacons and two pre-planned events led to significant disruption and traffic issues on the A470 last January.
Cars parked on the side of the road in the National Park area caused "substantial difficulty" to emergency services vehicles which were called to a three-vehicle crash and created a hazard to pedestrians who had to walk in the road.
Sgt Owen Dillion, of the roads policing unit, said: "Last January, we issued fines to over 100 vehicles parked illegally in one weekend. We really hope not to do the same this year."
A yellow "be aware" warning of snow across Wales remains in place on Thursday night, with a risk of snow, sleet and ice on Friday.