Heavy, drifting, snow is expected to fall across much of Wales later on Tuesday and overnight.
There was a short burst of snow in Wrexham on Tuesday morning
Snow has been blamed for a number of road accidents in north Wales, where over an inch has already fallen.
But the weather is expected to worsen, and spread south, with as much as 20cms of snow falling on high ground.
West Wales is predicted to be the worst affected areas, however, the royal Senedd opening in Cardiff on Wednesday should not be affected.
Cwrtnewydd Primary School in Ceredigion was the first school to close after falling foul of the weather, but others are expected to follow suit on Wednesday.
BBC Wales weather forecaster Dereck Brockway said the cold snap is likely to continue for the next few days.
"It'll get worse later tonight, with up to 20cms of snow falling on high ground," he said.
"Ceredigion, Pembrokeshire and Carmarthenshire and party of south west Wales will be the worst affected, with parts of mid and south Wales, mainly the valleys, too.
"Monmouthshire and Cardiff will largely escape. The snow should not affect the Senedd opening, but if you are travelling any distance to the ceremony, then the advice is to leave plenty of time for your journey."
Mr Brockway warned motorists across Wales not to travel unless they have too.
"But if you do have to, take plenty of warm clothing, a warm drink and a spade - just in case, especially if you driving through west Wales," he said.
The cold spell , with a mix of sunshine and snow showers, is expected to last into the weekend.