Up to 8in (20cm) of snow could fall in some areas between 00:05 GMT on Friday and 18:00 on Saturday.
Rail passengers on other services are urged to check their train status ahead of travel.
Gwynedd council said whenever snow or ice was predicted priority routes were pre-treated with salt and grit.
Denbighshire council said it was prepared for the wintry weather with salt stocks at its three depots and its highways team geared up to work around the clock to keep the main A and B roads and roads connecting villages open.
Welsh Water warned customers their pipes could freeze or burst when temperatures fall, costing thousands of pounds worth of damage.