Dolgellau, Bala, Blaenau Ffestiniog and Trawsfynydd in Gwynedd are severely affected, with tens of cars trapped.
North Wales Police said snow ploughs and gritters were on their way to the scene, and people leaving their cars were putting their lives at risk.
It comes as the clean-up takes place across large parts of Wales following Storm Ciara.
Dafyn Jones, 48, from Pwllheli, Gwynedd, was trapped in his work’s van in freezing conditions on the A4212 between Bala and Trawnsfynedd for more than an hour on Monday.
He said: “You could see conditions were deteriorating, it got worse and worse but by then you’ve committed.”
He said the hold-up had been caused by a delivery lorry getting trapped in the snow and a driver abandoning their car.
The A477 Cleddau Bridge in Pembrokeshire is closed to all traffic and pedestrians because of the weather.
Major and minor routes, including the A494, A4212 and A470, have been left impassable because of the snow, with council workers trying to clear them.
Some roads remain closed due to the flooding, including the A547 at Abergele in Conwy, and some minor roads in the Conwy Valley.
The A5 remains partially blocked following a landslip at Llyn Ogwen, Gwynedd, between Bethesda and Capel Curig.
North Wales Police said the Gwynedd south area was experiencing a “major weather event”, with large amounts of snow making major and minor routes impassable.
Insp Matt Gedde said: “If you are trapped, please stay in your car and keep your mobile phone charged.
“Stay warm and only call our control room in an emergency, as we are aware of the situation and are working to get to stranded motorists as soon as we can.”
Meanwhile, there are 18 vessels sheltering in the bay around Moelfre, Anglesey, because of the strong westerly winds, Holyhead coastguard said .
Winds hit 93mph (150km/h) on the Llŷn Peninsula in Gwynedd on Sunday, with the rail network effectively shut all day across Wales.
Six flood warnings are in place across Wales, which means flooding is expected and immediate action required.
There are a number of flood alerts, meaning flooding is possible and people should be prepared.
In Wrexham county, the River Dee remains high, with full flood warnings in force around the area.
People at a caravan park in St Asaph, Denbighshire, had to be evacuated when river levels rose.
Police evacuated Spring Gardens Caravan Park and properties in Llys y Felin with residents placed in a “rest centre” set up in the leisure centre on Sunday.
A £6m flood defence scheme was opened in the city in 2018.
However, a severe flood warning in the area – which means there is possible danger to life – has now been downgraded.
The storm cut power to thousands of homes, with engineers across Wales working around the clock to reconnect supplies.
Western Power Distribution said it restored electricity to all bar two properties by Monday morning.
“We restored supply to 86,000 customers yesterday, that was in some extremely challenging conditions – winds in excess of 90mph,” said Sean Sullivan, the firm’s network supply manager for Wales.
“It was particularly difficult especially for our staff out on the ground repairing things. It was 20 or 30 times our normal fault value, so it was a tremendous achievement.”
SP Energy Networks said at the height of the storm, 20,000 homes across north Wales were affected.
Because of spray and bad weather, Aberystwyth Promenade was closed on Monday between Terrace Road and Constitution Hill.
One town to suffer widespread flooding has been Llanrwst, in the Conwy Valley.
“It’s pretty devastating,” said Osian Deiniol, who runs the Blas Ar Fwyd food deli store in the town.
He told BBC Radio Wales’ Breakfast show he was left picking bottles of wine out of water, while trying to save thousands of pounds worth of stock in 11 fridges.
“It’s heartbreaking. The deli has been on the high street for 32 years and is part of the fabric of the town,” he said.
“It was flash floods. But the power of social media, the community rallied around, people were volunteering to come and help – people who were flooded themselves.
“It’s devastating, but the community really came to our aid.”
The Conwy Valley rail line remains under water in the area, with a bus replacement service operating between Llandudno Junction and Blaenau Ffestiniog.
Network Rail and Transport for Wales have both urged all rail passengers in Wales to check before they travel, with a number of other lines also affected.
A rail replacement bus service is operating on the Cambrian Line from Machynlleth to Aberystwyth and between Machynlleth to Pwllheli.
Services could also face some delays west of Swansea into Carmarthenshire and Pembrokeshire.
Planes and ferry services
Ferry services bringing fans home from the Six Nations match in Dublin on Saturday are now operating again, as are flights from the Irish Republic.
Cardiff Airport has reported a number of cancellations for flights to Amsterdam, as Storm Ciara arrives in the Netherlands.
A number of schools in north Wales were closed on Monday.
Ysgol Bro Idris in Dolgellau was closed for the day because of falling snow.
Ysgol Dyffryn Conwy in Llanrwst was closed by flooding, while Ysgol y Creuddyn in Conwy was shut due to a “significant risk that more slates will fall”.
Three primary schools in the county are also closed – Eglwysbach, Penmachno and Talhaiarn.
In Merthyr Tydfil, Ysgol Coed Y Dderwen is also closed due to damage to scaffolding which was surrounding the school.