Police have urged motorists to take care after Storm Bert arrived in Northern Ireland.
A yellow weather warning for wind, rain and snow is in place for much of Saturday.
There are already reports of several road closures this morning.
Met Office Chief Meteorologist Jason Kelly said: “Storm Bert started to arrive overnight on Friday and into Saturday, initially over Northern Ireland.
“As we go through the first part of Saturday morning, it will start to show its hand across Scotland, north Wales and northern England, with the potential for some heavy snowfall, especially over higher ground.”
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In Northern Ireland, wind and rain is already causing disruption with fallen trees and flooding.
On the higher ground the rain is still turning to snow, with reports of very difficult driving conditions, according to Trafficwatch NI.
Roads affected include the Coleraine Mountain Road, Windyhill Road in Limavady, the Glenpark Road, Omagh and the Glenshane Pass.
A fallen tree has led to Dergbrough Road being closed in Co Tyrone, while in Co Down the Belfast Road near Newtownards is partially shut due to a fallen tree and utility lines, as is the Springvale Road.
The Coast Road in Ballygally is also obstructed due to a rock fall.
The PSNI issued a specific warning to drivers in Co Down.
“Motorists are urged to take extra care on a number of roads in the Newtownards area due to the impact of poor weather conditions this morning,” it said.
“There is a fallen tree on the Belfast Road, and there is also a report of a fallen tree on the Springvale Road, between Ballyhalbert and Ballywalter, at its junction with the Inishargie Road. Both lanes are currently blocked as a result of this.”
P&O Ferries said it had cancelled the 4am sailing between Larne and Cairnryan in Scotland’s south west this morning.
The Met Office warning for snow and ice is in place until 11am today.
A separate yellow warning for strong winds is in place until 7pm. The forecaster said gusts in excess of 70 mph were possible along some exposed coasts of Northern Ireland and western Scotland.
Meanwhile, in the Irish Republic, more than 30,000 homes are without power, according to RTE.
Donegal appears to be the hardest hit county in terms of power outages so far, with around 12,000 people currently without power.
Across the Republic, a series of weather alerts are also in place.
A ‘Status Red’ rainfall warning was put in place for Galway and Cork from midnight until 10am today.
A Status Orange rainfall warning for Waterford, Kerry, Clare, Mayo, Sligo and Leitrim is in place until 10am today.
Donegal will be covered by a Status Yellow snow, ice, rain and wind warning until midday.
Yellow wind, rain and snow warnings cover other areas on Saturday and into Sunday.
An amber alert for heavy snow and ice is in force between 7am and 5pm on Saturday in areas across Scotland, where 10-20cm is likely on ground above 200 metres and potentially as much as 20-40cm on hills above 400 metres.
The weather warning covers parts of Angus, Perth and Kinross, Stirlingshire, Aberdeenshire and some of the Highlands, Argyll and Bute, the Borders, Dumfries and Galloway, East Ayrshire and South Lanarkshire.
A second amber warning will be in place between 7am and midday on Saturday covering parts of Yorkshire and the north east of England.
Over the weekend, Wales and the South West are at risk of seeing 75mm of rain widely, and potentially more than 100mm over the higher parts of South Wales and Dartmoor.
Watch: Snow arrives in Northern Ireland as temperatures fall below 0