Weather warnings have been put in place for all parts of the UK as Storm Fionn threatens to bring snow, strong winds and icy conditions.
The Met Office has issued amber alerts for snow and ice in central Scotland and Northern Ireland with as much as 25cm expected to fall on higher ground. Less serious yellow warnings for snow have been issued for the rest of Scotland and northern and western England.
A Met Office spokesperson said: "Snow showers will be heavy and frequent through the rest of Tuesday, gradually easing off during the early hours of Wednesday, with a further slow improvement expected through Wednesday daytime as showers become less heavy and less frequent.
"Some high ground is likely to see 15-25cm building up, especially above 200-300 metres. At low levels throughout the warning area, 5-10cm will be more typical.
"Showers will be accompanied by hail and lightning at times, while strong winds bring the potential for temporary blizzard conditions and drifting of lying snow, mainly over high ground."
Elsewhere, very strong winds expected to reach 80mph will affect almost the whole country on Wednesday night (17 January).
The winds are expected to cause travel chaos with roads closed and train services cancelled. Other disruption could include power cuts and the loss of mobile phone coverage.
The Met Office has also warned about flying debris in high winds.
Large waves are expected to hit some coastlines as a result of the storm - the sixth named storm of the season.
Five flood alerts are in place: the Lower River Soar in Leicestershire; North Sea coast at Bridlington; Somerset coast at Porlock Weir; the tidal River Waveney from Ellingham to Breydon Water in Norfolk; and the tidal River Yare, from Thorpe St Andrew to Breydon Water.
Drivers are also being advised to take care on the roads because of ice hazards.
Simon Henrick, head of news at Green Flag, said: "After a variable and relatively mild winter period, the changing weather conditions may take some drivers by surprise this week so it's important that drivers do all they can to stay safe in the more challenging conditions.
"By thoroughly checking their cars before leaving home, approaching the roads with caution and keeping plenty of distance between cars, drivers will decrease the risk of encountering a problem on the roads this week."