Snow and ice has gripped parts of the UK as a cold snap dubbed the "mini beast from the east" sweeps in.
Parts of eastern England and Scotland have already seen snow, with more expected in the Midlands and Wales.
More than 100 flights to and from Heathrow Airport have been cancelled and drivers are warned to take care.
Amber warnings of a possible risk to life affect north-west England, Yorkshire, the Midlands, London and south-east England from the afternoon.
Met Office yellow "be aware" warnings are also in place across much of the UK until Sunday.
The heaviest snow showers have affected north-east England into the North Midlands, as well as parts of southern England - including Hampshire, Sussex and Kent.
Temperatures dipped to a low of -3.2C in Loftus, North Yorkshire, during the afternoon.
But strong winds made temperatures feel as cold as -7C or -8C for some.
Gusts of up to 70mph are expected in northern England and parts of Wales, forecasters say.
The Met Office said: "Travel delays on roads are likely, stranding some vehicles and passengers. And some delays and cancellations to rail and air travel are likely."
The cancellations at Heathrow have mostly affected short-haul routes, although BA services to New York and Chicago have been disrupted.
A BA spokesman said: "During cold weather conditions aircraft have to be de-iced prior to departure to ensure that they are safe, and additional measures are being taken to ensure the safety of our operation is maintained at all times."
Multiple weather warnings in force this #StPatricksDay, with #snow, strong #winds and a bitterly cold feel for all of us. #Spring very much on hold for now. Stay #weatheraware pic.twitter.com/XuT8yJaOuY— Met Office (@metoffice) March 17, 2018
Other cancelled flights from Heathrow include services from Lufthansa, Aer Lingus, TAP Air Portugal and KLM.
About 15,000 passengers have been affected by the disruption, according to the Independent's travel editor Simon Calder.
He told the BBC: "If you're booked to travel tomorrow, don't be too surprised if you get a text or an email later saying, 'by the way, we've rebooked you on a different flight'."
The Heathrow Express service between London Paddington and the airport has also been disrupted. Trains have been running once every 30 minutes in either direction. This service will continue throughout Saturday evening, according to National Rail.
There have been no severe disruptions at Gatwick Airport, however passengers are advised to check their journeys before travelling.
Gatwick is currently operating a normal service, however the weather is predicted to deteriorate and all passengers are strongly advised to check with their airline for the latest flight information before leaving for the airport.— Gatwick Airport LGW (@Gatwick_Airport) March 17, 2018
National Rail has advised passengers to check for service disruptions before travelling.
Great Western Railway services between Newbury in Berkshire and Bedwyn, Wiltshire and between Westbury and Chippenham are disrupted.
Great Northern said up to 8cm of snow could fall on its network from the evening and warned services could be cancelled at short notice.
Meanwhile, motorists have been advised to avoid trans-Pennine roads where possible.
Highways England's Richard Leonard said: "Our gritter drivers will be out treating our roads around the clock but it is still important to drive to the conditions when snow is forecast.
"Drivers should plan their journeys, monitor weather reports and pack a snow kit of blankets, food, water and a shovel if they really need to travel."
Among other disruption:
- The annual Boat Race of the North, where rowing teams from Durham and Newcastle universities compete on the River Tyne, was cancelled due to strong winds - while conditions on the A1 motorway left Durham's boat club unable to transport its boats to the race.
- The weather also prevented Prince William and his wife Catherine from attending the Six Nations rugby match between France and Wales in Cardiff. The Prince is vice-royal patron of the Welsh Rugby Union. The Duke and Duchess had earlier withstood snowy conditions to watch a St Patrick's Day parade of the Irish Guards in west London.
- Various sporting events including the Weston Super half marathon and a road race in Eastleigh have been cancelled.
- The coastguard rescued 11 people, including nine teenagers, from two rowing boats and a safety boat after they capsized on the River Stour in Dorset.
- Derbyshire Police said the A57 Snake Pass had been closed due to snow while "multi-car accidents" were also causing congestion on the A53 and A515 near Buxton.
The Met Office expects snow showers to become more frequent between 16:00 GMT on Saturday and 09:00 on Sunday.
Amber warnings are also in place for south Wales and south-west England between 00:05 and 18:00 GMT on Sunday.
Snow and winds are expected to ease by Monday, although overnight frost and some ice will occur.
Temperatures are expected to be back around average from Tuesday, as conditions return to more typical levels for mid-March.
It comes after a cold spell nicknamed "the Beast from the East" saw much of the UK and Europe blanketed in snow at the start the month.
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