Blizzard conditions expected to cause travel chaos across UK A dog enjoys the snow in Herrington country park near Sunderland. Blizzard conditions will wreak travel havoc across much of the UK on Friday, with heavy snow and driving wind expected. Worst hit will be parts of south Wales, where the Met Office has issued the highest-level red warning as up to 30cm (12in) of snow is predicted to fall from about 1am on Friday and continue throughout the day. Much of England, especially in the south, can expect up to 10cm of snow on lower ground and up to 20cm on higher ground, forecasters warned. In London, 5cm is expected to start falling from 9am, settling because daytime temperatures will barely rise above freezing after plunging to -5C at night. Train services throughout England and Wales are expected to be severely disrupted, with some flights at risk of cancellation and schools facing the threat of closure. South West Trains is operating a revised timetable on a number of routes on Friday, including London to Bournemouth and Weymouth, London to Salisbury and Portsmouth and London to Reading. East Coast trains advised its passengers to check before travelling and lifted ticket restrictions on journeys arriving or departing from London's King's Cross after 10am. The Southeastern train company said the weather was "likely to disrupt our train services," while the passenger watchdog Passenger Focus appealed for "a good flow of information." The AA said the greatest risk of disruption to road travel would be in Wales, the west Midlands, southern England and Northern Ireland. The Highways Agency warned drivers to avoid unnecessary travel, check forecasts and allow extra time for journeys. Steve Crosthwaite, head of the agency's national traffic operations centre, said: "During periods of severe weather, we suggest people consider whether their journey is essential. They may want to delay travel until conditions improve. The Met Office forecaster Helen Chivers said: "We have upgraded our weather warning for snow to red, the highest level, for part of south Wales, especially the Heads of the Valleys area and up into the Brecon Beacons, where we are expecting 30cm of snow combined with strong southeasterly winds. So it's going to be blizzard conditions there and some drifting as well. Once it starts snowing, it is likely to carry on, not only across south Wales but across much of southern England through most of the morning. The whole of England and much of Scotland and Northern Ireland will have seen snow by the end of Friday, except Cornwall, where it is likely to rain. An amber warning has been issued to cover the rest of Wales up into Lancashire, across the Midlands, down to London and south-east England, across the whole of central southern England and down into Devon. A yellow warning, the next level down, has been issued for the remainder of the UK, except Cornwall. It will be deeply unpleasant. In the amber areas, people should be prepared to change their travel plans because of the potential for disruption to roads, rail and possibly air transport," said Chivers. But in the red warning, people should take action to avoid all non-essential journeys if possible and make other plans for the day. Among areas worst affected will be Merthyr Tydfil, Pontypridd and Ebbw Vale, she said. The snow will peter out on Friday night and during Saturday. By Saturday morning, it will mostly be left in the eastern parts of England. Most places will have a mainly dry, but cold, weekend. London fire brigade urged caution during the "Arctic blast." Its warning follows two serious fires this week caused by householders taking extreme measures to keep warm. British Gas said it was expecting a "huge spike of activity" for its home service engineers this weekend, predicting they would attend about 20,000 callouts. A mountain guide near Fort William Photograph: Andrew Milligan/PA