Scotland's transport minister has warned of potentially "extreme" weather conditions as heavy snow sweeps across the country.
Humza Yousaf said the amber warnings in force for many areas for Wednesday and Thursday could be upgraded to red.
The current warning covers Central, Tayside, Fife, Grampian, Highlands, south west Scotland, Lothian and Borders and Strathclyde.
Forecasters said high ground could see up to 40cm (16in) of snow.
All schools in the Scottish Borders will be closed on Wednesday, and possibly Thursday as well, Scottish Borders Council has announced.
East Renfrewshire, Falkirk and Stirling councils have also decided to shut all schools and nurseries in advance of the weather while Fife Council is closing dozens of its schools and nurseries.
Mr Yousaf advised anyone in parts of Scotland covered by the amber alert to avoid travel on Wednesday.
He told BBC Scotland's John Beattie programme: "There is a possibility that the amber warning could be in some areas upgraded to a red.
"That is a warning for snow that we have never seen since the modern system has come into place in Scotland.
"We are right on the cusp, the strongest possible amber warning - and right on the cusp of potentially becoming a red. That means a high likelihood of a high impact so that could be not just treacherous but frankly dangerous if you travel."
The Met Office said that by Thursday morning a series of snow showers would see 5-10cm (2-4in) fall widely in eastern and central Scotland and the north east of England.
The chief forecaster's assessment added: "Where showers become organised though, most likely across Scotland, areas of 15-25cm are likely, locally as much as 40cm.
"Strong winds will lead to drifting of snow and severe wind chill, while lightning could be an additional hazard, particularly near coasts."
The amber warning has been extended until 18:00 on Thursday, with the snow expecting to extend further westwards than initially thought. It now covers parts parts of Lanarkshire, Glasgow, Renfrewshire, North Ayrshire and Dunbartonshire.
Trains, planes and ferries are also likely to be affected, with a wind chill that could see parts of the UK feeling as cold as minus 15C (5F).
Deputy first minister John Swinney has chaired a meeting of the Scottish government's Resilience Team to ensure preparations are in place to deal with the potential impact of the weather.
Police Scotland urged drivers to be vigilant and keep updated on the latest forecasts.
Ch Supt Stewart Carle, head of road policing at Police Scotland, said: "With the wintry weather on its way, I would urge drivers across Scotland to start to take extra care on the roads for their journeys.
"Snow is already falling in some areas, and drivers should make sure they are prepared for their journey with warm clothing, food and drink, sufficient fuel and a charged mobile phone.
"There could be significant traffic delays so please start to plan your journey now and consider if you really need to travel when conditions are poor."
What the weather warning colours mean
- Yellow: Severe weather expected. Yellow means you should plan ahead thinking about possible travel delays, or the disruption of your day-to-day activities.
- Amber: Be prepared for disruption. There is an increased likelihood of bad weather affecting you, which could potentially disrupt your plans and possibly cause travel delays, road and rail closures, interruption to power and the potential risk to life and property.
- Red: Extreme weather is expected. Red means you should take action now to keep yourself and others safe from the impact of the weather. Widespread damage, travel and power disruption and risk to life is likely. You must avoid dangerous areas and follow the advice of the emergency services and local authorities.