The "be prepared" amber warning is in place for wide parts of the country, including Strathclyde, Central, the Lothians, south west Scotland, Tayside and Fife from 7am until 10pm.
A deepening area of low pressure is expected to track east across the United Kingdom during Wednesday night and early Thursday (17-18 January).
Another ice warning covers the western half of Scotland from midnight to 10am on Saturday, where further snow showers are also forecast.
Exisiting yellow warnings for snow in the north of the United Kingdom for Wednesday overnight into Thursday have also been upgraded to Amber.
Persistent snow and ice has caused treacherous driving conditions across the north of the United Kingdom and brought the M74 in Scotland to a standstill.
"That in practice means that all travel should be avoided on those parts of the trunk road affected by the amber warning, namely south and south-west Scotland for the duration of the amber warning", he said.
According to the latest Transport Scotland statistics, more than 1m vehicles are now registered in the 11 council areas affected by the measure. Ice will also form in some areas overnight.
All schools in the Borders have been closed today and will remain closed tomorrow, Scottish Borders Council has confirmed. Police in Dumfries and Galloway have warned against "all but essential travel".
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Almost 30 schools and nurseries were closed in South Lanarkshire and there are also a number of schools shut in East Ayrshire, Stirling, Falkirk and Perth and Kinross.
A spokesman said: "Ice is expected to form on untreated roads, pavements and cycle paths across parts of southeast Scotland and northeast England from late Thursday afternoon".
Level 3: High risk of disruption for road journeys - police advise that there is a high risk of disruption for road journeys.
The Met Office is forecasting "persistent and heavy snow" in the regions between 9pm and 5am, with likely delays on the roads and "a risk that some vehicles and passengers could become stranded".
Scotland, Northern Ireland and the north of England may continue to be affected by wintry conditions into Friday evening, though the Met Office predicts it will be less severe.
"Motorists should consider avoiding peak journey times and whether they can delay or avoid travelling".
Disruption and cancellations are also expected to rail and air travel, with some rural communities at risk of becoming cut off and left without power.