On Tuesday, the Met Office issued a yellow warning for snow in Scotland but has upgraded it to an amber "be prepared" warning with up to 15cm of snow likely in some parts.
In parts of northern England, East Anglia, north Wales and the Midlands, amber warnings for wind are in place.
Forecasters have warned road, air and ferry services could be affected.
Building damage is also possible as the storm, likened to a "weather bomb" by forecasters, unleashes its power on the country.
In southern Scotland 10 to 15cm of snow is expected to fall between 02:00 GMT and 18:00 GMT on Thursday.
On higher ground, snowfall could reach 20 to 30cm throughout the day across Falkirk, South Lanarkshire, East Ayrshire, North Lanarkshire, Edinburgh, Dumfries and Galloway and the Lothian region.
Gusts will reach 50 to 60mph across north Wales, north-west England, the Midlands, Yorkshire and East Anglia.
The strongest winds are expected to be "short-lived" and gone by the evening.
Heavy rain is also likely throughout the day, with snow expected over high ground across north Wales, north-west England, the Midlands, Yorkshire and East Anglia.
Forecasters say there is also a risk of flooding in Northern Ireland, and possibly at lower levels of northern England, and the far south of Scotland.
However, the Environment Agency says it has not issued any flood warnings for the UK.
More rain and wind is expected to continue through to the weekend and into next week but will not reach the same level as Storm Doris.
Doris is the latest storm to be named by the Met Office, and follows Angus in November and December's Barbara and Conor.
Monday was the warmest day of the year so far with Kew Gardens in west London recording temperatures of 18.3C.
Parts of the UK are set to face snow and winds of up to 80mph when Storm Doris arrives on Thursday.