A yellow weather warning issued by the Met Office has been upgraded to the more serious rating of amber.
The warning is still yellow until 3pm today but then becomes amber until 6am tomorrow.
Heavy snow is forecast throughout today and tomorrow.
Another yellow warning is in place between 6am and 9pm tomorrow.
The Met Office says high ground may see snow accumulations of 20- to 40 cm in places, with drifting and blizzard conditions due to the strong to gale force southeasterly winds.
At lower levels, 10 to 15 cm of snow is likely in places.
It says the public should be aware of the potential for severe disruption to transport and to power supplies.
Heavy snow is being forecast for throughout today and temperatures are not expected to reach above 0 degrees celsius.
More heavy snow is being predicted for tomorrow morning, becoming lighter in the afternooon and evening but with temperatures ranging between -1 and -2.
Sunday is expected to be cloudy but more light snow is being forecast on Sunday night, Monday and Tuesday,