More than 40 schools across counties Londonderry and Tyrone have been closed as snow and ice made travel treacherous.There has been heavy snow in several places, including Sion Mills, Limavady and Maghera.
The Roads Service said it used snow ploughs to keep some routes clear and has gritted the main roads, but that some minor roads are icy.
Several school bus services have been cancelled.
Wintry scenes around Londonderry and Strabane
Stephen Smith from Transport NI said snow ploughs and salters have been dispatched.
"The main through routes were salted last night and again this morning. They are all passable with care, a lot of the roads haven't experienced much traffic yet and it will take the action of traffic to turn the salt to brine and eventually remove it from the roads."
SDLP councillor Martin Reilly told BBC Radio Foyle the snow has had a knock-on effect on travel in Derry.
"There seems to be very heavy tailbacks from the city side to the waterside on the Foyle bridge and that seems to have had a domino effect.
"There were a number of cars that were struggling to come off the bridge onto the caw roundabout. Anybody that is thinking of travelling across the Foyle should go via the Craigavon bridge."
A number of school bus services in Derry have also been cancelled including the Reddans and McGonagles service to Thornhill.
Most Ulsterbus services are back to normal although the 212 Derry/Belfast service is experiencing delays of around half an hour and the Strabane to Castlederg service has been cancelled.
Passengers should check Translink's Ulsterbus website for the latest travel updates.
Snow covered hills around the north west
Snow is one can deep in places
Derry city was a winter wonderland on Tuesday morning
Christmas tree anyone?Some country roads have seen six to eight inches of snow
People woke up to find their cars under a white blanket