Environment Canada says the mixed precipitation is expected to continue throughout the day, creating "hazardous" conditions for motorists across southern and central Ontario.
The national weather agency previously said that some areas could see snow and ice pellet accumulations of between five and 10 centimetres.
Several collisions have been reported across the GTA and Niagara Region, including a fatal crash in the Fort Erie-bound lanes of the QEW near St. Catharines.
OPP Sgt. Kerry Schmidt confirmed that at least three dozen collisions are currently on the go on GTA highways alone.
"Today is one of those days… it doesnt look terrible and that is the danger because when it doesnt look completely treacherous, people are going to think that it is manageable and they can head out there. But with that freezing ice and those pellets coming down, it can freeze on contact and it can cause a complete skating rink," he said in a video update posted on social media.
"Over the next couple of hours, we are going to see these conditions deteriorate even more."
He told CP24 on Sunday afternoon that new collisions continue to pop up as OPP officers try to clear the backlog.
"Drivers (are) going too fast, losing control on the snow and slush on the on-ramps and off-ramps," he said.
"We are seeing multiple single-vehicle crashes, spinning out and some serious crashes as well."
Officers responded to a 12-vehicle crash near Davenport and Avenue roads on Sunday morning and while Toronto police say road conditions in the area are "treacherous," so far no injuries have been reported.
Mark Mills, the road operations superintendent for the City of Toronto, said salt trucks and snow plows are out, adding that today will be an all hands on deck event.
For today, please avoid travel if possible. If you need to travel today, please plan ahead, he said at a news conference on Sunday morning.
If you drive, please slow down. Drive to the conditions of the roadway. Please give yourself time and space for our cyclists, pedestrians.
Mills said the main goal of road operations today is to make sure that the sidewalks, streets, and highways are clear for the Monday morning commute.
It is a test with these mixed precipitation events, he said. We will have to be diligently patrolling.
With extreme winter weather conditions today, please drive with extra caution on our roads. Be sure to give yourself plenty of time to get to where youre going and remember to drive safe and look out for pedestrians. pic.twitter.com/Zb2jR0WH4G
At Toronto Pearson International Airport, freezing rain started to fall at around 8 a.m. and flight delays and cancellations have been reported.
Officials at both Pearson Airport and Billy Bishop Toronto City Airport are urging travellers to check their flight status before leaving the house today.
Metrolinx is reporting some delays on GO bus routes today due to the messy conditions. The TTC says while there may be some delays on bus routes, subway and streetcar service is running normally.
Environment Canada is also warning of strong wind gusts of up to 70 kilometres per hour, resulting in blowing snow and reduced visibility.
High winds are likely to blame for falling glass at a busy intersection in the Financial District early Sunday morning.
Fire crews were forced to shut down the intersection of King Street West and Yonge Street due to glass falling from a building in the area.
The inclement weather has also prompted officials in Burlington and Mississauga to cancel their respective Santa Claus parades today.
Environment Canada has issued a weather alert calling for several hours of freezing rain today. The freezing rain warning comes as a low pressure system approaches the area. The precipitation is expected to taper off to freezing drizzle or flurries this afternoon. Several millimeters of ice accretion are possible over these regions with some areas possibly receiving a few centimetres of ice pellets in addition to the freezing rain. Strong wind gusts up to 70 km/h coupled with ice accretion on tree branches and power lines could cause power outages. Surfaces such as highways, roads, walkways and parking lots will become icy, slippery and hazardous. Slow down driving in slippery conditions. Watch for taillights ahead and maintain a safe following distance. Beware of branches or electrical wires that could break under the weight of ice. Freezing rain warnings are issued when rain falling in sub-zero temperatures creates ice build-up and icy surfaces.
A strengthening low pressure system is expected to bring a messy mix of weather to the area beginning early Sunday morning.
Some areas could see several hours of freezing rain or ice pellets before transitioning to light snow or freezing drizzle Sunday afternoon. Light snow is then expected to continue through Sunday night before tapering off Monday morning.
There remains some uncertainty as to the track of the low; however, there is the potential that some areas could see total snowfall or ice pellet amounts of 5 to 10 cm by Monday morning.