Toronto is under a winter storm warning as snow continues to fall, snarling traffic during the evening commute on Monday.

Environment Canada issued the warning around 5:30 p.m. The federal weather agency says winter storm warnings are issued when "multiple types of severe weather are expected to occur together."

The season's most significant winter storm arrived in the city at around noon today, and it's expected to blanket Toronto with 15 to 25 centimetres of snow by Tuesday morning. The highest amounts are expected closer to Lake Ontario because of added moisture from the lake.

A man walks his dog through Toronto's winter storm on Monday evening. Environment Canada says the area could see up to 25 cm of snow by Tuesday morning. (John Rieti/CBC)

Scarborough RT shuts down

The TTC has closed the Scarborough RT (Line 3) due to the weather conditions and will run shuttle buses instead.

Customers can use a TTC fare to travel to and from downtown using the GO line. The TTC says fares can be used at five GO Stations including Rouge Hill, Guildwood, Eglinton, Scarborough and Danforth.

Approximately a third of all flights leaving or arriving at Pearson International Airport have been cancelled, while dozens of flights at Billy Bishop Airport have also been cancelled or delayed.

Toronto Pearson is warning that delays and cancellations are possible into Tuesday morning as a residual effect from the storm. They are asking travellers to check the status of their flights online before heading to the airport.

Other commuters in Toronto should also brace for a messy trip home Monday afternoon.

Approximately one third of all flights in or out of Pearson International Airport have been cancelled on Monday night. (Max Perry)

The national weather agency is forecasting the heavy snow to persist until around 5 a.m. Tuesday.

Winds gusting to 50 km/h are expected to blow the snow around, making for treacherous driving conditions during the afternoon commute.

Stay home if possible, OPP warns

OPP Sgt. Kerry Schmidt says the highways in the Greater Toronto Area were crawling even before the start of rush hour.

He says as of 3:30 p.m. police were investigating 20 crashes, including one that involved a transport truck.

"As it gets dark, it's going to be even more treacherous," he said. "So make sure you throw your headlights in the full on position."

He's also encouraging people to stay at home if possible.

The city of Toronto plans to start plowing main roads between 5 p.m. and 7 p.m. on Monday evening, said Mark Mills, the superintendent of road operations. The plowing will continue throughout the night.

Our <a href="https://twitter.com/cityoftoronto?ref_src=twsrc%5Etfw">@cityoftoronto</a> snow clearing equipment, including salt trucks and plows are getting ready to help make our roads safer before & during tonight’s storm. <br><br>Follow <a href="https://twitter.com/TO_WinterOps?ref_src=twsrc%5Etfw">@TO_WinterOps</a> for updates & visit <a href="https://t.co/nz0FgQrc9c">https://t.co/nz0FgQrc9c</a> to track real-time street salting & plowing in your area! <a href="https://t.co/t9h0TYNYrf">pic.twitter.com/t9h0TYNYrf</a> —@TO_Transport

"This'll be the biggest event so far this season," Mills said outside a salt storage building.

Sidewalks will be plowed starting at around 6 p.m.

Mills advised drivers to use extra care and to expect some longer than normal travel times this afternoon.

The TTC has also activated its winter preparedness plan in anticipation of the snow.

Commuters wait for a bus at Kennedy Station on Monday night. Some riders say they waited two hours at the station. (Adrian Cheung/CBC)

Some early closures have already been announced, including Sheridan College and the University of Toronto's Scarborough and Mississauga campuses. The city of Brampton has also cancelled all recreation programs this afternoon.

Due to worsening weather conditions, U of T Mississauga will close as of 4 p.m., Monday, Jan. 28, 2019. <a href="https://t.co/6dDB1077jK">https://t.co/6dDB1077jK</a> <a href="https://t.co/HpU3WwwBwH">pic.twitter.com/HpU3WwwBwH</a> —@UTM

While some commuters may be dreading the snow and cold temperatures, Toronto Raptors centre Serge Ibaka bundled up for what he called a "beautiful two minute walk."