People across the GTA are dealing with a messy commute as a fresh bout of winter weather descends on the region.

A snowfall warning from Environment Canada is in effect for the entire GTA and most of southern Ontario. The snowfall warnings are in effect across an area from Kingston in the east to London in the west, and as far north as Sudbury.

An Alberta Clipper tracking over southern Ontario is expected to dump a fresh blanket of snow across the province.

The snow started falling in the GTA before noon and the storm is expected to leave between 15 to 20 centimetres on the region by tonight.

“Total snowfall accumulations of 15 to 20 cm are likely, with the highest amounts expected closer to Lake Ontario where extra moisture from the lake may boost snowfall amounts,” Environment Canada said in its weather advisory.

The national weather agency said there will likely be a “significant impact” on driving conditions during both the evening commute on Monday and the morning commute on Tuesday.

“Prepare for quickly changing and deteriorating travel conditions. Surfaces such as highways, roads, walkways and parking lots may become difficult to navigate due to accumulating snow,” Environment Canada said.

Police dealing with collisions

The messy weather made for a slow commute at the start of the evening rush hour.

Ontario Provincial Police said they were dealing with 25-30 collisions around the GTA as of 5 p.m.

“It’s all travelling very slowly. The lanes are covered in snow,” OPP Sgt. Kerry Schmidt told CP24. “It’s hard to even see the lanes – where (you) should and shouldn‘t be.”

The slick roads have sent vehicles around the GTA sliding and Schmidt said there have been some highway closures because of jackknifed tractor trailers.

“This is just one of those days where it’s going to take you a little longer or a lot longer to get home,” Schmidt said.

He advised people to turn their headlights on, make sure to clear snow off your vehicle before driving and use extra caution on the roads.

“You need to just pay complete attention,” Schmidt said. “Be attentive, be alert, give yourself that following distance so you can stop if you need to.

He also advised people to stay home if they don’t need to go out.

Mark Mills, the city’s road operations supervisor, told CP24 Monday evening that plows are expected to start clearing arterial roads sometime between 5 p.m. and 7 p.m. now that at least seven centimetres of snow has fallen.

“This rush hour will slow us down a little bit with this congestion,” Mills said.

[Find out where the snow plows are in Toronto]

He estimated it will take anywhere from three to six hours to clear the main roads. Sidewalk clearing is expected to get underway between 6 p.m. and 8 p.m. and plows are expected to begin clearing local roads around midnight, with that effort lasting into tomorrow afternoon.

City crews were out in full force earlier today preparing for the heavy snowfall.

“Our first line of defence is always salting. So we will concentrate on salting on the expressways, main arterial roadways, collector roads, then we will get into the local roads,” Mills said earlier Monday.

“If you have flexibility in your schedule today or the opportunity to leave work a little early to beat the p.m. rush, that might be a suggestion or to use public transit today.”

Transit and flight delays, cancellations

The TTC said Monday afternoon that Line 3 (Scarborough RT) was being shut down due to weather conditions.

“There will be no train service on the entire Line 3,” TTC Spokesperson Stuart Green said. “Those trains will not be in service. Instead there will be 17 shuttle buses running and those buses will be stopping at every station along the route.”

While some problems have also been reported on surface routes, the rest of the subway system is operating without any significant delays.

TTC CEO Rick Leary said earlier Monday that extra trains and staff would be out across the city today after weather and a host of other issues caused big delays across the subway system last week.

"Last week was very disappointing for the TTC. We disappointed our customers and that is disappointing to us. This morning, however, we are in a much better position," he said.

"Things are moving much better this morning. We’ve put extra trains and staff out on the system to help accommodate our customers."

The messy weather is also causing flight delays and cancellations. More than 200 departing flights have been cancelled at Pearson International Airport today. Passengers are being advised to check for updates before heading to the airport.

“Airlines will be making decisions about delays and cancellations, so we suggest that passengers confirm their flight plans before travelling today,” the airport wrote on Twitter.

Snow prompts some university closures/ recreation cancellations

The TDSB said Monday that due to the weather, all board-related activities and meetings, including continuing education classes and school permits are cancelled as of end of school day. The TCDSB also said that all permits will be cancelled this evening.

Dufferin-Peel Catholic District School Board has cancelled all adult and continuing education classes, as well as permits for school and board facilities for tonight.

All City of Toronto recreation programs are cancelled as of 6 p.m. tonight. Community centres will remain open for permits. Artificial outdoor ice rinks will not be supervised, but may be open depending on ice conditions. Artificial Outdoor Ice Rinks will not be supervised, but may be open depending on ice conditions

York University has cancelled classes and operations at both of its campuses this evening.

Due to the snow storm, all of Sheridan College's campuses as well as the University of Toronto's Mississauga campus will be closed at 4 p.m. U of T Scarborough will close at 5 p.m. U of T says its St. George campus will remain open and classes and exams will proceed as usual.

Classes at Ryerson University are cancelled as of 6 p.m. and the Humber College campus will close as of 6 p.m.

George Brown College also said that it will close at 4 p.m. due to the heavy snowfall. The college said all night classes will be cancelled. Lab School Child Care Centres will be closing at 4 p.m. as well, the school said. Further updates about Tuesday are expected to be posted on the school’s website by 4 a.m.

The City of Brampton has cancelled all scheduled recreation programs as of 3 p.m. Fitness centres will remain open, but fitness classes are cancelled.

OCAD University will be closed as of 6 p.m.

Seneca College has cancelled all continuing education classes tonight. All community recreation programs, varsity, campus recreation and fitness activities are also cancelled at all campuses as of 6 p.m. All campuses will remain open.