All lanes of the westbound 401 closed for several hours at Guelph Line due to an early morning multi-vehicle collision, just one of many reported.
Ontario Provincial Police urged motorists to leave room between their vehicles and adjust their driving speeds to suit the conditions.
CBC meteorologist Jay Scotland says accumulations in southern Ontario will generally be low.
However, a special weather statement is in effect for parts of the Halidmand region as well for much of eastern Ontario, including Ottawa for as much as five to 10 centimetres.
Snow squall watches are also in effect from Huron through Mount Forest, Barrie, Parry Sound to western Algonquin Park plus Niagara and the Picton area for strong squalls and locally heavy lake effect snow to develop.
Toronto is expected to get a total of two centimetres of snow, before it tapers off by mid- to late afternoon.
Ottawa and eastern Ontario are being cautioned to expect five to 10 centimetres by the evening as a low-pressure system tracks south of the province.
Environment Canada said snow accumulation is expected to be less along the St. Lawrence River as it may become mixed with or change to rain Monday afternoon.
In Quebec, early-morning snow was falling in Montreal and will spread to the north and east.
Five to 10 centimetres are expected north of the St. Lawrence River, while areas south of the river will get two to four centimetres before the snow becomes mixed with rain in the afternoon.