Toronto has declared its first extreme cold weather alert of the winter in a bid to protect the city's most vulnerable residents from the dangerously frigid temperatures expected to take hold Monday evening.

The warning comes as Environment Canada predicts a sudden blast of winter weather across southern Ontario. Temperatures in Toronto are expected to drop to -4 C in the afternoon before plunging to 16- C by the evening.

With the wind chill factor, the temperature is expected to feel more like -25 C.

Because extreme cold weather can be life-threatening to people who spend much of their time outdoors, the city's alert means emergency shelters are expanding their capacity by 172 spaces and outreach workers will focus on convincing homeless people to seek shelter indoors.

Transportation, including TTC tokens, will be made available to help homeless people get out of the cold.

An extreme cold alert is called when Environment Canada predicts a temperature of -15 C or lower, without the wind chill, issues a wind chill warning or predicts extreme weather conditions such as a blizzard.

People seeking shelter from the cold, or those who know someone who is, can call city services at 311.

Environment Canada has also issued a special weather statement for much of southern Ontario and is warning residents to expect strong winds and about two centimetres of snow in most areas.

Snow squall warnings and watches remain in effect in parts of southwestern Ontario, and snow fall levels could reach as high as 15 centimetres in the Grey-Bruce region and Muskoka.