For the second time this week, city officials issued an extreme cold weather alert as frigid temperatures are expected Friday night.

Environment Canada said temperatures will plummet to a low of -20 C this evening into early Saturday morning, with wind chill values near -30. The national weather agency cautioned that Friday's weather conditions could pose a risk for frostbite.

In anticipation of the cold weather forecast for the next 24 hours, Toronto's Medical Officer of Health issued an extreme cold weather alert Friday morning. The alert will stay in effect until further notice.

It follows the one that was issued by the city on Monday when the temperature felt like -20 C with the wind chill.

Extreme cold weather alerts are typically issued when temperatures are forecast to reach -15 C or colder or when the wind chill is forecast to reach -20 C or colder. Other weather-related factors may also be considered.

The alert prompted the city to open its warming centres at 129 Peter St., 5800 Yonge St. and at the Exhibition Place, Better Living Centre at 195 Princes' Blvd.

A fourth warming centre also opened Friday evening at 1684 Queen Street East.

Warming centres usually stay open until noon on the day an alert is terminated.

The city also opened emergency 24-hour respite sites at two community centres Friday afternoon.

The sites at Warden Hilltop Community Centre (25 Mendelssohn St.) and Masaryk-Cowan Community Centre (220 Cowan Ave.) will provide 89 physically distanced spaces to the those experiencing homelessness who are struggling to find a warm and safe place to stay, the city said.

"Although temporary, spaces at these locations will continue to operate until further notice," the city said.

Frigid temperatures are expected to stay this weekend. On Saturday, it will be mainly sunny with only a high of -12 C and a wind chill of -18 in the afternoon.

The low will be -13 C with a wind chill of -19. It will also be sunny on Sunday, with the temperature reaching a high of -5 C.

Snow is in the forecast on Monday as Ontario students return to in-person learning. The high will be -4 C.