Toronto is under its first heat warning of the summer and it came just minutes before the start of fall.

Environment Canada issued the warning for Toronto and much of southern Ontario just before 3:30 p.m. Friday. Fall officially began at 4 p.m.

An alert issued Friday afternoon advises people of "extreme heat," with the humidex making it feel close to 40 C this weekend. That means added risks to young children, pregnant women, older adults and those working outdoors.

The unseasonably warm temperatures are expected to continue into next week, the weather agency says.

Latest heat warning since 2001

Friday's temperature has already reached a forecasted high of 27 C with the humidex making it feel like 30 C.

The mercury is expected to climb even higher into the weekend with Saturday's high expected to be 31 C, Sunday's 32 C and Monday's 30, all with sunny skies.

A heat warning is issued when very high temperatures or humidity could bring a heightened risk of heat-related illnesses such as heat stroke or exhaustion.

There have been just three other heat alerts in 2017 — on June 11, June 12 and June 17 — all before the official start of summer.

Friday's heat warning is the latest into the season Toronto has seen since 2001. The latest alert the city saw previously came on Sept. 11, 2013.