Temperatures have crossed the 30-degree mark amid a late-spring heat wave in Toronto, and Environment Canada has just slapped the first heat warning of the season on the city, expected to last through the end of the week.

The government weather and climate agency issued the heat warning for the City of Toronto just after 2 p.m. on Wednesday, cautioning the public of a "two-day heat event beginning Thursday" and into Friday that will see scorching heat combine with humidity values for temperatures that feel in the mid-30s.

Maximum temperatures are expected to soar to between 29 and 31 degrees on Thursday — values already being seen by Wednesday afternoon — while minimum temperatures aren't expected to fall below 17 to 20 degrees.

Cooler temperatures are expected to bring some relief on Saturday, making for a pleasant weekend with the mercury dropping to a much more manageable mid-20s.

Until then, Environment Canada warns of increased risks for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors, while advising everyone to stay hydrated.

So, if the heat wave is already here, why does the heat warning only apply to Thursday and Friday?

It all comes down to bureaucratic technicalities in a similar fashion as the oft-criticized extreme cold warnings that trigger the opening of emergency warming centres.

Heat warnings are only issued for this specific region when forecasts call for two or more consecutive days of daytime maximum temperatures reaching 31 C or higher, and overnight temperatures no lower than 20 C.

As the overnight low temperatures tonight are expected to be near 15 C, well below heat warning criteria, the current hot weather — despite being roughly equal to what is expected tomorrow — is not included in the heat alert.

There are, of course, exceptions to this overnight low rule, such as if daytime heat exceeds 40 degrees C. That is an extremely rare occurrence, even amid climate change.

The record temperature set in Ontario was 40.6 during a deadly multi-day Toronto heat wave in 1936, and challengers to this record have been few and far between in the generations since.

And with forecasts calling for potentially cooler temperatures this summer, this current heat wave might be the closest we get to that mark this year.