Toronto Public Health receives weather forecasts from Environment and Climate Change Canada (ECCC) in advance of issuing a Heat Warning. Extended Heat Warnings are issued if conditions are forecasted to continue for 3 or more days. Heat warnings from Environment Canada are based on the following triggers:

Forecast Temperature High (°C) Forecast Temperature Low (°C) Forecast Humidex Duration (days) Action
≥31 ≥20 ≥40 2 Medical Officer of Health will issue a Heat Warning
≥31 ≥20 ≥40 3+ Medical Officer of Health will issue an Extended Heat Warning