A heat wave descending on parts of Ontario this week is expected to bring dangerously high temperatures and humidex values, Environment Canada warns.

The weather agency issued a heat warning starting Monday for an area that spans London to Ottawa, stretching up to Sault Ste. Marie and as far north as Fort Albany.

Temperatures in the Greater Toronto Area and Ottawa could feel as hot as 45 C with the humidity. Regions further north are also forecast to see humidex values that could feel like the low to mid 40s, Environment Canada said.