The agency says that beginning Saturday and continuing throughout the Canada Day long weekend, daytime highs are forecast between the low thirties into the mid forties with humid values factored in.
The heat wave is supposed to start on Friday and the temperatures will go up along with the humidex levels in the Maritimes and southern Ontario, and Quebec.
"What we're seeing in parts of central Ontario are probably very close to record amounts of humidex", said David Phillips, senior climatologist from Environment Canada, noting Canada Day revelers on Parliament Hill in Ottawa could witness the hottest July 1 in 55 years.
Environment Canada also warns to watch out for serious medical conditions such as heat exhaustion and heat stroke, and to seek medical help if anyone is suffering from any of the symptoms of a heat-related medical emergency.
The temperatures will be "sultry" and "oppressive", he told CTV Barrie on Thursday.
Phillips suggests the more heat-sensitive of the population take it easy.
"Uncertainty about when will the heat wave still persists".
The weather agency says there will be "little relief from the heat" with overnight lows in the low 20s.
The health unit advises people to prepare for the heat by staying hydrated, staying cool, and to check air quality.
"Don't wait until you're thirsty to drink water", Roy said.
Should the hot weather continue, the City of Toronto could also issue an extended heat warning, which would trigger extended hours at public swimming pools.
Current forecasts expect temperatures to be over 30 C for all of next week.