The temperature was recorded on Sunday 27 June, beating the previous national high of 45°C (113°F), which was set in Saskatchewan in 1937.
It was confirmed by Environment Canada, which issued alerts for British Columbia, Alberta, and parts of Saskatchewan, Yukon and the Northwest Territories.
Forecasting temperatures near 40°C in several regions - 10-15°C hotter than normal - it said: "A prolonged, dangerous, and historic heatwave will persist through this week."
However, it said it expects temperatures to begin cooling on Tuesday.
The US National Weather Service has also issued similar warnings about a 'dangerous heatwave', with temperatures soaring in parts of states including Washington and Oregon.
It said in a statement: "The historic Northwest heatwave will continue through much of the upcoming week, with numerous daily, monthly and even all time records likely to be set."
The daytime high in Portland climbed to 44.5°C (112°F), the hottest temperature recorded there since the National Weather Service began keeping daily records in 1940.
In Seattle, meanwhile, temperatures reached an all-time high of 40°C (104°F), surpassing a 2009 record of 39.4°C (103°F).
The National Weather Service said this week's weather 'will likely be one of the most extreme and prolonged heat waves in the recorded history of the Inland Northwest'.
It was caused by an extended 'heat dome' over the Pacific Northwest, which the National Geographic defines as 'an area of high pressure that parks over a region like a lid on a pot, trapping heat'.
Andrea Bair, the climate services program manager for the National Weather Service's western region, told the outlet that the heat dome helps sustain the heatwave.
"When you get high pressure over the West, it keeps that warm air over the West," she said.
"A heat dome is basically that trapping dome. The heat event itself is the heat wave, lasting several consecutive days and nights that are well above normal."
While Bair said it is common for areas of high pressure to sit over the West during both winter and summer months, she explained it was 'unusual to have heat events this early'.
Featured Image Credit: PA