This heat wave is unprecedented, but Mikey has the right of it in that it is caused by a high pressure front traveling through the pacific northwest and western Canada. This would have happened regardless of climate change, though it's uncountable that global warming has contributed to just how ridiculously high the temperature is in many places. Lytton, a small village in British Columbia, for example broke Canada's all time heat record three days in a row, topping out at an insane 49.5C (roughly 121.3F), and other records are being broken all over the place, including here in my city where it hit 39C for the first time ever and is expected to hit 40+ today. Temperatures would not be this extreme had global warming not been at play, even though it didn't cause the heat wave itself.