In late June to early July 2021 a heatwave of unprecedented magnitude impacted the Pacific Northwest region, lands colonially named British Columbia (BC) and Alberta (AB) in Canada, and Washington (WA) and Oregon (OR) in the United States. Many locations broke all-time maximum daily temperature records by more than 5°C. The standing Canadian national temperature record was broken on three consecutive days, at multiple locations, with the highest temperature of 49.6°C recorded, 4.6°C higher than the previous Canadian record. Weather forecasts provided advanced notice of the severity of the event, while sub-seasonal forecasts showed substantially increased likelihood of atmospheric blocking and high temperatures with 10-day lead times, and some skill out to 18 days. The impacts of this event were catastrophic. Estimates indicate at least 900 attributable deaths occurred across BC, WA and OR, likely the deadliest weather event in Canadian1 history. The heat contributed to mass-mortalities of marine life, reduced crop and fruit yields, river flooding from rapid snow and glacier melt, and a rapid increase in wildfires—the latter contributing to devastating landslides in the months following.