Was it in fact that high of humidity in BC during this event? I think it wasn't, right? When I looked at Portland's weather on the day it was really hot there, the humidity was around 20%.
At 49C you need a dewpoint of almost 70 (!!) to get 20% relative humidity. thats nasty -- during the heatwave in portland the dewpoint was between 63-68 most days. Usually its ~55. For context, the dewpoint on the east coast during the storm season usually is between 70-75.
In the interior of BC it’s typically quite dry during the summer. It’s basically high desert at least in the south.