The news articles about the BC rains keep referring to an “atmospheric river”, which is a new term for me.
What does it mean? 
It’s a tropical plume !
Here in the Seattle area, we get atmospheric rivers, sometimes called Pineapple Expresses, that are large weather patterns consisting of wet, warm air that blow up here from the tropical pacific. They dump a ton of rain (our usual rain is closer to drizzle, but during a pineapple express, it’s legitimate rain) over many hours or even a couple of days. And it’s warm, so the snow level usually rises to 8 or 9 thousand feet.
Here’s how my newspaper explained it:
https://www.theguardian.com/world/2021/nov/17/what-are-atmospheric-rivers