It has been a LONG time ago (late 1960's-eary 1970's) since I did much in BC (Rockies, Selkirks, Bugaboos, etc). Went up for a week around 4th July once and spent most of the time playing pool in local bars because the rain was torrential. I recall LOTS of rain, particularly in June-early July. Used to be that late August-early Sept was better weather. It would not be unusual to have a week of no sunshine to dry things out.

I know down jackets have come a long way since then - they now come with water resistant outer material. Down is great as a sit-around in the evening jacket, but if out for weeks at a time, I am not sure I would count on it staying dry 100%. I have taken a down jacket in wet conditions, but it was always a luxury item, that I could survive without, albeit in less comfort. Also, with the rain, as silly as it sounds, what about bringing an umbrella? Or at least have a tarp to set up where you plan to cook.

I would certainly have some back-up gear. Are you traveling in a car or using public transportation? Or doing all walking?