You asked about weather, which has been answered yet.
Expect plus 20 degree (celcius), snow, freezing nights and hard rain. There is no such thing as a weather forecast up there, certainly not a consistent one. Don't forget that you're still coastal influenced, and September is beginning to enter the rainy season.
Expect the rivers to blow out, and you won't be disappointed when they do. It's pretty well a guarentee. Staying mobile helps, but every year there is at least one rain event that knocks out everything. The name brand terrace river is particularly susceptible to rain, probably the most likely to blow out of any of the rivers in northern BC.
And despite being pretty "remote" (which is a bit laughable when you actually look at google maps), do not expect solitude. Expect it to be busy. Mid sept to mid oct is prime time and is exceptionally busy
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