Telling employer you might fail drug test for weed while living in legal state but moving to illegal state.

Pretty simple, I have a drug test coming up in a state where marijuana is illegal and I currently live in CO a legal state. I've been marijuana free for almost 3 weeks(two days to the 3 week mark) and not sure when the screening is but it sounds like it'll be soon but I want to try to get it to 4 weeks. In the event that I can't and I fail the at home drug tests (which would likely mean I fail a lab test.) Would it be a bad idea to let my employer know that I could test positive? Obviously I would stop when I move states but I also don't want my employer to know I smoke weed and possibly get my offer rescinded. Not sure what to do from here.

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