It is unlikely to "wash the crystals" off, but a lot of wind and rain can break brances, and those branches will contain crystals just lost...ya know? I don't grow outdoors really...but I know that vegetables outdoors that get a lot of rain towards the end up tasting washed out...like the rain washed all the taste right out of them...you ever get some produce that looks good but tastes like nothing, or has a slight dirt flavor? Overwatering at the end of the cycle. I don't know if this means that marijuana grown under very wet conditions at the end of flowering would make the smoke tasteless, or the smoke less potent...I just don't know...but with other plants too much water weakens the flowers and that just doesn't bode well for the overall quality of the final product.