A strong, driving rain (like what seems to happen in norcal every october) can and will knock some trichomes off the buds. It depends on wind, amount of rain and how long it lasts. If the weather clears up soon after(like 3 days is usually about it if you're close to harvest) and has a nice breeze to dry things out and you shake the water off them immediately after the rain stops, then you should be fine. This is the primetime when mold will take a foothold. Light rain for a couple hours followed by clear, dry and breezy will not be a problem. The rain does worry me though. All strains react differently to rain and some are much more suseptable to mold than others.

Hail is always bad news.

BTW I know THC is not water soluable. The force of the rain hitting the trichomes will likely cause some of them to fall off. It probably won't make much of a difference with potency. Mold is the big issue with rain.