An old-school method of determining "ripeness" is to look at the shape of the trichromes. When they look like a ball on a stem, you're close. When most of those spheres have collapsed slightly, making a mushroom shape, harvest. If a large portion of the spheres have fallen off their stems, you're a bit late.

The current method is to watch the trichromes turn milky, then amber.

I've seen buds get rained on for a day or two, and continue to mature nicely. If you have a forecast of many rainy days, then I'd be watching them like a hawk. Several sunny days after a rain probably won't give you any issues.

Some growers do a complete harvest when they see any mold. It can take over fast. You can also just harvest the dense buds, leaving the rest to swell over the following days.