Dry weather = solid 14 volts at all times.
Wet weather = shitty ~12.5 at idle, ~13.5 at low cruise, approaches 14 at highway.

Comments and suggestions?

 

The first step is to clean and grease (dielectric grease) all major electrical connections. Battery lugs, main ground, starter ground, alternator feed (at alternator and main fuse panel), etc. Likely a corrosion/moisture issue. Don't forget the plug at the back of the alternator.

If that doesn't do it, then it's possible the regulator in the alternator is on it's way out.