Do I have an issue here?
 

Yesterday I was driving home from work and noticed my voltage was dropping, it was raining. When I had the radio, lights, wipers, and AC on the voltage would drop to the mid point on the gauge and when I turned the AC and radio off it went back to where it normally sits just above the top line and below the red area.

I tested it this morning with my volt meter and it appeared to be normal, 12.5V car off, 13.8-14V car running, 13.1V with all electrical accessories on.

Do you think the rain was actually interfering with the electricals or do you think the gauge was just acting up? I know the gauges on these cars are notorious for being inaccurate.

Sounds like the battery is no longer holding a full charge.