You live in an area that gets a little snow once in awhile. When the snow on the roof begins to melt because of the warm air in the attic, the resulting stream of water, flowing under the snow, flows down to the roof overhang area (where the gutter is). Because that area is cold, not heated, the water freezes. As the thawing, freezing cycle keeps going, the ice formed has to go somewhere. A dam forms at the gutter, so the ice begins backing up - under the shingles. It gets to the warm attic area again, and because it is slightly warmer than the snow pack it is travelling under, it melts again. As this cycle repeats itself it soaks the wood where you see the dip.
The reason for this can almost always be traced to improper or inadequate attic ventilation. In most cases the problem involves the intake (cornice, soffit) vents. Go to AirVent.com. Watch the videos. If needed, give them a call.
Unfortunately, you are looking at a major repair, possibly a new roof.