Could wind be causing my high efficiency furnace exhaust to back up into the house?
I found a wind neutral pipe cap (right in photo) and installed it six years ago, and at least five major PNW storms ago.  It has performed flawlessly and required no maintenance.

It has a geometric baffle design which neutralizes excess air infiltration/exfiltration by balancing air flow somehow, regardless of direction.  I think it is this one.