Well... it happened. Looks like my good 31 just went bad on humidity.
My first sign was when I noticed this evening the FARS dewpoint exceeded the "bad" 31 in my passive shield. That was my "oh shite" moment.
I just measured a 10% humidity error with my Kestrel. Today has been quite brisk with well-mixed conditions, overcast, NW winds. The airport, which is about 15 miles inland (normally difficult to compare to except on days like this), is reporting a 50 degree dewpoint (I was showing 53.3 vs 49.6 on my Kestrel).
What preceded this? Fog and mist last night. Reached 94% humidity for the first time since July 1st.
Maybe it will snap back once we get a dry spell (tomorrow?). But I'm not holding my breath.