You have described three of my four antennas. My practical experience is that wet asphalt shingles absorb quite a bit more signal than the same when dry. Wet snow is even worse.
To be safe, subtract 15 to 25 dB from your NM (noise margin).
What to do with NM
I am reliably receiving signals with predicted NM's of 33 to 72 dB using attic mounted antennas. The fourth antenna, outside, is my only hope for
reliable reception of the weaker signals.