Well, after a long, slow, steady rain, the local RH has reached 100% here at all four of our airports: KAVQ (Marana), KTUS (Tucson), KDMA (USAF) and KRYN (Ryan), but none have declared "fog" yet.
My station (SHT31 sensor) got to 99%RH this morning, but hasn't hit 100% (yet). No fog visible.
ADDENDUM: 100%RH recorded between 22:40 pm last night and 09:40 am this morning...accompanied by visible FOG!