We visited that area in late September 2019. Had breakfast at Paradise Cove Beach Cafe (Sand Castle of Rockford Files), then drove over the mountains into Simi Valley to the Reagan Library. Sunny as could be at the beach, but the fog up in the Santa Monica mountains was so thick we had to slow way down. I would hate to have been flying in it.
It happens a lot, and usually burns off late morning/early afternoon. I’ve also been on top of the ridge when the fog is coming in and it’s really eerie. It just flows over the mountains and slides down the other side. You can see for miles straight, but you can barely see your feet.