This feels like the most plausible situation to me. I live in LA on the other side of the valley and remember being surprised that it was so densely foggy/chilly late this morning, right before I went inside and heard the news. It would only be worse closer to the coast like they were.

A few weeks ago I was up in Santa Barbara and drove into the mountains and encountered really thick fog, so thick I couldn't see in front of me and had to keep braking until it passed. Unfortunately being in a helicopter... that's not a possible thing to do. It's fucking terrifying. News reports say searching for the black box and interpreting the results could take upwards of a year but hoping for the sake of the families of all those onboard that they get closure much more quickly.