I live about 80 miles/130km SE from the crash site, but the terrain and microclimates are similar. I was mountain biking and there was a low-lying fog layer about 100-200 feet thick, with a broken around 3000-5000 feet (my estimate).
Another mountain biker and IFR-rated pilot was interviewed. He was first on the crash scene and said there was very dense fog with 3-4 feet of viz. They heard the S76 just before and during impact but did not see it. However, witnesses in the general area (I know those accounts turn out inaccurate) saw the helo "falter" and "sputter", and then steeply descend. So there seemed to be good visibility elsewhere.
The coastal scud in that area blows in from the west along Hwy 101 and hugs low-lying terrain. My very early speculation- they were VFR on top over a patchy ground fog layer in the canyon. A mechanical issue forced a descent into IFR and terrain.