One of the oddities of the nearby river system* is that it can produce a heavy fog bank: warm water mixing with cold air, but limited to a single valley because it is also mixing with a another river branch which will generally have colder water.*
Consequently it isn’t uncommon to start out at a 1000 feet with clear blue sky, then see this ahead and below the next hill:
Which will become this:
And end up as this at 400 feet:
Before clearing out completely in about three miles.
*It isn’t a valley: two main rivers, several minor rivers, four (technically five) reservoirs, and two distinct river types squeezed into a few miles.
*It really depends on how much water is needed and where.