Deep inversions in the Rogue, Umpqua, Illinois and Applegate Valleys held low clouds all day, like a lake caught in between mountains. The clouds contain moisture (and colder air in and below them), so the temperatures above the inversion are warmer and don’t mix well under high pressure.
As the high pressure zone moves eastward, the flow will push slightly stronger winds over the low cloud deck, and mix it up on the edges. This will help to break up the clouds in the afternoons on Friday, Saturday, Sunday and Monday…at least allowing for some sunshine to mix in.
Areas of valley fog will also be present through Monday, along with building air quality problems. Inversions are notorious for holding the vehicle, industry and wood burning pollution near the valley bottoms. So, the only thing that will clear it out is a storm from the Pacific Ocean or a pocket of very cold air above. Both of those things will happen beginning on Tuesday…and continuing into Thursday.