Why does the current warm spell in the Pacific Northwest include high, thin clouds?

Question

I've noticed this during summers (lived here since birth). Sometimes there will be cloudless blue skies, other times when it gets warm there will be a layer of thin clouds, often with higher humidity levels and uncomfortably warm nights. I took a picture of what I mean on Friday.

My guess is monsoonal flow that can't develop into thunderstorms due to high pressure aloft.