I found mountain NorCal extremely pleasant in the summer when I went. Highs 85-90 in Mt Shasta area with lows in the 40s-low 50s, zero humidity at all and an exceptionally blue sky. The Redding heat bowl is not all of inland NorCal.
Granted, the wildfire smoke there can get legitimately horrid in bad years, but I have to say the normal conditions there seemed heavenly to me.