Southern California is currently in the midst of a heat wave, which is an unwelcome stress for stay-at-home laws during the Covid-19 pandemic. Indeed, the California coast sees its warmest temperatures in well into the fall, peaking in September-November, often with a secondary peak through April-May. This is also the peak of the wildfire season. Go north about 300-600 miles to Washington or Oregon (and southwestern BC) and the warmest times of the year (plus the peak of the wildfire season) are smack dab in the middle of early July through to mid-August, like in most of the CONUS excluding the Desert Southwest. NOAA's map of when the average "warmest day of the year" is in the CONUS shows this difference quite clearly. Why the difference? I know that the California coast is Csa/Csb while Washington and Oregon and BC are Cfb, but I don't think that explains everything.
I am not an expert, but ...
the pacific ocean has seasonal patterns:
You can think of the water coming from the North being colder, the one from the South warmer. On top of that, then you have associated rainfall etc. that will affect the climate (it can work both ways, warmer water may mean more rain, then more cloud coverage so less solar radiation so slightly colder days).