I've ridden the Pacific Coast twice during that time window:
- 4th of July week 1998, riding from Eugene, OR to San Francisco. In 1998, I rode the entire Pacific Coast but broke it up into three one-week trips. Memorial Day week I went from Vancouver BC to Eugene. It rained a lot during that week. 4th of July week I went from Eugene, OR to San Francisco and Labor Day week I went from San Jose to San Diego.
- 1st week of August 2010, riding from Portland OR, over to the coast and then down to the coast to San Francisco Bay Area. I needed to be in the Bay Area for work and decided to use my bicycle as my travel method and Amtrak on the way back.
Not during your time window, but I also cycled from Portland down to California border at end of May 2013. It rained more than an inch in Brookings on May 27th 2013, when I was there:
https://www.wunderground.com/history...ic=&reqdb.wmo=
As far as the weather was concerned: it was nice weather during both the July (1998) and August weeks (2010). Earlier in May (1998, 2013) I had some solid rain days.
As far as traffic was concerned: it was still fine during all the weeks I cycled including the July/August weeks.
I agree, at least for the drier months. In general, my favorite parts of the Pacific Coast are from Astoria down to Pismo Beach, CA. If you were going in wetter months or even May, I would favor Monterrey down to Santa Barbara.