"There are about 60-70 days of measurable precipitation per year in most of the cities between SF and Silicon Valley, meaning some days it’s barely anything, some days it won’t be happening while you’d be riding, and really we usually only get 3 big storms per winter that drop a meaningful amount of rain.
What I’m saying is, go board somewhere with an indoor if it’s too hot to ride, or go coastside where it’s not too hot. But don’t worry about rain. 5 years of drought… remember? 
I think I missed 3 days of riding last winter, total.
That said, you may want to list your priorities in order. For me turnout in a pasture 24/7 is number 1, as I have a trainer who comes to me and trailer out to a clinican monthly. I don’t need a trainer day-to-day. I board coastside."
Last year was wet for this area of California and the average was about 48 inside indoor days. Thats said many barns have allocated schedules as indoors are not as popular as in some areas which means sharing, Make Sure if you move to a barn with an indoor that you can use it inside your schedule.
You can board anywhere as so many coaches come to horse park and as long as your at a place you like you can haul to HPW.
DO NOT count on riding outside in this area if it is raining. WE ARE ON CLAY and it is SNOT when wet which is what made last year a disaster for many and that includes horse park. The XC is closed much of the winter.
WEB:
Bec Braiting (comes often) Direct coach
Paula Wong
Tory
HPW:
Liz Hall (very direct)
John Robertson (very nice Australian)
Isola:
Amanda Fisher
http://www.horsepark.org/trainers.php
There are a lot of good coaches. I will add more this evening but good luck
