My father's family is from the southern United States and they all, including my dad, who has lived in California all of his life, pronounce it warsh. My sisters and I pronounce it wash. I was wondering if the reason my dad got that "correction" was because his whole family talks like that but I didn't get it because my mother has a General American accent (Well my dad's is almost General American except for this) as do most of the people where I live including the workers at the daycare I spent my early years.
You normally pick up features from your peers, not your parents. What kind of community did they live in? On the one hand here were enough "Okies" moving to California in the early part of last century that they formed their own communities, but their accents tended to be stigmatised so status-aware members would seek to shed them at the earliest opportunity.