You have to be a registered voter to get sent a ballot without requesting one, though right?, and 80% of Washington state's voters are registered (which btw is pretty impressive), which would favor Hillary, unlike the caucus-system, which has same day registration.
Erm... if you're suggesting that Sanders will do worse in the primary than the caucus, then, sure. But to say that Clinton is 'favored' among registered voters would suggest that she'll win this primary... and frankly, I'd be shocked.
After all, there are plenty of 'soft' Sanders supporters who didn't make it to the caucus.