In the past, there was data on early voting in multiple states before Election Day came. In California there is a primary which I believe there is early voting data available publicly in but I have tried to find it but it didn't show up. What is the early voter turnout right now in the primary in California?
Political Data is hosting a public primary ballot tracker here, presumably based on data obtained directly from the Board of Elections or a private interface with the VoteCal database. I don’t believe this data is made available to the public by the BoE, so this is probably your best bet.
As of the latest update (May 20th), 818,663 of the 22,008,488 posted ballots had been returned; a turnout of roughly 3.72% so far. 10,292,039 of these were posted to registered Democrats, with a return rate of about 4.16%, while 5,263,297 were posted to registered Republicans, with a return rate of 3.96%.