Pre Emergents need to be applied a bit before (2 to 3 weeks) the target "weeds" germinate and that depends on the "weed" and soil temperature. Talk with the people at your local UC Cooperative Extension Service about just when to do this.

Here is a link that might be useful: UCDavis on premergents

There is a rule of thumb that says use preemergent when the forsythia are in bloom. That might work for you. We don't have forsythia where I live so I can't really fine tune that any. Possibly a better time in my neighborhood would be when the heavy spring rains hit. Weed seeds need moderate to warmer temps and they need a steady supply of moisture. You can control your watering, but rain can't be helped. If steady rain comes to you for more than 2 days, and you are entering early spring, then apply preemergent immediately. Immediately means before the rain stops.

Just out of curiosity where are you in NoCal and what is your soil like (sandy, clayey, etc.)?