Between 6 a.m. and 2:30 p.m. a total of 40 crashes were reported, and all were due to the wet conditions, CHP officials said.
The incidents were all non-injury or minor-injury crashes, and many were solo-vehicle spinouts.
A CHP officer was at the site of one crash around noon when a BMW, driven at unsafe speeds by Joseph Sucatre, 62, of Santa Rosa, went out of control and crashed into the officer's car, CHP officials said. No one was injured in the collision.
A four-car crash a short time later shut down the northbound U.S. Highway 101 connector to eastbound state Highway 12 for about 40 minutes.
The CHP is reminding drivers to slow down, increase following distance, and turn on headlights when it's raining.