Precipitation variability in our climate is quite high. Compare the chart above with that for Central Park , New York. The statistical average of the recorded data. The average position of the storm track often is characterized by heavy precipitation. Since the storm track is most often over northern California and not southern California, the northern portion of the state gets more rain.
Mountain ranges for California play a significant role in impacting the precipitation patterns.
It appears that mountains interact with air and make it easier for clouds and precipitation to form. Rainfall is heaviest in the mountains. There is a relationship between rainfall and its topography. You can clearly see this from the graph, which points this out. As in all maps of quality we see a grid made up of latitude on longitude lines.