A study that looked at two-decades worth of rainfall data has found that the majority of California’s rain arrives due to atmospheric rivers. Furthermore, about 10-30% of the rainfall measured at locations across the state comes from one major storm. The study analyzed rainfall rates at 176 weather stations across California for this study.
Rainfall is distinctly different between northern and southern California, with the line demarcated at 37.5°N. Northern California averages about twice as many rainfall events as southern California. Researchers found that, on average, there are 25 – 45 rainfall events each year in Northern California. Of those, about 40% – 50% are atmospheric rivers which contribute an average of 79% of extreme-rainfall along the northern coast and 76% in the norther Sierras.
Southern California tends to also experience lower median rainfall totals than northern California (10–14 mm versus 10–22 mm per event) as well as shorter rainfall durations (<5–11 hours versus 10-14 hours for northern California). Atmospheric rivers contribute to 68% of extreme-rainfall accumulations in southern California.
Lamjiri, M. A., Dettinger, M. D., Ralph, F. M., Oakley, N. S., & Rutz, J. J. (2018). Hourly Analyses of the Large Storms and Atmospheric Rivers that Provide Most of California’s Precipitation in Only 10 to 100 Hours per Year. San Francisco Estuary and Watershed Science, 16(4).