Mitigating the impacts of extreme precipitation is particularly complicated in California, where more than 50% of precipitation typically falls in less than 120 hours annually, and where droughts and floods are extreme and frequent. Lack of reliable precipitation datasets with high temporal resolution has limited investigation of key (hourly) aspects of important storm characteristics that strongly modulate their impacts.

Here, a newly-available quality-controlled long-term dense network of hourly precipitation observations in California is used with hourly to multi-day precipitation observations in the United States (U.S.) to study extremes across the U.S. and through time. Recent advances in atmospheric river (AR) monitoring and cataloging enabled not only the confirmation of ARs as primary sources of extreme precipitation along the U.S. west coast but also recognition of their lesser- known contributions to the eastern U.S. extremes. The coexistence of ARs and hurricanes, which has not yet been explored, is identified here to contribute substantially to eastern U.S. extreme precipitation.

Storm duration, more so than hourly precipitation rates, is found to strongly modulate precipitation totals, especially in the western U.S. These findings emphasize the importance of improving forecasts of storm duration, which has high practical importance as longer storms are more likely to yield severe floods over large areas.

A unique scaling method, the R-CAT scale, is applied to daily precipitation records to provide a basis for placing the extraordinary nature of several recent precipitation extremes in the context of historical storms. While confirming the increase in intensity and frequency of extreme storms in the eastern U.S., new results are found in the western U.S. identifying significant declines in annual maximum 3-day precipitation totals and frequency of R-CAT storms in this region. Results here also provide a deeper perspective on these overall trends, in that a significant shift towards more temporally uniform precipitation during the most extreme storms is identified in the eastern U.S.

These findings provide improved scientific foundations for the development and implementation of effective hazard-mitigation, climate-adaptation, and water-management strategies in California and across the Nation.

https://doi.org/10.15447/sfews.2018v16iss4art1

California is regularly affected by floods and droughts, primarily as a result of too many or too few atmospheric rivers (ARs). This study analyzes a 2-decade-long hourly precipitation data set from 176 California weather stations and a 3-hourly AR chronology to report variations in rainfall events across California and their association with ARs. On average, 10–40 and 60–120 hours of rainfall in southern and northern California, respectively, are responsible for more than half of annual rainfall accumulations. Approximately 10% to 30% of annual precipitation at locations across the state is from only one large storm. On average, northern California receives 25 to 45 rainfall events annually (40% to 50% of which are AR-related). These events typically last longer and have higher event-precipitation totals than those in southern California. Northern California also receives more AR landfalls with longer durations and stronger Integrated Vapor Transport (IVT). On average, ARs contribute 79%, 76%, and 68% of extreme-rainfall accumulations (i.e., top 5% events annually) in the north coast, northern Sierra, and Transverse Ranges of southern California, respectively.

The San Francisco Bay Area terrain gap in the California Coast Range allows more AR water vapor to reach inland over the Delta and Sacramento Valley, and thus influences precipitation in the Delta’s catchment. This is particularly important for extreme precipitation in the northern Sierra Nevada, including river basins above Oroville Dam and Shasta Dam.

This study highlights differences between rainfall and AR characteristics in coastal versus inland northern California — differences that largely determine the regional geography of flood risks and water reliability. These analyses support water resource, flood, levee, wetland, and ecosystem management within the catchment of the San Francisco Estuary system by describing regional characteristics of ARs and their influence on rainfall on an hourly time-scale.