UC San Diego

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.