Synoptic Drivers of Extreme Precipitation Days in the Upper Yuba Watershed of California

Extreme precipitation in California is associated with a range of impacts including flooding, landslides, and erosion. These events, which primarily occur in the cool season, may be difficult to forecast at a small scale due to complex terrain and mesoscale forcings. In order to better understand the meteorological mechanisms resulting in extreme precipitation the watershed scale, this study identifies and describes the key synoptic and dynamic patterns and processes associated with extremes in the Upper Yuba watershed of California. We use the self-organizing maps (SOMs) approach to identify and characterize the synoptic meteorological patterns associated with extreme precipitation days in the watershed. We use a nine-node SOM to cluster integrated water vapor transport (IVT) for extreme precipitation days into clusters with similar IVT patterns. These clusters then represent different storm types consisting of different variations of atmospheric river (AR)-like features, including corridors of high water vapor transport and a zonally-oriented jet stream across the North Pacific. Extreme precipitation patterns also depict anomalously low 500 hPa geopotential heights and a surface cyclone offshore of the states of Washington and Oregon in most cases. Results show that extreme precipitation within the watershed is possible from a range of storm types with different configurations. Precipitation intensity and duration within the watershed also vary with storm type, which may lead to different impacts. This study will not only aid in better understanding the key meteorological drivers of extreme precipitation in the Upper Yuba watershed, but may provide a basis for improved hydrometeorological forecasting and extension to other societally important watersheds.