The Connection between Extreme Rainfall and Hydrologic Extremes in the San Francisco Bay Area

It is important to understand the connection between extreme rainfall and hydrologic extremes, especially in regions with complex terrain, e.g., California. The Advanced Quantitative Precipitation Information (AQPI) project is being implemented to improve monitoring and forecasting of precipitation, streamflow, and coastal flooding in the San Francisco (SF) Bay Area. As part of the AQPI project, NOAA and the Cooperative Institute for Research in the Atmosphere (CIRA) at Colorado State University are assessing the performance of the gap-filling X-band radar Quantitative Precipitation Estimation (QPE) and its impact on streamflow forecasts. In this study, several heavy rainfall events occurring in the mountainous regions of the SF Bay Area are used to explore the sensitivity between extreme rainfall and hydrologic extremes. The rainfall events were part of a series of storms in 2019 that produced over $150 million in flood-related damages and resulted in a presidential disaster declaration. The gap-filling radar QPE is used as the rainfall source. The NOAA National Water Model (NWM), built on the WRF-Hydro community modelling system, is implemented for the hydrologic simulations. The sensitivity of rainfall characteristics and antecedent moisture conditions on the occurrence hydrologic extremes is demonstrated in several Bay area watersheds.