Assessment of NOAA National Water Model Operational Short-Range Streamflow Forecasts: A February 2019 Case Study in San Francisco Bay Area

In August 2016 the NOAA Office of Water Prediction (OWP) launched a new operational hydrologic prediction service based on the National Water Model (NMW) in the continental United States (CONUS). Skilful hydrological forecasts at short-range lead times would be extremely beneficial for decision-making in flood forecasting, especially in small watersheds that are not covered by the National Weather Service River Forecast Centers. However, for stakeholders to make the best use of the NWM, they need to know something about the overall skill of the forecasts in their watersheds of concern. The purpose of this study is to examine quantitative predictability of the NWM operational streamflow forecasts and to answer the following overarching questions: what is the forecast reliability depending on lead time? and what are the strengths and weaknesses of the forecasts compared to persistence forecasts?

The San Francisco (SF) Bay area is especially prone to flood threats and is selected as an application domain. As a case study, an event on February 10-22, 2019 is selected. This event was part of a series of storms that occurred in January and February 2019 and produced over $150 million in flood related damages, resulting in a presidential disaster declaration. This study focuses on the short-range streamflows out to 18 hours. In general, the NWM performed well in providing peak flow forecasts that can support flood mitigation decision making, even for small watersheds. As expected, the NWM showed better performance than persistence forecasts at shorter lead times, but it showed a decreasing trend in skill at longer lead times. Results also showed that the reliable lead time varied with water management influences, and drainage size.