Subseasonal Forecasts of Water Vapor Transport Associated with Atmospheric River over the Western United States

There is a growing demand for sub-seasonal to seasonal (S2S) forecast of Atmospheric River (AR) activities in the water resource and flood risk management, especially in the western United States. The water vapor transport associated with AR plays a direct and dominant role in the precipitation over the western United States when ARs strike the mountain topography. This study provides a multi-model forecast skill assessment of water vapor transport associated with AR over the western United States at sub-seasonal time scale. The data used to assess the forecast skill in this study are from three operational forecast centers, the National Centers for Environmental Prediction (NCEP), the Environment and Climate Change Canada (ECCC), and the European Centre for Medium-Range Weather Forecasts (ECMWF), which contribute to the S2S Prediction Project database. With the focus on the sub-seasonal time scale, the forecasts of weekly total integrated water vapor transport (IVT) and IVT associated with AR at week-1 (1-7 days lead) out to week-4 (22-28 days lead) are compared with the corresponding model climatology to obtain the weekly mean anomaly forecasts. These weekly anomaly forecasts for week-1 to week-4 are evaluated with different reanalysis datasets, and are compared to the climatology forecast that serves as a benchmark. The impacts of large-scale circulation background (such as ENSO and MJO) on the forecast skill are also explored. In addition, an experimental near real-time forecast product for AR activity over the western United States at S2S time scale will be introduced. This project is a research collaboration between the NASA Jet Propulsion Laboratory (JPL) and the Center for Western Weather and Water Extremes (CW3E) at University of California San Diego (UCSD), with support from the California Department of Water Resources, and is one of the S2S pilot application projects.