Experimental Subseasonal Forecasting of Atmospheric Rivers over the Western United States during Winter 2017–18 and 2018–19

There is significant demand in the water resources and operational forecasting applications communities for reliable forecasts of atmospheric rivers (ARs) and their associated hydrological extremes at subseasonal-to-seasonal (S2S) lead times (2 weeks to 2 months in advance). This demand is particularly high over the western United States, where AR frequency and duration project strongly onto extreme floods, Sierra Mountain snowpack amount, and watershed levels. To help meet this demand, an experimental near real-time forecasting framework for AR activity over the western U.S. at 0- to 3-week lead time has been created. Weekly outlooks of AR activity at these lead times are produced from three operational forecast centers: Environment and Climate Change Canada (ECCC), the European Centre for Medium-Range Weather Forecasts (ECMWF), and the National Centers for Environmental Prediction (NCEP). Categorical verification statistics for AR occurrence during Winter 2017-2018 are presented. Caveats regarding this verification are discussed and explored, and a description of the Winter 2018-2019 AR forecasting framework is presented.