How do Secondary Frontal Waves Associated with Landfalling Atmospheric Rivers Lead to Forecast Challenges and can Precursor Synoptic Conditions Lead to Early Detection?

Atmospheric Rivers (AR) generate both socioeconomic hazards and benefits. In Northern California, they provide approximately 50% of annual precipitation but are responsible for 90% of damaging floods in some watersheds. Atmospheric Rivers are often associated with strong polar fronts and can make landfall during largescale extratropical patterns that favor development of additional baroclinic storm systems. When secondary waves develop along the existing frontal zone associated with a landfalling AR, significant uncertainty in the forecast of AR duration and strength can result. In this study, we identify 2 dozen moderate to strong AR that impacted California’s Russian River Watershed in the last decade using the Bodega Bay Atmospheric River Observatory. We separate events into those on which a secondary wave develops during the period of landfall and those that do not. From these events, we use atmospheric reanalyses and in-situ observations to identify synoptic and dynamic factors that are necessary precursors to secondary wave development. We also use official quantitative precipitation forecasts and estimates from the California Nevada River Forecast Center to assess forecast skill in both event scenarios. The above methods are used to answer the questions “Is the development of a secondary wave on a landfalling AR associated with a less skillful forecast?”; and “Can large scale patterns be used to aid early detection of a secondary development scenario?”