California Winter Precipitation Predictability: Insights From the Anomalous 2015–2016 and 2016–2017 Seasons

California recently experienced two unusual winter seasons. Following a failed rainy season despite the strong 2015–2016 El Niño that typically brings heavy rains, California unexpectedly experienced one of its wettest winter seasons on record during the 2016–2017 La Niña. The seasonal forecast systems were unable to predict these unusual winter precipitation patterns. We examine the ability of a state‐of‐the‐art forecast system to capture the large‐scale atmospheric circulation patterns that influence the track of storms, which bring a majority of winter precipitation to California. We show that the seasonal forecast systems can reproduce the large‐scale circulation differences in the North Pacific‐American domain, but random atmospheric variability can still easily prevent the accurate prediction of regional scale precipitation in extratropical regions such as California. Further, we identify the role of a natural pattern of large‐scale variability in the atmosphere that affects weather in the middle and high latitudes, referred to as the Arctic Oscillation, in controlling the accuracy of California precipitation forecasts. Prioritizing improvements in the representation of these patterns, the processes by which they are predicted and their influence over other regions in the forecasts systems can help improve seasonal predictions, which is important for the management of California's water resources and infrastructure.