On the Multiscale contributors to quantitative precipitation forecast uncertainty in the US West Coast

Wintertime heavy precipitation and flooding events along the US West Coast are associated with intense onshore water vapor transport by atmospheric rivers (ARs). Although it has been widely recognized that the uncertainty in AR forecasts is a major contributor to the uncertainty in quantitative precipitation forecast (QPF) along the US West Coast, from the perspective of the atmospheric general circulation, there are multiscale contributors to the QPF uncertainty, depending on the forecast lead time and the forecast model domain size. It remains a major forecasting and risk-management challenge to understand and quantify the multiscale interactions between uncertainties in forecasts of the upper-level jet in the North Pacific and the genesis and evolution of extratropical cyclones, whose AR-induced moisture transport directly contributes to heavy precipitation events. In this presentation, we leverage an ongoing effort to evaluate NOAA's newly developed AR forecast system to untangle the interactions among the multiscale controllers that contribute to the QPF uncertainty along the US West Coast. We will show using precipitation and atmospheric analysis datasets that the QPF uncertainty along the US West Coast, dependent on the forecast lead time and the model's forecast domain size, can be linked to the uncertainties in the forecasts of convective activities in the tropical Pacific, the interaction between tropical convection and upper-level jet in the North Pacific, and the genesis and evolution of extratropical cyclones associated with the upper-level jet.