Winter Precipitation Predictions in the Contiguous United States: Challenges for Two Seasonal Forecast Models, SEAS5 and CFSv2

Abstract Significant dynamical control of three seasonal-mean large-scale circulation modes, known as the Pacific–North American (PNA), West, and South modes, on winter precipitation over the contiguous United States (CONUS), through regulating atmospheric river (AR) frequency, is evident in reanalysis. The PNA mode affects changes in precipitation and the frequency of ARs over the Pacific Northwest, Midwest United States, and Florida, while the West and South modes influence the western and southern United States, respectively. The capability of operational seasonal forecast models to replicate these modes remains unclear, and evaluating their skill in this specific aspect is our primary goal. To achieve this, two operational model ensembles, the European Centre for Medium-Range Weather Forecasts’ fifth generation seasonal forecast system (SEAS5) and the National Centers for Environmental Prediction’s Climate Forecast System, version 2 (CFSv2), are assessed to gauge their overall skill in forecasting the observed compound effects of large-scale circulation and ARs on CONUS winter precipitation. We find that the two models, initialized on 1 December, are able to capture some spatial features of the observed modes during December–February (DJF). However, they fail to reproduce their temporal variations, especially those associated with the West mode, because in reality, it is directly driven by barotropic instability related to the subtropical jet stream over the North Pacific. This inherent limitation, arising from the West mode’s insensitivity to slowly varying sea surface temperature changes, constrains models’ skill in DJF precipitation forecasts over the western United States. Significance Statement We evaluate the performance of two seasonal forecast systems in predicting the leading three large-scale circulation modes across the contiguous United States, given their crucial role in shaping winter precipitation variations by modulating atmospheric river activity. The West mode significantly impacts winter precipitation variability over the western United States (U.S.), while both models struggle to accurately forecast its observed variations and thus perform poorly in predicting western U.S. precipitation. This difficulty results from the internal origin of the West mode. These factors indicate that western U.S. precipitation may be highly unpredictable in the setting of current seasonal forecasts, reflecting both the inherently limited predictability of precipitation over the region and the models’ biases in simulating the internal characteristics of the West mode.