Seasonality in the Predictive Skill of Southwest United States Precipitation in the Seasonal Forecast Systems

Abstract We examine seasonality in the skill of 8 seasonal forecast systems to predict the Southwest United States (SWUS) precipitation between October and April. The models share similarities in the large seasonality they exhibit in their prediction skill over the 1993–2016 period: from October to December, the skill is low if not non‐existent, before it gets higher in all the models, with a peak in February‐March in the model with an extended hindcast period (European Centre for Medium‐Range Weather Forecasts, ECMWF, 1981–2024 period). Using ECMWF, further analyses of daily precipitation illustrate how the model is able to better predict the SWUS precipitation over the February‐March forecast target window, versus November‐December. Higher precipitation in early February compared to early November is a contributing factor, as is the annual cycle in tropical eastern Pacific sea surface temperature, and the associated El Niño Southern Oscillation teleconnection with the extratropics.