Representation of East Pacific Easterly Waves in Subseasonal‐To‐Seasonal Forecasts of the Unified Forecast System

Abstract East Pacific easterly waves (EPEWs) are critical precursors to tropical cyclogenesis and regional rainfall yet are poorly captured in subseasonal‐to‐seasonal (S2S) forecast models. This study evaluates how EPEWs are simulated in NOAA's Unified Forecast System (UFS) Prototypes 5–8 during May‐October relative to ERA5 reanalysis, using wave‐tracking algorithm and process‐based diagnostics to identify biases that increase with forecast lead time. UFS Prototypes exhibit strong differences in EPEW activity and moisture‐vorticity coupling strength, with all Prototypes except for Prototype 7 exhibiting excessive EW activity relative to reanalysis that becomes more vigorous with forecast lead from weeks 1–2 to weeks 3–5. Variations in the background thermodynamic environment are a dominant driver of differences in EW activity across Prototypes, with the strength of meridional moisture gradients showing the strongest correlation with EPEW track density. In particular, Prototype 7 has weaker mean moisture gradients and EW activity compared to reanalysis. Composite diagnostics reveal EPEW convective structures evolve from relatively bottom‐heavy to more top‐heavy with increasing lead time, accompanied by accelerated convective adjustment timescales and exaggerated moisture‐vortex instability, with overall biases being most exaggerated for Prototypes 5, 6, and 8. The vorticity maintenance budget shows excessive horizontal advection and vortex stretching contributions across all Prototypes, while the moist static energy maintenance budget reveals an excessively strong column process associated with excessive longwave radiative heating and surface latent heat flux contributions. However, these maintenance budget terms are generally more muted for Prototype 7. These findings highlight the critical need for improved parameterizations in S2S prediction systems.