Role of the Tropics in State‐Dependent Improvements of US West Coast NOAA Unified Forecast System Precipitation Forecasts

Boreal-wintertime hindcasts in the Unified Forecast System with tropics nudged toward reanalysis improve United States (US) West Coast precipitation forecasts at Weeks 3–4 lead times when compared to those without nudging. To diagnose origin of these improvements, a multivariate k-means clustering method is used group into subsets by their initial conditions. One cluster characterized an initially strong Aleutian Low demonstrates larger improvements nudging others. The greater for this are related model errors simulating interaction between and teleconnection patterns associated Madden-Julian oscillation (MJO) El Niño-Southern Oscillation (ENSO). Improving tropical intraseasonal precipitation, especially during early MJO phases under non-cold ENSO, may be important producing better US Coast.