Impacts of Tropical Forecast Errors on Week 3–4 Extreme Precipitation Predictions over California during Winter 2022–23
Journal Article
Overview
abstract
Abstract; This study examines ensemble forecast experiments in which model prognostic variables are nudged toward reanalysis values in the tropics to assess the effects of tropical errors on week 3–4 predictions of two long-lasting extreme precipitation events in California during winter 2022–23. For both events, the first spanning late December to mid-January and the second spanning late February to early March, nudging yields significantly improved predictions for the large-scale flow over the North Pacific; however, the impacts on the California precipitation forecast differ between the events. Comparison of the results for the two events highlights that subseasonal prediction of California precipitation extremes requires accurately forecasting predictable signals linked to tropical forcing, as well as extratropical synoptic-scale dynamics. For the December–January event, which is poorly predicted in the nonnudged “control” forecast, nudging results in especially large improvements in the week 3–4 California precipitation forecast combined with decreases in ensemble spread. These improvements correspond to improved prediction of the onset, persistence, and phasing of the North Pacific wave pattern. This pattern fosters landfall of successive cyclones and atmospheric rivers over California. For the February–March event, nudging yields improved prediction of the amplitude and persistence of a blocking ridge over the eastern Pacific but mixed results for the California precipitation downstream. In this latter event, the ensemble spread in the nudged forecasts remains large, and the precipitation forecast accuracy depends strongly on the representation of midlatitude synoptic-scale Rossby wave breaking near the U.S. West Coast on the eastern flank of the blocking ridge.; ; Significance Statement; We analyze weather model experiments to assess the effects of forecast errors in the tropics (10°S–10°N) on predictions of two extreme precipitation events in California during winter 2022–23. We specifically focus on forecasts generated 3 weeks before the start of each event. Correcting tropical errors produces large improvements in forecasts of the weather patterns associated with each event. However, the impacts of tropical error corrections on the California precipitation forecast differ between the events, reflecting contrasting influences of tropical processes. For the earlier event, the precipitation forecast is improved substantially; for the later event, the forecast is only modestly improved due to errors in midlatitude weather conditions that are not strongly controlled by tropical processes.;
