Impacts of tropical forecast errors on weeks 3–4 extreme precipitation predictions over California during winter 2022–23.

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Bibliographic Details
Title:	Impacts of tropical forecast errors on weeks 3–4 extreme precipitation predictions over California during winter 2022–23.
Authors:	Moore, Benjamin J.¹ (AUTHOR) bem006@ucsd.edu, Dias, Juliana¹ (AUTHOR), Hoell, Andrew¹ (AUTHOR), Tulich, Stefan^1,2 (AUTHOR), Gehne, Maria^1,2 (AUTHOR), Albers, John¹ (AUTHOR), Baggett, Cory³ (AUTHOR), LaJoie, Emerson³ (AUTHOR)
Source:	Monthly Weather Review. Jun2026, Vol. 154 Issue 6, p1-23. 23p.
Subjects:	Long-range weather forecasting, Atmospheric rivers, Rossby waves, Rainstorms, Ensemble learning
Geographic Terms:	California, Pacific Ocean, Pacific Coast (U.S.), North Pacific Ocean
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 weeks 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, 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 non-nudged "control" forecast, nudging results in especially large improvements in the weeks 3–4 California precipitation forecast as well as 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, 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 West Coast on the eastern flank of the blocking ridge. [ABSTRACT FROM AUTHOR]
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Database:	Engineering Source

Description
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 weeks 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, 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 non-nudged "control" forecast, nudging results in especially large improvements in the weeks 3–4 California precipitation forecast as well as 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, 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 West Coast on the eastern flank of the blocking ridge. [ABSTRACT FROM AUTHOR]
ISSN:	00270644
DOI:	10.1175/MWR-D-25-0133.1