From California’s Extreme Drought to Major Flooding: Evaluating and Synthesizing Experimental Seasonal and Subseasonal Forecasts of Landfalling Atmospheric Rivers and Extreme Precipitation during Winter 2022 – 2023

October 24, 2023

CW3E researcher Mike DeFlorio, along with co-authors from CW3E, the National Institute of Water and Atmospheric Research (NIWA), the University of Arizona, the University of Graz, the International Research Institute for Climate and Society (IRI), the NASA Jet Propulsion Laboratory, the NOAA Climate Prediction Center, and the California Department of Water Resources (CA DWR), recently published a paper entitled “From California’s Extreme Drought to Major Flooding: Evaluating and Synthesizing Experimental Seasonal and Subseasonal Forecasts of Landfalling Atmospheric Rivers and Extreme Precipitation during Winter 2022 – 2023” in the Bulletin of the American Meteorological Society (BAMS). This research contributes to both the S2S (Subseasonal-to-Seasonal) Prediction of Extreme Weather and Atmospheric Rivers (AR) Research and Applications Priority Areas in CW3E’s 2019-2024 Strategic Plan, and represents a substantial cross-institutional collaborative effort to diagnose observations and evaluate S2S forecasts of ARs, circulation regimes, and extreme precipitation during last year’s historic winter over California and the broader western U.S. region.

The key objectives of this paper are to:

1. Diagnose observations before and after the 3-week active AR period over California during December 2022 – January 2023, and provide context for how AR landfalls during this period substantially alleviated multi-year drought over California
2. Evaluate experimental seasonal and subseasonal forecasts of ARs, circulation regimes, and precipitation over the western U.S. and North Pacific Ocean during Winter 2022 – 2023
3. Introduce experimental western U.S. seasonal and subseasonal forecast synthesis products that summarize and aggregate key information across institutions and methods to provide situational awareness guidance to end users

Figure 1 below (Figure 5 from the BAMS article) illustrates the substantial impact of nine landfalling ARs during December 2022 – January 2023 in alleviating multi-year drought conditions across California and Nevada. During the October 2019 – November 2022 period when drought was still widespread, the equivalent of 1-2 normal water years’ worth of precipitation did not fall across much of Northern and Central California, and Southern California was also in a precipitation deficit. However, by the end
of January 2023, many parts of Central California (including the Sierra Mountain Range), as well as coastal Southern California, were no longer in deficit mode, as indicated by the white regions in panel c). Conditions improved dramatically across Northern California, though many areas remain in deficit compared to normal conditions for the 3.5 year period shown in panel c).

Figure 1: Figure 5 from DeFlorio et al. 2023 (BAMS). Deviations from 1981-2010 normal of precipitation accumulated during (a) October 2019 (beginning of recent drought) through November 2022, (b) total precipitation between December 2022 and January 2023, and (c) between October 2019 and January 2023, with missing or extra overall precipitation expressed in terms of normal water-years. White areas in panel (c) are regions where the net 3.5 years of precipitation anomalies were no longer in deficit mode by the end of January 2023. Precipitation amounts illustrated here are the PRISM 4-km resolution monthly datasets.

During the past several winters, CW3E has been developing experimental seasonal and subseasonal forecast synthesis products in close coordination with stakeholders at CA DWR. An example of the CW3E experimental subseasonal forecast synthesis product applied to the period of interest in this study is shown below in Figure 2 (Figure 12 from the BAMS article). These experimental synthesis products will be included in CW3E’s weekly S2S outlooks during the Winter 2023-2024 season.

Figure 2: Figure 12 from DeFlorio et al. 2023 (BAMS). CW3E experimental subseasonal (weeks 2-4 lead time) synthesis forecast product for December 22, 2022 00Z subseasonal dynamical ensemble forecasts. The regions include Washington/Oregon (WA/OR), Northern California, Central California, and Southern California (each row). Forecast results from three models (NCEP, ECCC, ECMWF) are shown respectively in each column. The superscripts indicate the different types of subseasonal products being considered in the synthesized forecasts. High confidence is determined when there is a ≥75% probability of a pattern conducive to above normal, below normal, or near normal conditions, and if the majority (>50%) of the forecast products agree on the sign of the anomaly. Low confidence is determined when there is a <75% probability of a pattern conducive to above normal, below normal, and near normal conditions, and >50% of the forecast products agree on the sign of the anomaly. If the individual forecast products disagree on the sign of the anomaly, the synthesized forecast is classified as uncertain.

DeFlorio, M.J., A. Sengupta, C.M. Castellano, J. Wang, Z. Zhang, A. Gershunov, K. Guirguis, R. Luna Niño, R.E. Clemesha, M. Pan, M. Xiao, B. Kawzenuk, P.B. Gibson, W. Scheftic, P.D. Broxton, M.B. Switanek, J. Yuan, M.D. Dettinger, C.W. Hecht, D.R. Cayan, B.D. Cornuelle, A.J. Miller, J. Kalansky, L. Delle Monache, F.M. Ralph, D.E. Waliser, A.W. Robertson, X. Zeng, D.G. DeWitt, J. Jones, and M.L. Anderson (2023), From California’s extreme drought to major flooding: Evaluating and synthesizing experimental seasonal and subseasonal forecasts of landfalling atmospheric rivers and extreme precipitation during Winter 2022 – 2023. Bulletin of the American Meteorological Society, in press, doi:10.1175/BAMS-D-22-0208.1.