Abstract 257

Abstract ID: 257

The transition from California’s extreme drought to major flooding: Evaluating CW3E’s S2S forecasts of the onslaught of landfalling atmospheric rivers and associated extreme precipitation in December 2022 – January 2023

Lead Author: Mike DeFlorio
Center for Western Weather and Water Extremes, Scripps Institution of Oceanography, University of California San Diego, United States of America

Keywords: Atmospheric rivers, S2S, Extreme precipitation, Predictability, Forecast synthesis products

Abstract: The western United States, and in particular California, experienced a historic run of nine consecutive ARs in three weeks’ time during the December 2022 – January 2023 period. Following three years of extreme and exceptional drought across the region from 2020-2022, intense ARs that made landfall across California in late December 2022 and early January 2023 were responsible for bringing reservoirs back to historical averages, as well as producing damaging floods and debris flows. Throughout this period, the Center for Western Weather and Water Extremes (CW3E) and collaborating institutions developed an expansive suite of peer-reviewed experimental subseasonal-to-seasonal (S2S) prediction tools for ARs, circulation regimes, and precipitation anomalies across the western U.S. region. In this study, we examine experimental seasonal forecasts of precipitation over the western U.S. issued by various institutions and agencies at the beginning of Water Year 2022-2023, and discuss potential sources of error and uncertainty in their underlying prediction systems. We then examine the remarkable shift during the December 2022 – January 2023 period from widespread ridging/dry conditions across the North Pacific and western U.S. to persistent troughing and record wetness dominated by the presence of ARs. For both the experimental subseasonal and seasonal forecasts, we introduce new forecast synthesis products developed at CW3E to aid in the interpretation of S2S forecast information across different methods, predictands, and lead times. These S2S forecast synthesis products are included regularly in CW3E S2S outlooks and can provide situational awareness guidance to western U.S. water managers and other applied end users across the region.

Agniv Sengupta (CW3E)
Christopher M. Castellano (CW3E)
Jiabao Wang (CW3E)
Zhenhai Zhang (CW3E)
Alexander Gershunov (CW3E)
Kristen Guirguis (CW3E)
Rosa Luna Nino (CW3E)
Rachel Clemesha (CW3E)
Ming Pan (CW3E)
Mu Xiao (CW3E)
Brian Kawzenuk (CW3E)
Peter B. Gibson (NIWA)
William Scheftic (U. Arizona)
Patrick D. Broxton (U. Arizona)
Julie Kalansky (CW3E)
F. Martin Ralph (CW3E)
Luca Delle Monache (CW3E)
Duane E. Waliser (NASA JPL/CalTech)
Andrew W. Robertson (IRI)
Xubin Zeng (U. Arizona)
Jeanine Jones (DWR)
Michael L. Anderson (DWR)