DeFlorio, M. J., A. Sengupta, C. M. Castellano, J. Wang, Z.
Zhang, A. Gershunov, K. Guirguis, R. Luna Nino, R. E. S. 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, sub judice.
California experienced a historic run of nine consecutive landfalling atmospheric rivers (ARs) in
three weeks. time during winter 2022-2023. Following three years of drought from 2020-2022,
intense landfalling ARs across California in December 2022 . January 2023 were responsible for
bringing reservoirs back to historical averages and producing damaging floods and debris flows.
In recent years, the Center for Western Weather and Water Extremes and collaborating
institutions have developed and routinely provided to end users peer-reviewed experimental
seasonal (1-6 month lead time) and subseasonal (2-6 week lead time) prediction tools for western
U.S. ARs, circulation regimes, and precipitation. Here, we evaluate the performance of
experimental seasonal precipitation forecasts for winter 2022-2023, along with experimental
subseasonal AR activity and circulation forecasts during the December 2022 regime shift from
ridging/dry conditions to persistent troughing and record AR-driven wetness over the western
U.S. Experimental seasonal precipitation forecasts were too dry across Southern California
(likely due to their overreliance on La Niń and the observed above normal precipitation across
Northern/Central California was underpredicted. However, experimental subseasonal forecasts
skillfully captured the regime shift from dry to wet conditions in late December 2022 at 2-3 week
lead time. During this time, an active MJO shift from phases 4&5 to 6&7 occurred, which
historically tilts the odds towards increased troughing/AR activity over California. New
experimental seasonal and subseasonal synthesis forecast products, designed to aggregate
information across institutions/methods, are introduced in the context of this historic winter to
provide situational awareness guidance to western U.S. water managers.