CW3E Event Summary: 26-29 January 2021

February 2, 2021

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An Atmospheric River and a Strong Surface Cyclone Produced Significant Impacts Across much of California

  • The AR made landfall at ~00 UTC 27 January in association with a powerful surface cyclone over the Northeast Pacific Ocean
  • Over the next few days, the AR gradually propagated southward along the coast of California, but not before pivoting and stalling over Central California
  • Some areas in Central California experienced AR conditions for nearly 48 consecutive hours, resulting in an AR 2 (based on the Ralph et al. 2019 AR Scale)
  • A strong low-level jet in the warm sector of the surface cyclone led to the development of a narrow cold-frontal rainband (NCFR)
  • Intense rainfall on recent burn scars caused damaging debris flows in Central and Southern California
  • More than 7 inches of precipitation fell in portions of the Sierra Nevada, Central California Coast Ranges, and western Transverse Ranges, with the highest observed amounts in western Monterey and San Luis Obispo Counties
  • Several feet of snow accumulated across the Sierra Nevada, resulting in closures of major highways

Click images to see loops of GFS IVT/IWV analyses

Valid 0000 UTC 26 January – 0000 UTC 30 January













Summary provided by C. Castellano, J. Cordeira, J. Kalansky, N. Oakley, and F. M. Ralph; 2 February 2021