CW3E Event Summary: 10-14 December 2021

December 16, 2021

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Atmospheric River Brings Heavy Rain and Snow to the Western US

  • An atmospheric river (AR) made landfall over the Pacific Northwest on 10 Dec and gradually moved south along the US West Coast
  • When the AR reached the San Francisco Bay Area, it temporarily stalled and began to re-intensify as it interacted with an upper-level trough to the west
  • AR conditions persisted for at least 48 hours over the Bay Area and the foothills of the Northern Sierra Nevada, resulting in an AR 2/AR 3 (based on the Ralph et al. 2019 AR Scale)
  • AR 1 conditions were observed in coastal Southern California
  • More than 5 inches of storm-total precipitation fell across the Pacific Coast Ranges, the Cascades, the Sierra Nevada, and the Southern California Transverse Ranges
  • Some locations in the California Coast Ranges reported more than 10 inches of precipitation
  • Several feet of snow fell in the Olympic Mountains, the Cascades, the Northern Rockies, and the Sierra Nevada
  • Low freezing levels supported significant snowfall accumulations below 6,000 feet in the Sierra Nevada as well as in the Southern Oregon and Northern California Coast Ranges
  • Intense rainfall on 13–14 Dec caused flooding and slides in the Bay Area and in Southern California

Click images to see loops of GFS IVT/IWV analyses

Valid 0000 UTC 10 December – 0000 UTC 15 December 2021










Summary provided by Chris Castellano, J. Kalansky, Shawn Roj, F.M. Ralph; 16 December 2021

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