CW3E AR Update: 20 October 2021 Outlook

October 20, 2021

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Multiple Atmospheric Rivers to Bring Heavy Precipitation to Northern California

  • A series of landfalling atmospheric rivers (ARs) will impact the western US this week into early next week
  • AR 4/AR 5 conditions (based on the Ralph et al. 2019 AR Scale) are expected in coastal southern Oregon in association with the first and second ARs today through Friday
  • The strongest AR is forecasted to make landfall across Central and Northern California on Sunday, potentially bringing AR 4/AR 5 conditions to the San Francisco Bay Area
  • Inland penetration of this AR may bring AR 2/AR 3 conditions to portions of the interior western US
  • Yet another landfalling AR is forecasted to impact the US West Coast on 26–27 Oct
  • The first two ARs are forecasted to produce 2–5 inches of rainfall in portions of Northern California and southern Oregon
  • The fourth AR is forecasted to bring widespread precipitation to much of the western US, with the heaviest precipitation amounts in Northern California
  • Significant snowfall accumulations are also possible in the Sierra Nevada in association with the fourth AR
  • Portions of Northern California may receive more than 10 inches of total precipitation over the next 7 days

Click images to see loops of GFS IVT & IWV forecasts

Valid 1200 UTC 20 October – 1200 UTC 28 October 2021














Summary provided by C. Castellano, C. Hecht, B. Kawzenuk, J. Kalansky, and F. M. Ralph; 20 October 2021

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