CW3E AR Update: 29 November 2021 Outlook

November 29, 2021

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Atmospheric Rivers Produce Heavy Rainfall and Flooding in the Pacific Northwest

  • The first two atmospheric rivers (ARs) in a sequence of three ARs impacted the Pacific Northwest between 24 Nov and 29 Nov
  • The first AR produced AR 2 conditions (based on the Ralph et al. 2019 AR Scale) in coastal Oregon and Washington
  • The second AR produced AR 3/AR 4 conditions in northern coastal Oregon and coastal Washington
  • Some locations in the Olympic Peninsula and North Cascades received more than 10 inches of total precipitation from the first two ARs
  • Heavy rain falling on moist soils resulted in flooding and mudslides in northern Washington and southern British Columbia
  • This sequence of storms, which began less than two weeks after a series of destructive storms earlier this month, led Environment and Climate Change Canada to declare a “red alert” for British Columbia





Unsettled Weather to Continue through the End of November in Pacific Northwest

  • The third AR in a sequence of ARs is forecasted to make landfall over British Columbia and Washington today
  • AR 4 conditions are possible across portions of coastal British Columbia, whereas AR 3 conditions are currently forecasted across coastal Washington
  • An additional 2–4 inches of precipitation are forecasted over the Olympic Peninsula and North Cascades during the next 3 days, with higher amounts (> 5 inches) possible in coastal British Columbia
  • The British Columbia River Forecast Center has issued Flood Watches and Flood Warnings ahead of the third AR

Click images to see loops of GFS IVT & IWV forecasts

Valid 1200 UTC 29 November – 0000 UTC 2 December 2021




Summary provided by C. Castellano, C. Hecht, B. Kawzenuk, S. Roj, and F. M. Ralph; 29 November 2021

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