CW3E Event Summary: 10-16 November 2021

November 17, 2021

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Atmospheric Rivers Produce Heavy Rainfall, Flooding, and Landslides in the Pacific Northwest

• Multiple strong atmospheric rivers (ARs) impacted the Pacific Northwest between 10 Nov and 15 Nov
• The first AR produced AR 3/AR 4 conditions (based on the Ralph et al. 2019 AR Scale) in coastal Oregon and southern coastal Washington
• The second AR produced AR 4 conditions in coastal Washington and northern coastal Oregon
• The intensification of a mesoscale frontal wave (MFW) led to a secondary pulse in moisture transport that prolonged the duration of the 2nd AR and brought borderline AR 5 conditions (max IVT > 1000 kg m⁻¹ s⁻¹; AR duration > 48 hours) to Tillamook County, OR
• Parts of the Olympic Peninsula and Washington Cascades received more than 15 inches of total precipitation from these ARs
• Heavy rain falling on moist soils led to widespread flooding and mudslides, particularly in northern Washington and southern British Columbia following the second AR landfall
• Strong winds and flooding during the second AR also created dangerous travel conditions, downed numerous trees, and caused widespread power outages in western Washington

Click images to see loops of GFS IVT/IWV analyses Valid 0000 UTC 10 November – 0000 UTC 16 November 2021

Summary provided by Chad Hecht, Chris Castellano, Shawn Roj, Brian Kawzenuk, Jay Cordeira, F.M. Ralph; 17 November 2021

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