CW3E Event Summary: 30 November-6 December 2023

11 December 2023

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Atmospheric Rivers Produce Heavy Precipitation and Flooding in Washington and Oregon

• A series of low-pressure systems and three atmospheric rivers (ARs) brought heavy snow and rain to much of the Pacific Northwest during 30 Nov – 6 Dec.
• While the first AR was associated with a cold system, the second and third ARs were characterized by the transport of very warm, moist air from the subtropical North Pacific into the midlatitudes.
• The third and strongest AR produced AR3/AR4 conditions (based on the Ralph et al. 2019 AR Scale) over coastal WA/OR, as well as AR2 conditions in portions of the interior Pacific Northwest.
• These storms produced 10–20 inches of total precipitation in the Olympic Mountains, Cascades, and OR Coast Ranges, as well as 4–8 inches of precipitation in the lowlands of western WA and OR.
• The cold storms (including the first AR) produced 1–3 feet of snow in the Cascades during 30 Nov – 2 Dec.
• Snow levels remained below 4,000 feet through the first AR, then rose above 7,000 feet during the second and third ARs, resulting in significant rain-on-snow.
• High reservoir inflows prompted dam operators to increase releases to ~3,000 cfs at Howard Hanson Dam after the third AR.
• Heavy rain falling on moist soils and fresh snowpack during the third AR led to widespread riverine flooding across western Washington and northwestern Oregon.
• The Skagit and Stillaguamish Rivers reached major flood stage on 5 Dec.
• Significant flooding was observed in multiple locations, requiring road closures, sandbagging, and multiple water rescues – to include a helicopter rescue by the US Coast Guard due to rapidly rising waters near Rosburg, WA.
• There were also numerous landslides and debris flows in Washington and Oregon, requiring the shutdown of various roadways, including a post-fire debris flow within the boundaries of the 2020 Holiday Farm Fire in Oregon.

Click images to see loops of GFS IVT/IWV analyses Valid: 1200 UTC 2 December – 1200 UTC 6 December 2023

Summary provided by C. Castellano, S. Bartlett, P. Iñiguez, and J. Kalansky; 11 Dec 2023

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