Flood — Snohomish, Washington

2021-11-12 to 2021-11-16 · near Cedarhome, Snohomish, Washington

Event narrative

Heavy rain and snow melt pushed a number of rivers above flood stage. Moderate flooding occurred on the Snohomish and Skykomish Rivers, with minor flooding on the Stillaguamish. Several pulses of heavy rain from the 3 atmospheric rivers sent the rivers above flood levels multiple times during the episode. Urban and small stream flooding also occurred.

Wider weather episode

An upper level trough Gulf Low continued off the west coast that brought a series of atmospheric rivers resulting in flooding across parts of western Washington. This second atmospheric river episode of the month was a set of three atmospheric rivers back-to-back-to-back. Water vapor transport into Western Washington averaged 160 - 200% of normal during the period of 11 November - 16 November. Heavy rainfall spread across the region on the night of the 11th through November 12, with the heaviest rain starting across Southwest Washington and slowly shifting to the north through the day. Rainfall totals ranged from 5 to 10 inches in spots leading to areal and river flooding, including major flooding.

Heavy, persistent rainfall associated with the Atmospheric River shifted northward across Western Washington on November 13 before stalling over northwest Washington November 14 through 16. Another 5 to 10 inches of rainfall fell across the northern half of western Washington. The snow level rose to 8000 to 10,000 feet, resulting in some snow melt contributing to runoff. Widespread minor or greater flooding occurred. Major flooding occurred on the Bogachiel, the three forecast points on the Nooksack, Skagit, and Cowlitz Rivers. Moderate flooding occurred on the Elwha, Skokomish, Skykomish, Snohomish, Snoqualmie, Puyallup, Nisqually, Newaukum Rivers and South Prairie Creek. Preliminary record flows occurred on Nooksack, Bogachiel, Calawah Rivers and Olsen Creek. Hundreds of water rescues and two fatalities took place during this time period. The heavy rainfall also triggered numerous landslides due to loading on the already-saturated soils.

Much of the reported damage amounts were for both episodes in November and some of the amounts for this episode were an estimated proportion for the month.

View location on OpenStreetMap → (48.3141, -122.3222)

Source: NOAA Storm Events Database, event_id 994114. Narrative written by the NWS forecast office that issued the report.