Author: Chris Castellano

CW3E Event Summary: 12-17 March 2025

20 March 2025

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Multiple Atmospheric Rivers Produce Heavy Rain, Heavy Snow, and Flooding in California and Oregon

The ARs:

• The first atmospheric river (AR) made landfall over Northern California on 12 Mar and quickly propagated down the coast, bringing a brief period of weak AR conditions to much of coastal California.
• A second and stronger AR made landfall over southern Oregon on 15 Mar before eventually drifting southward and dissipating over Central California.
• An AR 2 (based on the Ralph et al. 2019 AR Scale) was observed in coastal Oregon and far Northern California in association with the second AR.

Impacts:

• The first AR produced widespread rain over coastal California and heavy snow in the Sierra Nevada and San Bernardino Mountains.
• The second AR produced heavy rain in southwestern Oregon and far Northern California, as well as heavy snow in the Oregon Cascades.
• Total rainfall from these storms exceeded 10 inches near the Oregon/California border. Total snowfall exceeded 48 inches in portions of the Oregon Cascades and Sierra Nevada.
• Rain from the first AR caused roadway flooding and landslides in California.
• Heavy rain falling on saturated soils during the second AR caused severe flooding in southwestern Oregon.

Click images to see loops of GFS IVT/IWV analyses Valid 0000 UTC 12 March – 0000 UTC 18 March 2025

Summary provided by C. Castellano, S. Bartlett, S. Roj, and M. Steen; 20 March 2025

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CW3E Event Summary: 22-25 February 2025

4 March 2025

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Multiple Atmospheric Rivers Produce Heavy Rain and Flooding in the Pacific Northwest

The ARs:

• The first AR made landfall on Sat 22 Feb and produced AR 2–3 conditions (based on the Ralph et al. 2019 AR Scale) in southern coastal Washington and coastal Oregon.
• Significant inland penetration of the first AR also brought AR 2 conditions to interior portions of the northwestern US.
• The second AR made landfall on Mon 24 Feb in association with a strong low-pressure system and produced AR 1–2 conditions in coastal Washington and Oregon.

Impacts:

• At least 5–10 inches of total precipitation fell over much of coastal Washington and Oregon as well as the Cascades.
• Unusually warm temperatures and high freezing levels created favorable conditions for snowmelt and rain-on-snow during the first AR in the Cascades and interior portions of the Pacific Northwest.
• The combination of snowmelt and heavy rainfall led to significant riverine flooding in eastern Washington, northern Idaho, and eastern Oregon.
• Strong winds during the second storm caused widespread tree damage and power outages in western Washington and northwestern Oregon.

Click images to see loops of GFS IVT/IWV analyses Valid 1200 UTC 21 February – 1200 UTC 25 February 2025

Summary provided by C. Castellano, S. Roj, and J. Kalansky; 4 March 2025

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CW3E Event Summary: 31 January – 5 February 2025

12 February 2025

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Long-Duration Atmospheric River Produces Heavy Precipitation and Flooding in Northern California

The ARs:

• A weak atmospheric river (AR) associated with a tropical moisture export (TME) made landfall over Washington, Oregon, and Northern California late 30 Jan.
• A second AR associated with a much stronger TME made landfall early 1 Feb and stalled over Northern California for several days before finally dissipating early 5 Feb.
• An AR 4 (based on the Ralph et al. 2019 AR Scale) was observed in coastal Northern California, with some locations experiencing > 100 hours of continuous AR conditions.
• An AR 3 was also observed in northeastern California and northwestern Nevada due to inland penetration of the second AR.
• CW3E’s AR Reconnaissance field campaign carried out eight IOPs that sampled both ARs, as well as nearby essential atmospheric structures and regions of forecast sensitivity.

Impacts:

• At least 6–15 inches of total precipitation fell over the Northern California Coast Ranges, Klamath Mountains, southern Cascades, and Northern Sierra Nevada. A few stations recorded > 20 inches of precipitation.
• While much of the precipitation in California fell as rain due to high freezing levels, these ARs produced an estimated 2–6 feet of snow in the highest elevations of the Klamath Mountains, southern Cascades, Northern Sierra Nevada, and Rocky Mountains in Central Idaho.
• Heavy rainfall over multiple days caused riverine flooding in Northern California. The Pit River near Canby exceeded major flood stage.
• The National Weather Service received numerous reports of landslides, debris flows, and flooding in Northern California.
• Heavy snow and strong winds caused hazardous travel conditions in the higher terrain.

Click images to see loops of GFS IVT/IWV analyses Valid 1200 UTC 29 January – 1200 UTC 5 February 2025

Summary provided by C. Castellano, S. Bartlett, L. DeHaan, S. Roj, M. Steen, S. Yoon, and J. Kalansky; 12 February 2025

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