CW3E Event Summary: 18–27 December 2025

13 January 2026

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Multiple Atmospheric Rivers Produce Heavy Rain, Significant Snowfall, and Widespread Flooding

The ARs:

• A strong atmospheric river (AR) made landfall over the Pacific Northwest on 17 Dec and shifted southward along the coast of Oregon and Northern California through early 20 Dec.
• A second AR developed on 20 Dec in a similar region along a corridor of strengthening moisture transport, bringing AR conditions to Northern and Central California through 22 Dec.
• The third AR in this sequence developed on 23 Dec along a region of enhanced moisture transport that developed near a deepening low pressure, bringing impacts to California through 24 Dec.
• As the third AR was weakening over land on 25 Dec, a fourth storm developed with a mid-level trough and a closed low offshore of California, bringing AR conditions, forcing for precipitation, and low-level upslope moisture flux to mountainous terrain through 27 Dec.

Impacts:

• Total 10-day estimated precipitation amounts exceeded 15 inches in the southern Cascades, northern Sierra Nevada, and western Transverse Ranges from these ARs, with large portions of California received >20% of normal water year precipitation in these 10 days.
• Total 10-day estimated snowfall amounts exceeded 48 inches in the higher terrain of the Sierra Nevada and 72 inches along the Sierra crest, with the heaviest snow occurring during the third and fourth storms. In the Sierra Nevada, snowpack increased from ~7–30% of normal to 57–100% of normal between 22 Dec and 27 Dec.
• The National Weather Service received numerous reports of flooding and landslides in Oregon and California, with the NWS Los Angeles also confirming an EF-0 touched down in central LA on 25 Dec.
• Tragically at least four fatalities were reported in association with this active weather period; two related to flooding, one due to a falling tree, and another due to an avalanche following heavy snow.

Summary provided by S. Bartlett, C. Castellano, B. Moore, S. Roj, M. Steen, M. Warner, C. Hecht, B. Kawzenuk, A. Wilson, J. Cordeira, and J. Kalansky; 13 January 2026

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