CW3E AR Update: 17 December 2025 Outlook

December 17, 2025

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Additional Atmospheric River Activity Forecast over the Pacific Northwest and Northern California

  • Another strong atmospheric river (AR) is forecast to move onshore over the Pacific Northwest on early on Thu 18 Dec, with southwesterly IVT >900 kg m-1 s-1 forecast in the core of the AR, shifting southward along the US West Coast and bringing AR conditions to Northern California early on Fri 19 Nov.
  • A second, weaker AR is forecast to move onshore over Northern California late on Sat 20 Dec with southwesterly IVT >600 kg m-1 s-1 forecast in the core of the AR, with AR conditions eventually dissipating early on Mon 22 Dec.
  • CW3E’s West-WRF ensemble control member is forecasting AR3/AR4 conditions along coastal Washington and Oregon with the first AR and AR2/AR3 conditions along coastal Oregon and Northern California with the second AR.
  • CW3E’s GEFS and ECMWF ensemble AR Landfall tools are also highlighting the potential for additional AR activity over the US West Coast, with high confidence (~70–90% probability) in AR conditions over California beginning Wed 24 Dec and moderate confidence (~50–70% probability) over the Pacific Northwest beginning Sun 28 Dec although some forecast uncertainty remains.
  • The NWS Weather Prediction Center (WPC) is forecasting 72-hour precipitation amounts of 7–10 in. over the southern Cascades and Coast Ranges in WA, OR, and N. CA with the first AR (period ending 4 PM Sat 20 Dec), with an additional 5–7 in. forecast over the Coast Ranges in N. CA and the northern Sierra Nevada with the second AR (period ending 4 PM Mon 22 Dec).
  • WPC has also issued marginal risk (level 1 of 4; ≥5% probability of flash flooding) and slight risk (level 2 of 4; ≥15% probability) excessive rainfall outlooks (EROs) over western WA, OR, and Northern CA with the first AR (Thu 18 Dec–Sat 20 Dec) and over the Northern California Coast Ranges and northern Sierra Nevada with the second AR (Sat 20 Dec–Mon 22 Dec).
  • The NWS Northwest River Forecast Center (NWRFC) is forecasting significant stream level rises in Oregon with the first AR, with 17 stream gages forecast to rise above flood stage, 3 gages above moderate flood, and 2 gages to reach major flood stage.
  • Freezing levels will start relatively low over the Pacific Northwest, and then rise significantly with the first AR on 18 Dec. Over the Sierra Nevada, freezing levels are forecast to remain above 7,000 feet during the second AR, resulting in primarily rainfall.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 17 December 2025 – 1200 UTC 22 December 2025

Summary provided by S. Bartlett, C. Castellano, M. Steen and B. Moore; 17 December 2025

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For any unfamiliar terms, please refer to the American Meteorological Society Glossary.