CW3E AR Update: 12 December 2025 Outlook

December 12, 2025

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Atmospheric River Forecast to Bring Precipitation to Pacific Northwest and Northern California

  • A strong atmospheric river (AR) is forecast to develop over the N. Pacific and merge with an area of remnant moisture from the prior AR that is currently dissipating, eventually moving onshore over British Columbia & the Pacific Northwest on Sun 14 Dec.
  • This AR is forecast to bring another long-duration period (>48 hrs) of AR conditions to the Pacific Northwest with strong southwesterly IVT >750 kg m-1 s-1 forecast over the region between Mon 15 Dec–Tue 16 Dec. The AR is forecast to then shift south along the US West Coast and bring AR conditions to Northern California between Tue 16 Dec–Wed 17 Dec.
  • There are still model-to-model uncertainties during the first AR, with the ECMWF ensemble forecasting a brief break in AR conditions on 16 Dec, while the GEFS keep continuous AR conditions in coastal Oregon.
  • The GEFS and ECMWF ensemble control members are forecasting AR3/AR4 conditions along coastal Washington & Oregon and AR2/AR3 conditions in coastal northern California, but there is still uncertainty in the forecast AR duration and intensity.
  • The GEFS (50–70% prob.) and ECMWF (70–90% prob.) ensembles are both forecasting high probabilities for AR conditions over coastal Oregon and Northern California for the period beginning on 18 Dec, although some forecast uncertainty remains.
  • The NWS Weather Prediction Center (WPC) is forecasting 72-hour precipitation totals of 3–6 in. from this AR along the Coast Ranges in WA, OR, & N. CA, and in the Cascades and northern Sierra Nevada for the period ending at 10 AM on Wed 17 Dec.
  • The NWS WPC has issued a marginal risk (level 1 of 4; ≥5% probability of flash flooding) excessive rainfall outlook (ERO) over northern WA for Sun 14 Dec–Mon 15 Dec, marginal risk and slight risk(level 2 of 4; ≥15% probability) EROs over WA, OR, and far northern CA for Mon 15 Dec–Tue 16 Dec, and a marginal risk ERO over WA, OR, and CA for Tue 16 Dec–Wed 17 Dec.
  • The NWS Northwest River Forecast Center (NWRFC) is forecasting stream levels to rise in western Washington and Oregon beginning on Mon 15 Dec and continuing for multiple days with this next round of AR activity.
  • Freezing levels are forecast to begin relatively high over the Pacific Northwest, eventually lowering as the AR moves through, creating the potential for significant snowfall along terrain the Cascades after Tue 16 Dec.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 12 December 2025 – 1200 UTC 18 December 2025

Summary provided by S. Bartlett, C. Castellano, S. Roj, M. Steen, and J. Kalansky; 12 December 2025

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*Outlook products are considered experimental

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