CW3E AR Update: 1 December 2023 Outlook

December 1, 2023

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Trio of Atmospheric Rivers Forecast to Impact Pacific Northwest

  • An AR associated with a cold storm will bring a brief period of AR conditions tonight into Saturday
  • Two stronger ARs are forecast to make landfall late Sat 2 Dec and early Mon 4 Dec
  • AR1 conditions (based on Ralph et al. 2019 AR scale) are forecast during the first AR
  • Nearly all GEFS ensemble members are forecasting an AR4 over northern coastal OR due to a prolonged period (> 72 hours) of continuous AR conditions during the second and third ARs
  • A majority of ECMWF EPS members are forecasting a break in AR conditions between the second and third ARs, with AR2/3 conditions likely during the second AR, and AR3/4 conditions likely during the third AR
  • The NWS Weather Prediction Center (WPC) is currently forecasting 7-day precipitation totals ≥ 10 inches in the vicinity of the Olympic Mountains, Cascades, and Coast Ranges
  • The NWS WPC Extreme Rainfall Outlook highlights a Marginal Risk for flooding for western OR/WA Dec 2-6 with a Slight Risk for the Olympic Peninsula on Dec 5
  • Heavy rain, particularly during the second and third ARs, is expected to cause flooding on rivers in western WA and northwestern OR
  • Rain falling on fresh snowpack will likely increase flood risk near the Olympic Mountains and WA Cascades
  • NWS Seattle and Portland are forecasting at least 1-3 feet of snow in the Olympic Mountains and the Cascades through late Saturday evening
  • High snowfall totals in the forecast have led to major to extreme conditions forecast within the NWS Winter Storm Severity Index along the Cascades between 1-3 December

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 01 December 2023 – 1800 UTC 6 December 2023

















Summary provided by M. Steen, S. Bartlett, C. Castellano and P. Iniguez; 1 December 2023

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*Outlook products are considered experimental