CW3E AR Update: 1 December 2023 Outlook

December 1, 2023

Click here for a pdf of this information. Click here to provide feedback on these outlooks!

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

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental