CW3E AR Update: 5 December 2022 Outlook

December 5, 2022

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Potential for Multiple Atmospheric Rivers to Impact the US West Coast During the Next 7 Days

• Two atmospheric rivers (ARs) are forecast to make landfall along the US West Coast on 9 Dec and 12 Dec
• The first AR is forecast to bring a short period of AR 1 conditions (based on the Ralph et al. 2019 AR Scale) to coastal Southern Oregon
• The deterministic GFS and ECMWF have drastically different solutions for a developing area of low-pressure associated with the second AR, making it difficult to pinpoint the timing, IVT intensity, and landfall location
• The 00Z deterministic GFS is forecasting the second AR to make landfall in coastal Northern California and bring AR conditions to most of California, while the 00Z deterministic ECMWF is forecasting the second AR to make landfall in coastal Washington about 20 hours later with limited AR conditions into coastal Northern California
• There is a large amount of uncertainty in forecast AR activity associated with both ARs in GEFS and ECMWF EPS
• In comparison to the 00Z ECMWF, the 00Z GFS is forecasting greater 7-day watershed precipitation total throughout the mountainous regions of California, Oregon, and Washington

Click images to see loops of GFS IVT and IWV forecasts Valid 0000 UTC 05 December – 1200 UTC 12 December 2022

Summary provided by S. Roj, S. Bartlett, C. Castellano, J. Kalansky, and F. M. Ralph; 05 December 2022

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