CW3E AR Update: 20 December 2022 Outlook

December 20, 2022

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Potential for Multiple Atmospheric River Landfalls Over the US West Coast

  • A series of atmospheric rivers (ARs) is forecast to move across the Northeast Pacific and make landfall along the West Coast of North America during 22–27 Dec. Per Fish et al. (2019), these types of multi-AR events, known as β€œAR families,” are not uncommon during boreal winter.
  • Current model forecasts suggest that AR landfall during this period is most likely to occur over the Pacific Northwest and British Columbia, but there is considerable uncertainty in the timing, location, and intensity of these ARs
  • Models are also showing the potential for landfalling AR activity in California during 28–31 Dec, but there is lower forecast confidence given the longer lead times
  • The 00Z GEFS is showing a high likelihood of AR 3 or greater conditions (based on the Ralph et al. 2019 AR Scale) over coastal Oregon during the next 7 days
  • Some locations are forecast to experience AR conditions for more than 96 consecutive hours across multiple AR landfalls
  • These ARs are forecast to produce more than 5 inches of total precipitation over the Pacific Coast Ranges and Cascades during the next 7 days, with more than 10 inches possible in the Olympic Mountains
  • There are large model differences in forecast precipitation, with the 00Z ECMWF forecasting lower precipitation amounts in western Washington and higher precipitation amounts near the Oregon/California border compared to the 00Z GFS

Click images to see loops of GFS IVT & IWV forecasts

Valid 0000 UTC 20 December – 0000 UTC 28 December 2022









Summary provided by C. Castellano, S. Bartlett, J. Kalansky, S. Roj, J. Rutz, and F. M. Ralph; 20 December 2022

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