CW3E AR Update: 22 December 2021 Outlook

December 22, 2021

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Strong Atmospheric River and Multiple Upper-level Systems to Produce Heavy Rain and Snow in the Western US

  • AR activity and heavy precipitation is expected over much of the western US today through Friday
  • The 12Z GEFS control member is forecasting AR 2/AR 3 conditions over coastal Southern and Central California
  • AR 5 conditions are currently forecasted over south-central Arizona, where maximum IVT magnitude may exceed 1250 kg m -1 s -1
  • As the AR weakens, a series of upper-level shortwave troughs is forecast to move down along the US West Coast and bring additional periods of precipitation to the Pacific Northwest and Northern California
  • Freezing levels are forecasted to decrease starting on 23 Dec to 3000-4000 ft and remain below 5000 ft during the passing of the shortwaves
  • A second period of AR activity is possible over the southwestern US on 25–26 Dec
  • The NWS Weather Prediction Center (WPC) is forecasting at least 5–10 inches of total precipitation in portions of the Pacific Coast Ranges, the Cascades, and the Sierra Nevada over the next 7 days, with more than 10 inches likely in the higher terrain of the Sierra Nevada
  • At least 2–5 inches of precipitation are forecasted in Southern California and parts of the Rocky Mountains in Utah, Colorado, Idaho, and Wyoming with up to 2.5 inches in parts of Arizona
  • More than 8 feet of snow are possible in the higher terrain of the Sierra Nevada
  • Although significant flooding is not expected at this time, minor flooding is possible in the vicinity of the Central California Coast Ranges and the mountains in Southern California

Click images to see loops of GFS IVT & 500-hPa vorticity forecasts

Valid 1200 UTC 22 December – 0000 UTC 30 December 2021












Summary provided by C. Castellano, S. Roj, J. Kalansky, and F. M. Ralph; 22 December 2021

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