CW3E AR Update: 18 November 2024 Outlook

November 18, 2024

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Long-Duration Atmospheric River Event to Bring Heavy Precipitation to California and Oregon

  • A strong atmospheric river (AR) is forecast to make landfall over the US West Coast tomorrow (19 Nov) in association with a rapidly intensifying low-pressure system.
  • After the initial AR landfall, the AR is forecast to stall over Northern California for several days.
  • The location of AR landfall has continued to trend southward in recent model runs, with the strongest AR conditions and heaviest precipitation now expected in Northern California.
  • Active weather is likely to continue across California this weekend into early next week, but there is considerable forecast uncertainty in the duration and exact location of AR activity beyond Fri 22 Nov.
  • AR 4 conditions (based on the Ralph et al. 2019 AR Scale) are likely in coastal Northern California.
  • Some locations may experience AR conditions for > 72 consecutive hours.
  • The NWS Weather Prediction Center (WPC) is forecasting > 10 inches of total precipitation in portions of Northern California during the next 7 days.
  • Some watersheds in Northern California could receive > 20% of their normal total water year precipitation during the next 10 days.
  • Heavy rainfall will likely result in hydrologic impacts over portions of Northern California and southern Oregon.
  • The WPC has issued a moderate risk excessive rainfall outlook for Del Norte and Humboldt Counties Thu 21 Nov into early Fri 22 Nov.
  • Major winter storm impacts are likely in the Klamath Mountains, Southern Cascades, and Northern Sierra Nevada.

Click images to see loops of West-WRF GFS IVT and IWV forecasts

Valid 0000 UTC 18 November 2024 – 0000 UTC 25 November 2024

Summary provided by C. Castellano, S. Bartlett, J. Cordeira, J. Kalansky, and M. Steen; 18 November 2024

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*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.