CW3E AR Update: 18 December 2024 Outlook

December 18, 2024

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Family of Atmospheric Rivers Forecast to Continue Bringing Heavy Precipitation to US West Coast

  • A series of atmospheric rivers (ARs) and low-pressure systems are forecast to develop over the North Pacific and propagate toward the US West Coast over the next 8-10 days.
  • The first AR is forecast to make landfall early tomorrow (Thu 19 Dec) and bring AR 1 conditions (based on the Ralph et al. 2019 AR Scale) with limited impacts expected over the US West Coast.
  • A second AR is forecast to make landfall over Oregon and Washington on Fri 20 Dec. The core of the AR is expected to stay mostly offshore with a more southerly IVT direction, limiting precipitation impacts.
  • The third and fourth ARs are forecast to make landfall on Sat 21 Dec and Sun 22 Dec. However, IVT is expected to approach from a more southerly direction pushing the heavier precipitation towards Vancouver Island.
  • A fifth AR could potentially bring strong AR conditions (IVT > 750 kg m−1 s−1) to coastal Oregon with moderate AR conditions (IVT > 500 kg m−1 s−1) for coastal Northern California to Washington.
  • Extended range and subseasonal forecast products suggest that additional AR activity and wet conditions are likely to continue into late next week, especially over the Pacific Northwest.
  • The NWS Weather Prediction Center is forecasting 8-14 inches of total precipitation over the Olympic Peninsula and Northern California during the next 7 days.
  • Uncertainty in the forecast evolution of these ARs and shortwave troughs is driving uncertainty in forecast precipitation. In general, EPS is forecasting higher precipitation totals across coastal California with less across coastal Oregon and Washington over the next 10 days compared to GEFS.
  • Numerous stream gages in western Washington and Oregon are forecast to rise above action/bankfull stage over the next 10 days. The combination of high freezing levels and heavy rain will likely increase runoff and flooding.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 18 December 2024 – 0000 UTC 24 December 2024

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

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*Outlook products are considered experimental

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