CW3E AR Update: 14 September 2020 Outlook

September 14, 2020

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Update on the Atmospheric River forecast to make landfall over the Pacific Northwest

  • An AR that is forecast to make landfall early this week could bring much needed precipitation to parts of Washington, Oregon, and potentially Northern California.
  • As time has progressed closer to verification, ensemble agreement associated with AR landfall timing and initial IVT magnitude has increased.
  • The parent low pressure system is forecast to cut off from the upper-level flow and become quasi-stationary off the PNW coast.
  • Forecast duration of the event is currently exhibiting larger uncertainties as the cut-off low remains off the coast and moves higher IVT magnitudes onshore.
  • Forecast precipitation accumulations from the Weather Prediction Center have primarily decreased since our last outlook, though the ECMWF has trended towards a larger event (max. precip. accumulations of 1.5–2.4 in. over Coastal OR and WA).
  • Forecasts continue to suggest that a majority of CA will remain dry with lighter precipitation rates over far Northern CA.

Click IVT image to see loop of GFS analyses/forecasts

Valid 0000 UTC 14 September – 0000 UTC 24 September 2020







Summary provided by C. Hecht, B. Kawzenuk, C. Castellano, J. Kalansky, and F. M. Ralph; 14 September 2020

*Outlook products are considered experimental