Three atmospheric rivers set to impact Washington, Oregon, and Northern California

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A series of atmospheric rivers are slated to affect the Pacific Northwest, starting today and lasting until Monday. The most intense activity is forecasted for Sunday, October 15, 2023, with the potential for strong atmospheric conditions in coastal Oregon.

The first and second atmospheric rivers are forecasted to bring relatively weak conditions, with Integrated Vapor Transport (IVT) values lower than 500 kg m−1 s−1, to coastal Washington, Oregon, and Northern California today, October 13, and Saturday. Notably, the third atmospheric river, predicted to make landfall between Sunday and Monday, October 15 and 16, is anticipated to be the strongest, although the precise intensity remains uncertain.

The National Weather Service Weather Prediction Center (NWS WPC) is forecasting 51 – 127 mm (2 – 5 inches) of precipitation in the Olympic Mountains and North Cascades over the next five days. However, the ECMWF Ensemble Prediction System (EPS) is projecting higher IVT magnitudes in coastal Oregon and Washington compared to the NCEP Global Ensemble Forecast System (GEFS). There is a notable disagreement between the ECMWF/EPS and GFS/GEFS models in terms of precipitation amounts, particularly in the Olympic Peninsula.

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Image credit: NOAA/GOES-West, RAMMB/CIRA, The Watchers. Acquired at 22:20 UTC on October 13, 2023

Moreover, AR4/AR5 conditions are possible in coastal Oregon. This scenario involves IVT magnitudes potentially exceeding 1 000 kg m−1 s−1, and atmospheric river conditions lasting for more than 48 consecutive hours between the second and third atmospheric rivers.

Despite the high likelihood of strong atmospheric conditions during the third event—specifically with IVT values greater than 750 kg m−1 s−1—the south-southwesterly orientation of moisture transport is not expected to significantly amplify precipitation in most areas. Consequently, major hydrologic impacts are not anticipated.

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1 Multiple Atmospheric Rivers Forecast to Impact Pacific Northwest Through the Weekend – CW3E – October 13, 2023

Featured image credit: CW3E

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