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Strong atmospheric river to bring heavy rain and flooding to Pacific Northwest, U.S.

noaa goes west 1330 utc june 1 2024

A strong atmospheric river, predicted to hit the Pacific Northwest on June 2, 2024, is expected to bring heavy rainfall and potential flooding, particularly affecting coastal and mountainous regions. The event may last over 48 hours, posing significant flood risks.

  • The forecast calls for AR 3-4 conditions over coastal Oregon and AR 2-3 conditions over coastal Washington and coastal Northern California.
  • Historical data from 1959 to 2023 shows that central Oregon typically experiences 1 – 2 landfalling ARs per year in June.
  • Should the forecast AR4 verify, it would be only the fifth AR4+ at this location since 1959.

A strong atmospheric river (AR) is forecast to make landfall over the Pacific Northwest, bringing heavy rainfall and potential flooding to the region starting June 2.

The Center for Western Weather and Water Extremes (CW3E) has indicated a near 100% probability of AR activity affecting the Pacific Northwest and Northern California from Sunday, June 2, and continuing into early next week.

This AR system is forecast to develop in the Central Pacific, accompanied by surface cyclones and mid-level shortwaves, progressing over the Northeast Pacific towards the US West Coast. High levels of Integrated Water Vapor (40+ mm) are anticipated.

The Global Ensemble Forecast System (GEFS) predicts AR 3-4 conditions over coastal Oregon and AR 2-3 conditions over coastal Washington and Northern California. Most GEFS members forecast Integrated Vapor Transport magnitudes to exceed 250 units during both AR landfalls in central Oregon, resulting in a prolonged AR event exceeding 48 hours, with a higher ranking on the AR scale.

Historical data from 1959 to 2023 shows that central Oregon typically experiences 1 – 2 landfalling ARs per year in June.

If the AR4 forecast verifies, it would be only the fifth such event at this location since 1959.

The Weather Prediction Center (WPC) forecasts 50 to 127 mm (2 to 5 inches) of precipitation over the Olympic Peninsula, Washington/Oregon coasts, and Cascades for the 72-hour period ending at 05:00 LT on Wednesday, June 5.

The WPC has issued a marginal risk (level 1 of 4) for flooding in the Excessive Rainfall Outlook (ERO) for western Washington and parts of western Oregon for the 24-hour periods ending at 05:00 on Monday, June 3, and at 05:00 on Tuesday, June 4.

The greatest hydrologic impacts are expected on the western side of the Washington Cascades.

The Northwest River Forecast Center (NWRFC) is currently forecasting one station above minor flood stage along the Snoqualmie River and eight stations above action/bankfull stage, all in Washington, due to the anticipated precipitation.

Freezing levels are expected to remain elevated throughout the event, likely causing all precipitation to fall as rain. This condition increases the potential for greater runoff and, consequently, a heightened risk of flooding in the affected areas.

Residents living in affected region are urged to maintain a close watch on weather updates and heed official warnings.

References:

1 Latest Update on Strong AR Forecast Over Pacific Northwest – CW3E – May 31, 2024

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