Pacific Northwest braces for multiple Atmospheric Rivers starting Friday, October 13

Several Atmospheric Rivers (ARs) are set to influence the coasts of Washington, Oregon, and Northern California starting Friday, October 13, 2023, and lasting through the following week. This sequence of weather events is forecasted to bring considerable rain, though no rivers are expected to exceed flood levels.

Starting this Friday, October 13, and continuing into the early parts of next week, the Pacific Northwest region will experience the influence of multiple Atmospheric Rivers (ARs). The first and second ARs are expected to bring weak AR1-2 conditions to the Washington and Oregon coastal areas from Friday afternoon through Sunday, October 15. These projections are based on the Ralph et al. 2019 AR Scale.

As we move into the next week, the jet stream’s activity over the North Pacific Ocean is anticipated to remain heightened, creating conducive conditions for further AR activity. This is particularly the case for the time span between Monday, October 16, and Wednesday, October 18. Nevertheless, there’s a level of uncertainty surrounding the specifics of the third AR.

Assessments suggest a high likelihood (90%) of AR1 or stronger conditions occurring northward from Point Reyes, California, come Monday. The Washington and Oregon coasts are also under a moderate 70% chance of witnessing up to AR3 conditions. However, the possibility of experiencing AR5 conditions on the Oregon coast remains minimal, at 20%.

The week’s cumulative rain forecast indicates the potential of 50 – 130 mm (2 – 5 inches) falling across the coastal mountains and the Cascades. Some areas might see even higher localized rainfall. Notably, the snow levels are projected to stay predominantly elevated, mainly above 2 500 m (8 000 feet) throughout these weather events.

Despite these significant rainfall projections, the current forecast doesn’t predict any river surpassing its flood stage. In a more speculative vein, while current data provides some insights into the imminent weather patterns, there’s much greater uncertainty concerning the potential emergence of a fourth Atmospheric River towards the end of next week.

References:

¹ Several ARs to Move Across the Pacific Northwest During the Next Week – Center for Western Weather and Water Extremes – October 11, 2023

Featured image credit: CW3E