CW3E AR Update: 23 December 2024 Outlook

December 23, 2024

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Unsettled Weather Pattern to Continue Over US West Coast Through This Weekend

A series of atmospheric rivers (ARs) will continue to propagate across the North Pacific and impact the Pacific Northwest and Northern California over the next 7 days.
The first AR is forecast to make landfall early late today and bring AR 2/AR 3 conditions (based on the Ralph et al. 2019 AR Scale) to coastal Southern Oregon.
The second and third ARs are forecast to make landfall on Wed 25 Dec and Thu 26 Dec.
The fourth and potentially most impactful AR is forecast to make landfall on Fri 27 Dec.
As the fourth AR moves onshore, a mesoscale frontal wave may prolong the AR duration and bring a second pulse of stronger moisture transport to the region. There is considerable uncertainty in the evolution of this AR.
The NWS Weather Prediction Center is forecasting 7–15 inches of total precipitation over portions of western Washington, western Oregon, and Northern California during the next 7 days.
Model differences in the forecast evolution of these ARs is driving differences in forecast precipitation. Overall, EPS is forecasting much higher precipitation totals across western Oregon and Northern California during the next 10 days compared to GEFS.
A marginal risk excessive rainfall outlook (ERO) has been issued for the Southern Oregon and Northern California Coast Ranges, as well as the Northern Sierra Nevada foothills today into early Wed 25 Dec.
Marginal risk EROs have also been issued for coastal Washington, Oregon, and Northern California for Wed 25 Dec into early Sat 28.
Numerous stream gages in western Washington, western Oregon, and Northern California are forecast to rise above action/bankfull stage over the next 10 days, with the greatest potential for flooding in southwestern Oregon.