CW3E AR Update: 16 December 2024 Outlook

December 16, 2024

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Family of Atmospheric Rivers Forecast to Bring Heavy Precipitation to US West Coast

A series of atmospheric rivers (ARs) and low-pressure systems are forecast to develop over the North Pacific and propagate toward the US West Coast through the end of the week.
The first AR is forecast to make landfall early tomorrow and bring AR 2/AR 3 conditions (based on the Ralph et al. 2019 AR Scale) and heavy precipitation to coastal Washington and northern Oregon.
The second and third ARs are forecast to make landfall over British Columbia on Thu 19 Dec and Fri 20 Dec, with limited impacts expected over the US West Coast.
The fourth and fifth ARs are forecast to make landfall on Sat 21 Dec and Sun 22 Dec and potentially bring moderate-to-strong AR conditions (IVT > 500 kg m⁻¹ s⁻¹) and heavy precipitation to western Washington, western Oregon, and Northern California.
Extended range and subseasonal forecast products suggest that additional AR activity and wet conditions are likely to continue into next week, especially over the Pacific Northwest.
The NWS Weather Prediction Center is forecasting 10–20 inches of total precipitation over the Olympic Peninsula during the next 7 days.
Uncertainty in the forecast evolution of these ARs and shortwave troughs is driving uncertainty in forecast precipitation. In general, EPS is forecasting higher precipitation totals across coastal Washington and Oregon during the next 10 days compared to GEFS.
Numerous stream gages in western Washington and Oregon are forecast to rise above action/bankfull stage over the next 10 days. The combination of high freezing levels and heavy rain will likely increase runoff and flooding potential during the first AR.