CW3E AR Update: 20 December 2024 Outlook

December 20, 2024

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Family of Atmospheric Rivers Forecast to Continue Bringing Heavy Precipitation to US West Coast

A series of atmospheric rivers (ARs) and low-pressure systems are forecast continue impacting the US West Coast over at least the next seven days.
The first AR has made landfall today (Fri 20 Dec) with the second AR following shortly behind, making landfall early tomorrow (Sat 21 Dec) over the Pacific Northwest and Northern California coasts. These two ARs are forecast to bring light precipitation impacts to the US West Coast.
The third and fourth ARs are forecast to make landfall on Sun 22 Dec and Mon 23 Dec. Both ARs are forecast to bring moderate-to-strong AR conditions (IVT > 500 kg m−1 s−1) to the Pacific Northwest and Northern California coasts.
Following a lull in AR conditions, a fifth AR is forecast to make landfall Wed 25 Dec.
Extended range forecast products indicate wet conditions, potentially driven by continued AR activity, are likely to continue into next weekend.
The NWS Weather Prediction Center is forecasting 6-14 inches of total precipitation over the Olympic Peninsula and Northern California during the next 7 days.
ECMWF is forecasting higher precipitation totals across coastal California and Southern Oregon with less across Northern Oregon and Washington over the next 10 days compared to GFS.
The Weather Prediction Center issued a marginal risk excessive rainfall outlook (Level 1 of 4, 5% chance) for the Northern California and Oregon coasts in association with the fourth AR from 4 AM PST 23-24 Dec.
Stream levels are forecast to rise throughout Washington, Oregon and California over the next seven days, with numerous gauges in Washington and Oregon reaching bankfull levels as a result of the high freezing levels and heavy rain.