Particle.news
Download on the App Store
5 ARTICLES
·
13h ago

Pacific Northwest Faces Prolonged Atmospheric Rivers With AR4–AR5 Risk in Washington

Probabilistic guidance points to an AR4–AR5 setup for parts of Washington early next week, raising the odds of widespread river flooding.

Highway digital billboard during snow storm in the Sierra Nevada mountains that reads, "Unsettled Weather Expected, Carry Chains." Oncoming headlights from cars visible.
A map from the National Weather Service (NWS) shows areas at risk of flooding as an atmospheric river takes aim at the West Coast on December 6, 2025.
Satellite image of the first atmospheric river moving over the US West coast on December 5, 2025. Credit NOAA GOES-19, Zoom Earth, The Watchers
Image

Overview

  • A second, stronger atmospheric river is forecast to come ashore Monday through Wednesday, with ensemble guidance showing ~70–80% odds of AR4 and ~20–30% of AR5 in southern coastal Washington, and flood risk peaking December 9–10.
  • Forecast rainfall totals reach 4–7 inches across much of western Washington and Oregon, with more than 10 inches likely in the Olympics, coastal ranges and windward Cascades, including roughly 8.4 inches for the Duwamish watershed.
  • NWS Portland projects rivers draining the Coast Range and Willapa Hills have a 10–25% chance of reaching moderate flood stage and an 8–15% chance of major flooding.
  • The first atmospheric river is already producing rain, 30–35 mph gusts and high-elevation snow as NWS issues winter weather, wind, coastal flood, gale and small craft alerts.
  • WPC flags marginal to slight excessive-rainfall risks for December 8–10, with saturated soils and rising snow levels near 6,000 feet during the stronger event heightening threats of rapid runoff and landslides.