This blog post explains the science and real-world impacts behind the strong atmospheric river currently affecting the Pacific Northwest. We will explore what an atmospheric river is, why this event is classified as a high-end system, and what the latest data suggest about rain, snow, flooding, and landslide risks across western Washington, Oregon, northern Idaho, and far western Montana.
What Is Happening Over the Pacific Northwest?
A powerful corridor of moisture, known as an atmospheric river, is streaming into the Pacific Northwest. It is expected to persist through late this week.
This system is delivering multiple days of heavy rain to lower elevations and coastal areas. It is also bringing significant snowfall to the highest mountain peaks.
The Center for Western Weather and Water Extremes has rated this atmospheric river as a level 4 out of 5. This indicates a high-impact event with substantial hydrological consequences for the region.
A 3,500-Mile Conveyor Belt of Moisture
The current atmospheric river stretches more than 3,500 miles from Alaska’s Aleutian Islands all the way to Hawaii. This long, continuous plume acts like a conveyor belt, transporting vast quantities of Pacific moisture directly into the Pacific Northwest.
Because this flow is both deep and sustained, it favors prolonged rainfall rather than short-lived showers. This duration is a key driver of the increasing flood risk, especially as soils become progressively saturated.
Rainfall Extremes and Flooding Concerns
The first wave of moisture arrived Monday, delivering impressive rainfall totals, particularly in mountainous and coastal terrain. This initial surge has already set the stage for more serious impacts as the next round of moisture approaches.
First Round: Heavy Rain and Initial Flooding
The first round of rain dropped 4 to 7 inches across the Cascades, Olympics, and coastal ranges. This onslaught of precipitation has already produced:
These early impacts are typical when an atmospheric river first makes landfall, especially over complex terrain that efficiently wrings moisture out of the incoming air mass.
Second Round: Higher Flood Risk on Saturated Ground
A second, robust wave of moisture will arrive from Tuesday night into Wednesday, amplifying the flood threat. With soils already near saturation from the first event, additional rainfall will more rapidly translate into surface runoff and river rises.
Forecast guidance suggests:
Snow, Temperature, and Elevation Effects
Although this is a robust storm system, snowfall has been relatively limited compared to typical cold-season Pacific Northwest events. The culprit is anomalously warm air embedded in the atmospheric river.
Warmer-Than-Usual Temperatures Limit Snowfall
With warmer-than-normal temperatures for this time of year, snow levels are elevated, confining significant snowfall to only the highest peaks. At mid-elevations, precipitation is falling as rain rather than snow, which can accelerate runoff into rivers and streams.
This combination of high snow levels and strong rainfall is especially concerning because:
Rivers, Landslides, and Safety Risks
The hydrological response to this atmospheric river is now becoming apparent, with significant river rises and increased slope instability across the region.
Major River Flooding in Western Washington
Forecasts indicate major river flooding on several key rivers in western Washington, including:
Communities along these rivers should anticipate potential road closures, flooded low-lying areas, and possible impacts to infrastructure such as bridges and culverts. Continued monitoring of official river forecasts is essential.
Landslides and Debris Flows in Steep and Burned Terrain
Authorities are also highlighting the increased risk of landslides in steep terrain and debris flows in areas previously affected by wildfires.
Prolonged heavy rain weakens soil cohesion on slopes. This is especially true where vegetation has been removed or damaged.
Residents and travelers in vulnerable areas are urged to:
Here is the source article for this story: Powerful Atmospheric River Aimes For Pacific Northwest