Portland, OR – The National Weather Service in Portland is spotlighting the growing attention on Atmospheric Rivers (ARs) — powerful streams of moisture in the atmosphere that play a major role in the Pacific Northwest’s winter weather.
An Atmospheric River is described as a flowing column of concentrated water vapor, capable of producing significant rain or snow when it moves inland and encounters mountain ranges. As the moist air rises and cools, it releases its moisture as heavy precipitation.
While many AR events bring beneficial rainfall and snowpack to Oregon and Washington, stronger systems can lead to extreme rainfall, flooding, and landslides—especially in coastal and mountain regions. These powerful ARs can deliver the equivalent of Mississippi River–level water flow in the sky, driving multiple inches of rain in just 24–48 hours.
Forecasters emphasize that not all Atmospheric Rivers are severe, but residents should stay weather-aware as stronger systems can develop quickly, bringing both flooding hazards and rapid snowmelt in higher elevations.
The NWS recommends monitoring local forecasts and preparing for possible travel impacts, rising rivers, and power outages during more intense AR events.
