Maps Show Where Atmospheric River Threatens Dangerous Flooding, Snow

An active pattern of atmospheric river activity looks set to bring cold and wet weather to the West Coast over the next few days.

Through Thursday, November 30 to Saturday, December 2, there will be "atmospheric river activity arriving across the Pacific Northwest by the end of the week will bring heavy rain to the coastal ranges, and heavy snowfall to the Cascades," according to the NOAA's National Weather Service.

Atmospheric rivers are long, narrow regions in the atmosphere that transport water vapor from the Pacific Ocean. When atmospheric rivers make landfall, they typically release this water vapor as either rain or snow, at times causing extreme rainfall or flooding.

"An active pattern of atmospheric River activity looks to set up along the West Coast through at least this weekend with a series of shortwaves and influx of moisture overspreading the Pacific Northwest/northern California," the NOAA has forecast in its latest short range public discussion. "Coastal/lower elevations will see moderate to locally heavy rain, with the heavier rainfall more likely Friday and then into the weekend."

Newsweek has contacted the NOAA for comment via email.

As such, widespread heavy rain and mountain snow are forecast to hit the Pacific northwest states of Oregon and Washington, as well as northern California.

A map posted by @US_Stormwatch on X, formerly Twitter, shows the atmospheric river expanding rapidly throughout Thursday, with between three to five inches of rain expected along the U.S. and Canada west coasts.

Another map from Accuweather shows that rain will be heaviest around Medford and Portland in Oregon and getting lighter as the system travels inland toward Idaho and Montana.

Forecasters predict that those in lower elevations can expect a mix of rain and snow. A rainfall forecast from Weather and Radar predicts that "by Sunday, many along the Pacific Northwest coast to the northwest California coast could see between three and five inches of rainfall."

The forecast also warns of potential flooding through the weekend. It continues: "Flash flooding is possible in these bursts of heavy rain as the storms march their way in, especially in areas with recent burn scars and drought-stricken grounds. Elevations below 3,500 feet can expect to see heavy rainfall and, starting on Saturday, excessive rainfall risks are in place."

However, for now the NOAA has said the threat of flooding "currently looks low with this initial rainfall, though will likely slowly increase as antecedent conditions become wetter."

Snow is expected in elevated areas, with the Cascade Range likely to see several feet through Saturday. Weather and Radar reports: "At this time, snow amounts within the winter weather advisories come to four to eight inches, but those higher than 3,500 feet and also in the winter storm watch regions can expect to see anywhere from 10 to 20 inches, with up to 30 inches possible above 5,000 feet by Saturday."

An atmospheric river has already drenched the West Coast this year, with California's state emergency operation centers activated and evacuations ordered in communities close to the Sacramento River. Four people died in the March 2023 storm, while another series of atmospheric river deluges lashed California from December through mid-January, killing at least 20 people, according to a report by Reuters.