Atmospheric River To Bring Heavy Rains, Snow Across Pacific Northwest

Following hot on the heels of a bomb cyclone that knocked out power to hundreds of thousands of people across the Pacific Northwest and claimed at least two lives, an atmospheric river storm is due to slam into the region.

The powerful weather system is expected to dump huge volumes of rain across Washington, Oregon, and Northern California, with intense wind gusts and heavy snow in mountainous regions.

Between 6 and 12 inches of rain is forecast in some areas, possibly triggering landslides, and upwards of 10 feet of snow is expected in California's Mount Lassen National Park.

"Atmospheric river will produce one last round of heavy rain/mountain snow and strong wind gusts across the Pacific Northwest today," the National Weather Service (NWS) said in a Short Range Forecast Discussion.

An aerial view of the flooded Los Angeles River during an atmospheric river storm on February 5, 2024, in Los Angeles, California (main) and satellite image of an atmospheric river hitting the West Coast this... Mario Tama/Getty Images / CIRA/NOAA

Flood Warnings, Flood Watches, High Wind Warnings, and Winter Storm Warnings have been issued across Northern California and southern Oregon, while Gale Warnings blanket the ocean along the California and Oregon coasts, with Storm Warnings off the Washington coast.

The storm has already caused between 10 and 15 inches of rainfall in some regions of California, resulting in flooding.

Atmospheric rivers are narrow, elongated corridors of concentrated moisture in the atmosphere. These phenomena act like "rivers in the sky," carrying vast amounts of water vapor, and can produce intense precipitation, including heavy rain or snow.

"While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. Exceptionally strong atmospheric rivers can transport up to 15 times that amount," NOAA states on its site.

According to the NWS, winds of up to 75 mph might be seen across Northern California and 65 mph in Oregon, which may blow down trees and power lines. This is especially true in the wake of the bomb cyclone that hit the area earlier this week, as soils are now saturated with water, making it easier for trees to be uprooted.

Heavy snowfall is expected in the Cascades and Sierra Nevada mountains, with heavy rainfall at lower altitudes.

"Heavy mountain snow is expected over the Washington Cascades and Northern Rockies through Saturday. Multiple feet of snow are likely to accumulate over portions of the northern Sierra (above 7000') by Saturday evening," the NWS stated.

The heavy rainfall is feared to trigger flash flooding and landslides, which could be life-threatening.

"Life threatening flooding is likely across portions of northern California", the NWS said. "Dangerous flooding, rock slides and debris flows remain likely today."

The NWS' San Francisco office advised in a forecast discussion that people should avoid driving through flood waters.

"Waters overtopping roadways may be deeper and swifter than they appear and can result in a dangerous situation developing. Flooding can be particularly hard to see in the dark so use extra caution when traveling at night. Remember - turn around, don't drown," the National Weather Service San Francisco office said.

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