Atmospheric River Triggers Winter Storm Warnings Across 11 US States

Due to the heavy snowfall, races at the Audi FIS Alpine Ski World Cup Men's Downhill were canceled in Beaver Creek, Colo., on Dec. 1, 2023. Alexis Boichard/Agence Zoom/Getty Images

Atmospheric rivers that have been lashing the Pacific Northwest of the United States have prompted winter storm warnings in 11 states, with more heavy rain, snow, and strong winds predicted in the days ahead.

The warnings are currently in effect for parts of Colorado, Idaho, Maine, Nevada, New Hampshire, New York, Oregon, Utah, Vermont, Washington, and Wyoming.

An atmospheric river is a narrow space in the atmosphere that transports significant amounts of water vapor. When they hit land, the vapor is released through rain, snow, or a combination of both. According to the National Oceanic and Atmospheric Administration, atmospheric rivers can vary significantly in size and strength.

On average, they carry roughly the equivalent of the water flow through the mouth of the Mississippi River. More powerful ones can transport up to 15 times more water vapor.

AccuWeather meteorologists predict that at least through Dec. 6, locations across Washington, Oregon, Northwest California, northern Idaho, and western Montana might face even more wild weather due to an atmospheric river overhead. An additional 1 to 2 inches of rain in most affected regions is being forecast. Coastal areas are expected to be hit with far more rain; a further 4 to 8 inches is being predicted.

Areas from northwest California to western Washington have already reported flooded roadways and travel disruptions, along with rock and mudslides. Additional rainfall in those areas could worsen the conditions.

Flood watches and advisories are in place for numerous counties across western Washington and Oregon through late on Dec. 6 due to the steady stream of rainfall that will continue to spread inland.

The highest population centers of the Pacific Northwest—Seattle, and Portland, Oregon—possibly could see a month’s worth of rainfall in the first six days of December as the prolonged stretch of rain continues, AccuWeather meteorologist Alyssa Glenny warned in a Dec. 4 report.

“Both cities are expected to receive an additional 2 to 4 inches of rain through [Dec. 6]; the historical average rainfall for the month of December is 5.72 inches in Seattle and 5.77 inches in Portland.” AccuWeather meteorologist Elizabeth Danco said.

“Looking ahead to later this week, a storm moving into the Northwest can bring colder air back into the region on [Dec. 7], allowing for temperatures to drop closer to the historical average,” she said. “However, the stormy pattern should shut off.

“Mainly dry conditions are expected to continue on [Dec. 8], but another round of rain and mountain snow can sneak back into the Pacific Northwest by late ... into [Dec. 9].”

AccuWeather says that while the expected precipitation might cause travel disruptions, road closures, and flooding, it could gradually help improve the drought conditions.

According to the U.S. Drought Monitor, locations across western Oregon and Washington state are enduring moderate to severe drought conditions. Cities such as Seattle have suffered from a relatively dry summer, setting the stage for the drought.

Increased Avalanche Danger

The Audi FIS Alpine Ski World Cup Men's Super G race at Beaver Creek Resort was canceled due to high winds and snow in Beaver Creek, Colo., on Dec. 3, 2023. Christian Petersen/Getty Images

Snow levels in the northwest mountains are also expected to rise significantly before warmer conditions set in early this week. According to Ms. Taylor, this brings some risk of flooding as well.

“Rising temperatures through Wednesday can lead to snow melting, especially in the Cascades where over a foot of snow has fallen,” she said.

“The snowmelt, in addition to the heavy rain Monday and Tuesday, can lead to flooding concerns and dangerous travel, especially in the mountains and foothills.”

Heavy rain falling in the mountains as temperatures rise could cause the snowpack on steep mountain slopes to become more unstable, increasing the risk of avalanches. The Northwest Avalanche Center has issued avalanche warnings for the slopes of the Washington Cascades, Mount Hood, Stevens Pass, and Snoqualmie Pass.

All are facing a level four-high avalanche danger through to at least Dec. 10. Travel isn’t recommended until after the weekend.

California Drought-Free

Most of these areas are familiar with massive rainfalls over a short period, especially from November to January. California, in particular, has been grappling with atmospheric river floods all year. Starting in March, forecasters warned that most of California was facing another atmospheric river storm system that would likely produce heavy rain.

The Nicasio Reservoir is seen at 100 percent capacity after a series of atmospheric river events drenched the state in Nicasio, Calif., on Jan. 12, 2023. Justin Sullivan/Getty Images

Again, a new series of atmospheric rivers flowed into California in September, prompting warnings of widespread heavy rain and flooding. The following months saw more than 9,000 people ordered to evacuate, and more than 54,000 without power, as parts of California were struck with yet another atmospheric river system. Heavy rain, thunderstorms, and strong winds resulted in floods on several major highways and small rural communities in the southern and central regions of the state.

California had been dealing with a prolonged drought for nearly three years during a strong La Niña event. However, after a shift toward neutral and now El Niño conditions, the state has been declared officially drought-free and is expected to stay that way until at least the end of January 2024.