Yet another atmospheric river to flood California later this week

A fresh bout of heavy rainfall could increase flood risk in California later this week, as yet another “atmospheric river” sweeps through the region.

A strong storm system over the Pacific Ocean will likely approach the Pacific Northwest coast on Thursday night and into Friday, dragging “an abundance of subtropical moisture” inland over Central California, according to the National Weather Service’s (NWS) Hanford branch. 

Accompanying heavy rains are expected to cause snowmelt in areas below 5,000 feet, with 4 to 6 inches accumulating in the Sierra Nevada foothills by Saturday night, the outlook stated.

“It now appears increasingly likely that a potentially significant and very likely warm atmosphere river will probably affect some portion of Northern or Central California,” University of California, Los Angeles climate scientist Daniel Swain said during virtual “office hours” on Monday.

“There’s a lot of uncertainty at this point regarding how intense it will be how prolonged it will be in the impacts it will likely have,” Swain continued. “However, confidence has grown that a warm rain event of some magnitude will occur later this week.”

Weather Channel meteorologist Domenica Davis made similar remarks on Tuesday, noting that “by Friday, we have this intense moisture flow that is going to be coming in, and this is going to bring widespread heavy rain.”

This latest episode of precipitation comes following a series of winter storms, with snow piling up in Sierra Nevada ski resort areas last weekend.

Sugar Bowl received about 60 inches of snow, while the Sierra Snow Lab recorded 48.5 inches and Palisades Tahoe amassed 36 inches, according to the NWS Sacramento office. 

“It’s been another remarkable week of California weather,” Swain said during his office hours.

The situation has become particularly noteworthy, he explained, because this accumulation occurred “on top of already extremely heavy snowfall” from previous weeks.

“In some cases, it’s approaching record snowpack accumulation levels for the time of year,” Swain said.

So much snow has piled up that some areas are beginning to experience structural issues, with buildings collapsing under the weight of the snow and ski resorts shutting down, according to Swain.

“Some of the chairlifts at the highest peaks are no longer that high above the snow itself,” he said.

With such enormous amounts of snow, the incoming deluge of rain could lead to what Davis described as a “quick melt” and creating “major concern” about flooding.

“Persons living near rivers and streams should closely monitor water levels and be ready to move to higher ground if the threat of flooding becomes imminent,” the NWS Hanford outlook warned.

“Now would be a good time to prepare and have an evacuation plan in place in the event high water becomes a threat to your safety,” the forecast added.

While Swain agreed that the storm system is likely going to increase flood risk later this week, he stressed that “this is not going to be a widespread catastrophic flood event.”

Swain said that he is most concerned about flooding in the Sierra Nevada foothills regions, particularly in urban areas where storm drains have been clogged by snow.

At higher elevations, Swain said that the rain influx could make accumulated snowpack heavier and potentially exacerbate existing structural issues.

“If you can, go out and try and remove some snow from structures that might be vulnerable,” he added.