California braces for more flooding, snowmelt from a warm new atmospheric river

LOS ANGELES — Another atmospheric river system has set its sights on California, raising considerable concern about flooding and structural damage as warm rain is expected to fall atop the state’s near-record snowpack this week, forecasters say.

“It now appears increasingly likely that a potentially significant and very likely warm atmospheric river event will probably affect some portion of Northern or Central California sometime between about late Thursday and Saturday,” UCLA climate scientist Daniel Swain said during a briefing Monday.

Last week, the odds of such a system developing were about 20%. By Monday, the chances had increased to “7 or 8 out of 10, if not higher, for a warm atmospheric river event of some magnitude,” Swain said. At least one more storm could follow this month.

The forecast comes as California is mired in remarkably deep snowpack amid one of its wettest winters on record. A series of nine atmospheric river storms hammered the state in early January, causing levee breaches, widespread flooding and nearly two dozen deaths.

In recent weeks, strong winter storms dropped piles of fresh powder across the Sierra Nevada and other areas, including the mountains of Los Angeles and San Bernardino counties, where some residents remain trapped behind feet of snow.

Officials said the bounty made a dent in the state’s extreme drought conditions and offered some hope for strained water supplies after three bone-dry years. But heavy snowpack can also become a hazard if it meets with warm rain that melts it too quickly.

“We’re going to see rain on top of snow, and for elevations of say 2,000 feet to about 4,000 feet, a lot of that snow is going to melt,” said Carlos Molina, a meteorologist with the National Weather Service in Hanford, Calif. “We’re going to basically lose a lot of the snow that fell from the previous storms. We’re looking at potential for flooding.”

Indeed, the highest likelihood of floodrelated impacts are in lower-elevation areas with unusually deep snowpack, Swain said. Small rivers and streams in those areas will see significant potential for runoff issues, as will some urban areas — particularly in places where storm drains are already clogged by snow.

There may also be problems at elevations above 5,000 or 6,000 feet, he said. Though snowpack in such areas is probably too deep and too cold to be melted by the incoming storm, it can become heavier as it absorbs more water. That could cause roof collapses and other structural issues.

“If you can go out and try and remove some snow from structures that might be vulnerable, do it,” Swain said. The state has already seen a spate of roof collapses from mounting snow, including a grocery store providing critical supplies in snowbound Crestline.

Scott Rowe, a meteorologist with the weather service in Sacramento, said the incoming low-pressure system is originating in the north but is expected to link with “very warm” subtropical moisture coming from Hawaii. Such storms are sometimes referred to as a Pineapple Express and are known to drop heavy moisture in California.

“We’re essentially transitioning from one storm track to another, where the moisture origins are coming from a warmer, juicier location,” Rowe said.

Though there is growing certainty that such a system will arrive, its precise timing, location and impacts will become clearer as the week goes on, forecasters said.

“Buckle up, it’s going to be quite the weather ride,” the weather service wrote in its forecast for the Central Coast and

San Francisco Bay Area. As much as 6 inches of rainfall are possible in the coastal ranges from Thursday morning into Saturday afternoon, and up to 2 inches are possible in other parts of the region, the agency said.

The weather service in Sacramento similarly warned of several inches of rain and higher snow levels. It also said there is as much as a 40% probability that small rivers and streams will rise, along with the risk of potential roadway flooding, particularly on Friday.

In the central part of the state, areas above 8,000 feet — including Yosemite National Park — could see up to 6 feet of snow, while areas from about 5,000 to 8,000 feet could see up to 4 inches of rain. The Central Valley floor could see 1½ inches of rain, and flooding along the Merced River is possible.