California braces for atmospheric river set to slam state

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 flood-related 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.

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