Swaths of California are once again bracing for days of heavy rain and snow, with flooding and avalanches threatening areas of the state.
The severe weather, caused by back-to-back atmospheric rivers starting on Thursday, comes on the heels of recent winter storms that dumped several feet of snow in parts of the state, leaving many stranded while others are still trying to dig their way out.
The atmospheric rivers, sometimes described as “rivers in the sky,” were expected to blanket higher elevations with heavy snow and to bring heavy rain across portions of northern and central California, with lighter rainfall ain the Los Angeles area. With the potential for up to seven inches of rain in some spots by the end of the weekend, forecasters said flooding was possible and that rain could cause mudslides. The rain is especially concerning in areas that have already received a near-record run of snow this winter, including at low elevations in the Sierra Nevada foothills.
The first atmospheric river is expected to move into California on Thursday, and another is forecast to arrive on Monday, said Bob Oravec, a forecaster with the Weather Prediction Center.
Mr. Oravec said that, unlike other storms this year, more of the precipitation is expected to be rain, prompting concerns about snowmelt in areas like the Sierra Nevada, where there is already snowpack.
“Besides the heavy rain that falls and runs off, there’s going to be a lot of snowmelt,” he said. “Flooding is definitely a concern as we go forward.”
“It’s going to be a heavy precipitation event,” Mr. Oravec added.
For higher elevations, the National Weather Service said there could be “significant avalanche danger” in places above 5,000 feet. Up to two feet of snow was possible at similar elevation levels in the Sierra Nevada, and areas south of Interstate 80 in Northern California could record up to five feet of snow, forecasters said.
The Weather Service in Sacramento said heavy snow on top of previous accumulation could cause roofs to buckle or collapse. After recent snowfall, forecasters instructed Northern California residents to consider removing the snow from their roofs to prevent buildup. Forecasters were also concerned that melting snow could flood roadways.
Bianca Feldkircher, a Weather Service meteorologist, said that Californians should prepare emergency kits and consider avoiding travel over the weekend.
“If you can postpone, postpone,” Ms. Feldkircher said in a video on Twitter.
Across the state on Wednesday, officials were taking steps to mitigate flooding and preparing to assist those who might end up trapped on roadways. The California Governor’s Office of Emergency Services said it was positioning swift-water rescue teams in Fresno and Tulare Counties in central California. The sheriffs’ offices in both counties issued evacuation warnings on Tuesday, urging residents to gather their belongings in case an evacuation is ordered.
The California Department of Water Resources said in a statement that it would release water from the Lake Oroville reservoir and that it would monitor the incoming severe weather in case more water needed to be released.
Mr. Oravec, of the Weather Prediction Center, said that managing reservoirs ahead of severe weather is important because “there’s huge amounts of snow that’s going to eventually melt and go into these reservoirs.”
“They’re trying to manage their water levels in these reservoirs to prevent more catastrophic flooding later,” he said.
Atmospheric rivers are not uncommon in California, but the state has been hit with a number of powerful storms recently, including one atmospheric river in January that prompted evacuation orders for more than 40,000 Californians and left more than 220,000 utility customers without power. That storm was part of a three-week succession of atmospheric rivers that dumped heavy rain across the state, damaging infrastructure and setting off flooding. The problems have been compounded with each new storm.
The severe weather events in California continued into February, when storms brought heavy flooding to Los Angeles County and whiteouts at higher elevations. Last week, Gov. Gavin Newsom declared a state of emergency in 13 counties affected by winter storms that unloaded as much as 10 feet of snow in parts of Southern California, leaving tourists and residents stuck for days.
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