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California faces new cycle of storms with escalating flood danger

A major atmospheric river is set to slam into the state starting Thursday, with heavy rain and high snow levels

Kelly Hatfield digs his truck out at his home in Truckee, Calif., on Friday. (Josh Edelson for The Washington Post)
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Much of California enjoyed a relative lull Wednesday ahead of what could be another series of strong storms in a winter that has seen a seemingly never-ending cycle of them. After an approximately two-week period in which atypical cold brought snow to unusually low elevations along with massive mountain snowfall, California is about to transition to a new phase of warmer storms with higher snow levels and rising flood risk.

The next storm is set to arrive in the form of an atmospheric river — extremely moist storms common to the West Coast that can tap into tropical moisture — on Thursday and last into the weekend. A flood watch is in effect Thursday afternoon through Sunday morning for much of Northern and central California, where heavy rain and melting snow could cause flooding at elevations below 4,000 feet. The watch includes the entire Bay Area, which was seeing only light showers Wednesday morning.

The National Weather Service placed parts of Northern and central California under Level 3 out of 4 flood risk, with the potential for 6 to 8 inches of rain across portions of the central California coast and Sierra Nevada below snow levels.

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“Rainfall totals will range from 1.5 to 4 inches. Locally up to 6-8 inches over favored peaks and higher terrain of the Santa Lucia Mountains where prolonged moderate to heavy precipitation and higher rain rates are currently forecast. Preexisting saturated soils will not be able to absorb excess rainfall,” the National Weather Service wrote in a forecast discussion.

Flooding is a major concern, as many rivers are running well above normal across central and Northern California, including the Merced River at Stevinson, Calif., which is expecting a record crest of 74 feet on Saturday.

Wind is a concern, as well. A wind advisory was issued for Thursday afternoon into Friday afternoon for many of the same areas across Northern and central California due to the possibility of 50 mph gusts that could blow down trees and power lines.

Sierra snow and rain

More snow was falling across the northern Sierra Nevada on Wednesday morning after up to 16 feet fell in two weeks. Snow levels are forecast to rise to elevations of 2,500 to 5,000 feet Thursday, and then to 6,000 to 8,000 feet Friday through Sunday.

Concerns were growing that the moderate to heavy rain expected Thursday into the weekend could be absorbed by the snow, making the snow even heavier. To avoid collapsed roofs, officials urged the removal of snow from roofs if safe to do so. Melting snow that refreezes on roofs can also result in ice dams, buildups of ice that can prevent water from draining and lead to a leaky roof.

Other safety tips included unclogging gutters and storm drains; checking tires, wiper blades, fluids and lights; having an emergency supply kit and chains; and avoiding mountain travel.

The Central Sierra Snow Lab reported that as of Wednesday morning, it had received more than 50 feet of snow (603 inches) so far this season, the fifth most on record.

More rain, less snow in Southern California

After unusually low-elevation snow in Southern California in recent weeks, this next storm is set to bring mainly rain to levels below 10,000 feet. The rain is forecast to be heaviest Thursday night into Saturday, with 4 to 8 inches possible along the coast and foothills in San Luis Obispo County, and 2 to 4 inches elsewhere in San Luis Obispo and Santa Barbara counties. Both counties could see stream and river flooding, especially the Sisquoc and Salinas rivers, from the combination of rain and significant snowmelt.

Lesser rainfall amounts of 0.75 to 1.5 inches are forecast for Los Angeles and Ventura counties, except 1 to 2 inches in the coastal mountain slopes.

Along with snow above 10,000 feet, there is a risk of avalanches above 5,000 feet.

A new cycle of storms

This next storm could be just the beginning of a new sequence of atmospheric rivers that could bring repeating rounds of rain and snow to California in the next couple of weeks, much like the parade of storms that slammed the state late December into January.

The atmospheric river arriving Thursday afternoon and night is expected to bring its heaviest precipitation through Friday afternoon. More moderate precipitation is expected over the weekend, before another atmospheric river hits Monday night into Tuesday.

Longer-range computer simulations — which are less certain — show the possibility of storminess every few days into the third week of March.

The consistent supply of precipitation this winter comes in contrast to the feast-or-famine nature of last winter, when an extremely wet and snowy December was followed by one of the driest months of January and February on record, sending the state even deeper into multiyear drought.

After all the rain and snow since late December this winter, the percentage of the state experiencing at least moderate drought has fallen to just under 50 percent, water levels in major reservoirs are nearing 100 percent of normal, and snowpack has reached 154 to 230 percent of normal. The concern now shifts to the risk of flooding, as warmer storms bring more rain and melting snow.