Another atmospheric river system has set its sights on California, raising major concerns about flooding and structural damage as warm rain is expected to fall this week on top of the state’s near-record snow pack, forecasters say.
“It now seems increasingly likely that a potentially significant and very likely warm river atmospheric event is likely to hit part of northern or central California sometime between Thursday and Saturday,” UCLA climate scientist Daniel Swain said at a briefing Monday.
Last week, the chance that such a system would develop was about 20%. By Monday, the odds had increased to “7 or 8 in 10, if not higher, for a warm river atmospheric event of any magnitude,” Swain said. At least one more storm could follow this month.
Atmospheric rivers rise up and over mountain ranges and drop their moisture.
(Paul Duginsky / Los Angeles Times)
The forecast comes as California is mired in a remarkably deep snow pack amid one of its wettest winters on record. In early January, the state was pummeled by a series of nine atmospheric river storms, causing levee breaches, widespread flooding and nearly two dozen deaths.
In recent weeks, strong winter storms dropped piles of fresh powder over the Sierra Nevada and other areas, including the mountains of Los Angeles and San Bernardino counties, where some residents are trapped behind feet of snow.
Officials said the bounty put a dent in the state’s extreme drought and offered some hope for strained water supplies after three bone-dry years. But heavy snow can also become a hazard if it encounters warm rain that causes it to melt too quickly.
“We will see rain on top of snow, and at elevations of, say, 2,000 feet to about 4,000 feet, a lot of that snow will melt,” said Carlos Molina, a meteorologist with the National Weather Service in Hanford, California. “We are basically going to lose a lot of snow that fell from the previous storms. We are looking at possible flooding.”
Indeed, the greatest likelihood of flood-related impacts is in lower elevations with unusually deep snow, Swain said. Small rivers and streams in those areas will see significant potential for drainage problems, as will some urban areas, especially in places where the drainage channels are already clogged by snow.
There may also be problems at elevations above 5,000 or 6,000 feet, he said. While the snowpack in such areas is likely too deep and cold to be melted by the oncoming storm, it could become heavier as it absorbs more water. That can cause roof collapses and other structural problems.
“If you can go out and remove some snow from structures that might be vulnerable, do it,” Swain said. The state has already seen a wave of roofs collapse from rising snow, including a grocery store delivering critical supplies in snowbound Crestline.
Scott Rowe, a meteorologist with the Sacramento Weather Service, said the incoming low-pressure system is coming from the north but is expected to be related to “very warm” subtropical moisture coming in from Hawaii. Such storms are sometimes referred to as a Pineapple Express and have been known to drop heavy moisture in California.
“We’re essentially transitioning from one storm track to another where the moisture is coming from a warmer, juicier location,” Rowe said.
While there is increasing certainty that such a system will come, the precise timing, location and effects will become clearer as the week progresses, forecasters say.
“Strap your seatbelt, it’s going to be quite a weather ride,” the weather service wrote in its forecast for the Central Coast and San Francisco Bay Area. Up to 15 cm of rain could fall in coastal areas from Thursday morning to Saturday afternoon, and up to 2 cm of rain could fall in other parts of the region, the agency said.
The weather service in Sacramento also warned of several inches of rain and higher snow levels. It also said there is as much as a 40% chance that small rivers and streams will rise, along with the risk of possible road flooding, especially on Friday.
In the central part of the state, areas over 8,000 feet — including Yosemite National Park — can see up to 6 feet of snow, while areas about 5,000 to 8,000 feet can see up to 4 inches of rain. The Central Valley floor can see 1½ inches of rain and flooding along the Merced River is possible.
While most of the storm is currently expected to affect northern and central California, southern California could also feel impacts, including possible flooding of rivers and small streams in parts of Santa Barbara and San Luis Obispo counties, David said. Gombert, a meteorologist with the weather service in Oxnard.
“The opportunities increase as you go north from Santa Barbara, but we don’t want to completely ignore LA and Ventura because there could be problems there,” he said.
Also concerning is the chance for more rain and snow in the mountains of San Bernardino County, where crews were still struggling to clear roads and help trapped people on Monday. Chances of precipitation there are expected to begin early Friday morning and last most of the day Saturday, said Elizabeth Schenk of the San Diego office of the National Weather Service, which covers the San Bernardino area.
Very high snow levels of 8,000 or 9,000 feet are likely in the area, “meaning most of the precipitation that’s going to fall will fall as rain,” she said, including amounts of up to 1½ inches.
“It’s not great,” she added, “because any rain that falls on the snow will accelerate the snow melt in that area, and how much snow they’ve already gotten in the last week and a half, that’s a lot of snow water equivalent.” , so that will potentially lead to a rapidly accelerating snow melt.
While the incoming storm is most likely to have moderate impacts, Swain said he feared it could prepare the state for bigger problems. Forecasts show that at least one more system could follow at close range.
“There’s a chance of bigger flooding if we get more successive warm atmospheric rivers after the first one,” he said. “There’s about a 1 in 3 chance, so the chance isn’t that small right now.”
There are also other factors that can increase the dangers. Scars from wildfire burns — such as those from the Dixie and Caldor fires, which burned in the Sierra in 2021 — can make runoff worse because they are known to have waxy soil that repels water. They also lack vegetation, making them more susceptible to debris flows. That could affect some downstream watersheds and tributaries during the storm, particularly along the Sacramento and San Joaquin river systems.
“We haven’t actually had a big, warm atmospheric river on top of a big snowpack in these regions since we had these big wildfires, so we could do some real-time science experiments over the next few weeks,” Swain said. .
In 2017, heavy rainfall and erosion damaged spillways at Oroville Dam, one of California’s largest reservoirs, causing more than 100,000 people to flee a potential wave of floodwaters. The crisis was averted, but left many on edge about what similar bursts of moisture could do.
Sean de Guzman, chief of water supply forecasting at the California Department of Water Resources, said Oroville — as well as the state’s other largest reservoirs, Lake Shasta and Lake Trinity — will continue to exist well below capacity and have more room to fill.
“We should see a greater amount of runoff, but nothing that the reservoirs shouldn’t be able to accommodate,” he told reporters Friday.
Earlier that day, the department’s third snow survey of the year found that statewide snow cover was 190% of the average for the date — just below a record set in the winter of 1982-83. In the southern Sierra, snow cover was near a record 263% of the average.
The deep snow came as a surprise after fall forecasts tended toward a drier-than-normal winter. Over the weekend, San Bernardino County officials acknowledged they were unprepared for the historic storms that swept through the mountains.
As of Monday, tens of thousands of Californians in several snowy counties were without power.
