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California storms left behind a ‘generational snowpack.’ What that means.

The snow is a boon for the state’s water supply but could pose a flood risk as the season progresses

Snow falls at Mammoth Mountain Ski Area in Mammoth Lakes, Calif., on Sunday. (Samantha Deleo/AP)
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California’s mountain snowpack is the largest it’s been in decades, thanks to a barrage of atmospheric rivers in late December into January. The snow is a boon for the state’s water supply but could also pose a flood risk as the season progresses.

Measurements completed last week show that Sierra Nevada snow water content is rivaling or outpacing the 1982-83 season, the biggest snow year in the past 40 years. Up to two feet of additional snow fell on the region this weekend.

“[It’s] fair to say this is a ‘generational snowpack,’ as many of the younger folks haven’t seen snow like this before, or if so, not in a long time,” Benjamin Hatchett wrote in an email. He is an assistant professor of atmospheric science at the Desert Research Institute in Reno.

Statewide snowpack is double the norm for the time of year. Some locations with longer records are ranking in the top 3 for early February snow since the 1940s and ’50s, according to Hatchett.

Most important, at least a full season’s worth of snow has fallen so far across the Sierra, and more storms could arrive in February and March.

“This is good news, but we now hope we'll continue building, or at least maintain, that snowpack such that we don't lose it to earlier-than-normal melt, as this implies that we can't store it as effectively in our reservoirs,” Hatchett wrote.

A ‘big boost’ for reservoirs

The recent extreme wet period put a sizable dent in the state’s drought.

Major reservoirs, many of which were severely depleted before winter started, are now near their historical average for the date. Those gains have prompted officials to increase planned water allocations to farms and cities served by the State Water Project, a storage and delivery system that stretches from the Sierra Nevada to Southern California. Lake Oroville, the backbone of the project, has added more than 1.2 million acre-feet of water since late December.

“Our reservoirs have gotten a really big boost from the nine atmospheric rivers that we had,” said Ellen Hanak, vice president and director of the Public Policy Institute of California’s Water Policy Center.

32 trillion gallons of water in 3 weeks: California storms by the numbers

California was lucky, she said, that last month’s strong atmospheric rivers were colder storms, with much of the moisture falling as snow, which will further fill reservoirs as it melts.

Mountain snow is crucial for capturing water from large storms because reservoirs can’t be completely full in winter — there must be space for flood protection from any incoming storms.

“It is basically free supplemental storage of water that you don’t have any place to put in the winter,” Hanak said.

However, the largest reservoirs still have a fair amount of empty space to fill, particularly those in far Northern California, which saw less rain and snow compared to areas farther south. Shasta Lake and Trinity Lake, for example, are only 57 percent and 32 percent full, respectively. And it will take more than one good year to replenish depleted groundwater and for drought-stressed forests and ecosystems to recover, she said.

Flood and drought concerns ahead

While this year’s huge snowpack is on balance very good news, there are some potential concerns as the season unfolds, including ongoing flood risk.

“A risk in having a large snowpack is that melt may occur faster than we’d ideally like,” said Erica Siirila-Woodburn, a hydrology research scientist at Lawrence Berkeley National Laboratory, in an email. “In some ways, big snowpacks could be a ‘ticking time bomb’ if warmer temperatures, higher rain-snow elevations, and rain-on-snow events cause accelerated snowmelt.”

A recent study by Hatchett, Siirila-Woodburn and others found that California’s wildfire-scorched mountainous areas are more susceptible to rapid snowmelt during winter dry spells.

Despite the current snow bounty, the region continues on a path toward a warmer, low-to-no-snow future with climate change, meaning that “more precipitation will fall in the form of rain and not snow and in more punctuated events like atmospheric rivers, creating less seasonal water storage and magnifying water stresses,” Siirila-Woodburn wrote.

Officials are warily eyeing recent dry weather with fears that the storm cycle could shut off again, as it did in January 2022. And any heat waves this spring could evaporate some of that snow water.

“There is a lot more that needs to play out over the course of the next several months to really capture our full water supply picture here in California,” Karla Nemeth, director of the Department of Water Resources, said last week during a monthly snow survey.

While there is no clear signal for wet or dry conditions for the coming months, a recent drought outlook suggests that a swath of the state could be drought-free by the end of April.

“The ideal season spreads storms out over the whole rainy season,” said Hanak of the Public Policy Institute of California. “It would be good to get more precipitation and it would be good if it stays cool — that’s what water managers are hoping for.”