A month into California’s peak storm season, the lack of wet weather is beginning to weigh on the state’s water supply.
The snowpack in the Sierra and southern Cascades, which provides as much as a third of the water used by California cities and farms, is about 55% of average for this time of year. It hasn’t been this low at this time since 2017, when the state was emerging from a five-year drought.
State water officials are scheduled to lug their snow gauges into the mountains Wednesday and confirm the measurement in the first snow survey of the season — the unofficial time for Californians to start worrying about water for the coming year.
Two more historically wet months lie ahead, and a few big storms could start to rebuild the snowpack. But the bleak December picture, on top of last year’s dry winter, is renewing concerns about drought and prompting water agencies to begin calling for people to conserve more.
“In California, it’s not if the next drought is coming, it’s when,” said Chris Orrock, spokesperson for the state Department of Water Resources, which conducts the snow survey. “We’re having below-average dry conditions now, and we’re continuing to monitor it.”
The U.S. Drought Monitor, a weekly report issued by the federal government and University of Nebraska at Lincoln, considers 95% of California to be in at least a moderate state of drought. That’s the state’s driest condition since March 2016.
California has always bounced between wet and dry times, and water managers have learned to deal with such swings. But the variability has grown with climate change, meaning there’s a higher probability that a given year will be either extremely wet or extremely dry.
“The chances of losing a storm on the shoulders of the season have gone up,” said LeRoy Westerling, a climate scientist at UC Merced, referring to the possibility of shortened wet seasons. “We’ve seen years where we lost our fall and spring storms.”
The missing wet fronts in autumn also have contributed to California’s record wildfire seasons.
This fall was one of the driest that Northern California has seen. For much of the season, a mass of high-pressure air hung off the West Coast, pushing the storm track to the north.
The lack of fall precipitation rounded out an extraordinarily dry calendar year in many parts of the state. San Francisco has recorded just 7.76 inches of rain since Jan. 1, putting this calendar year on track to be the third-driest since 1850, according to the private Golden Gate Weather Services.
In recent weeks, the high-pressure system over the Pacific that kept storms from hitting the coast has begun to weaken, and California has started to get a little more rain and snow. Much of the Sierra and north state even saw a white Christmas.
The National Weather Service is calling for a small chance of showers in the Bay Area on Wednesday night and likely on-and-off rain starting Friday and continuing at least through early next week.
Long-term forecasts revolve around La Niña. The climate pattern characterized by cool temperatures in the tropical Pacific tends to maintain a northward push on storms. The Climate Prediction Center, a division of the National Oceanic and Atmospheric Administration, says that as a result of La Niña, drier-than-average weather is probable in California over the next three months in all but the northernmost counties.
As it stands, the state’s water supplies are running behind average. California’s 154 largest reservoirs contain about 82% of the water they should hold at this time of year, according to the Department of Water Resources. Shasta Lake, the largest reservoir, is at 72% of its historical average. Lake Oroville, the second largest, is at 57%.
San Francisco’s water system, centered on Hetch Hetchy Reservoir in Yosemite, is an anomaly, holding slightly more water than average for this point in the season, according to the city Public Utilities Commission.
The snowpack over the next couple of months, which come spring will begin to melt, will determine how well storage holds up at California’s reservoirs. Snow levels in the north state, home of California’s biggest reservoirs, are most pivotal.
As of Monday, the snowpack in the northern Sierra and southern Cascades was 56% of average for this time of year. It was 59% in the central Sierra and 33% in the southern end of the range.
The low numbers have prompted the Department of Water Resources to project that water agencies that get water from state-run reservoirs will receive only 10% of their requested allocation next year. Although the allocation is subject to change, many water agencies aren’t taking chances.
The Zone 7 Water Agency, which relies on state supplies to provide water to more than a quarter million people in Pleasanton, Livermore and Dublin, is beginning to ask residents to go a little lighter at the tap. The supplier has enough water in storage to make it through whatever comes this winter, but water managers don’t want to get to the point where they’re scraping the bottom of the reservoir.
“Based on 2020 being a dry year, we’ve started light conservation messaging,” said Valerie Pryor, general manager for the district. “Obviously, two back-to-back dry years and multiple dry years is not what we prefer.”
Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander
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December 30, 2020 at 07:00PM
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Dismal California snowpack is bad sign for water supplies - San Francisco Chronicle
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