This week’s big storm soaked the Bay Area. It brought blizzards of much-needed snow to the Sierra Nevada. It triggered evacuations in the Santa Cruz Mountains and wrecked Highway 1 in Big Sur.
The soaking system — the region’s first atmospheric river storm of the winter — made an impressive dent in California’s very dry winter. But precipitation totals are still behind historical averages, experts noted Friday. And the state’s water picture, while improved, remains shaky with two months left to go in the winter season.
How much rain comes between now and the traditional end of the winter rainy season April 1 will determine whether there will be summer water restrictions and how moderate or ominous the wildfire season will be.
“We still have February and March,” said Roger Gass, lead forecaster with the National Weather Service in Monterey. “But we’re running out of months to make it up.”
In three days, the storm dumped 2 to 3 inches of rain on Bay Area cities — essentially doubling their rainfall totals so far this winter, from about 20% of the historic average to about 40%, depending on the location. It delivered 6 to 9 inches in the Santa Cruz Mountains and a staggering 16 inches in Big Sur.
“It definitely helped us get into a better situation,” Gass said. “But it obviously doesn’t make up for all the deficit. We wouldn’t want one storm system to do that. We would have had widespread flooding.”
The biggest beneficiary of the powerful system that swept down off the Gulf of Alaska late Tuesday night, blasting the Central Coast, was the Sierra Nevada. The source of one-third of California’s water supply, the statewide Sierra snowpack was 40% of its historical average on Monday. By Friday morning it had jumped to 66%, after more than 6 feet of snow dumped from Yosemite to Lake Tahoe. Some places got more, with 9 feet of new snow at Mammoth Mountain ski resort and 7 feet at Kirkwood and Dodge Ridge.
Another storm is forecast for Monday. Most of the impact will be in the North Bay, where Sonoma County could receive 2 or 3 inches of rain. Bay Area cities are expected to receive less than 1 inch. But the National Weather Service says that storm should bring in 2 feet of new snow to the Sierra, likely pushing the snowpack there up to about 75% of normal. After that, however, the forecast calls for dry weather for the next two weeks.
“Hopefully it’s not the only storm we get in February,” said Chris Orrock a spokesman for the state Department of Water Resources in Sacramento.
Of note: This week’s storm didn’t fill reservoirs. Every winter, most areas need about 12 inches of rainfall before the ground is saturated enough to get large amounts of runoff into the reservoirs.
“The ground has been dry. It really absorbed a lot of the water,” Orrock said. “The next storm might put a little more into the reservoirs and streams. But we’re not seeing that yet.”
Most of the state’s largest reservoirs remain at low — although not dire — levels. The biggest, Shasta Lake near Redding, was 46% full Friday, or 69% of hits historical average for that date. The second largest, Oroville, in Butte County, was 35% full, or 54% of its historical average. Some were in better shape. Lake Don Pedro, east of Modesto, was 68% full, or 99% of normal.
California’s water system is complicated. But it is based on a fundamental truth: Three-quarters of the rain and snow falls in the north, and three-quarters of the people live in the south. Rain is important all over the state. But from a water supply perspective, big winter storms in the north are most important, to fill up the massive reservoirs that provide water to millions of people and farmers in the summer.
To follow up on the 3 day #AtmoshpericRiver event, here are 72 hour rainfall totals across the #BayArea & #CentralCoast. Yes, that is more than 12" at many sites in the Santa Lucia Range above the #BigSur Coast. pic.twitter.com/sBhfbkEI6X
— NWS Bay Area (@NWSBayArea) January 29, 2021
Even with this week’s storms, the “eight station index,” a key measure of precipitation at eight watersheds from Shasta Lake across the Northern Sierra, stood Friday at just 51% of its historic average since Oct. 1.
Bay Area water agencies are watching closely.
The rains added about 5,000 acre feet — enough water for 50,000 people in a year — to the 10 reservoirs run by the Santa Clara Valley Water District in San Jose. Not counting Anderson Reservoir, which is drained for earthquake repairs, the reservoirs were 29% full on Friday, or 61% of normal for this time of year. The district has more than two years’ supply in its underground aquifers, however.
“The water supply situation is not bad. It’s acceptable,” said Bassam Kassab, water supply manager for the district, which serves 2 million people. “But people should always conserve. We have a semi-arid climate in California, and we never know when we are going into another drought.”
The staff at East Bay Municipal Utility District, which provides water to 1.4 million people in Alameda and Contra Costa counties, will re-start meetings of its drought planning committee in February. The district’s seven reservoirs were 72% full on Friday.
“We are definitely in a wait-and-see mode,” said spokeswoman Andrea Pook. “We need more of these storms.”
Last year was dry across Northern California. Rainfall levels were half of normal in most cities. The Sierra snowpack last April 1 was just 54% of normal. A series of freak lightning storms in August sparked the worst fires in recorded state history, burning 4.2 million acres, destroying 10,488 structures and killing 33 people.
With this winter off to another slow start, 95% of California was in at least a “moderate drought” on Thursday, according to the U.S. Drought Monitor, a weekly report from the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture and the University of Nebraska. Those numbers are expected to improve somewhat next week.
Will there be water restrictions this summer?
“My crystal ball is not working today,” Pook said. “We don’t really know what’s going to happen in the next couple of months. We are getting prepared in case we have to go in that direction. Ask me in April.”
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How the atmospheric river storm affected California’s water supply - Pacifica Tribune
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