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The most important snowpack survey of the season in the Sierra Nevada was recorded Tuesday at 161 percent of the historic average, the fourth best reading in 40 years and good news for the entire state.

California’s water supply for the next year is almost in ideal shape, and the rainy season isn’t over, with yet another atmospheric river storm forecast to arrive Friday.

“With full reservoirs and a dense snowpack, this year is practically a California water-supply dream,” Department of Water Resources Director Karla Nemeth said in a statement. “However, we know our long-term water supply reliability cannot rely on annual snowpack alone. It will take an all-of-the-above approach to build resiliency for the future.”

State water officials consider the annual April 1 snowpack reading in the Sierra Nevada to be the most important of the year for planning summer water supplies across California. The April survey typically takes place when the snowpack is at its deepest and the water content, a key indicator for water supply, is at its highest for the season, according to the DWR.

After more rain in the Bay Area and snow in the Sierra Nevada mountains Tuesday, Wednesday is expected to be dry. Thursday, a weak system is forecast to move through the region, just ahead of a weak atmospheric river on Friday that is expected to deliver a good soaking across the Bay Area.

Estimated rainfall totals from the ‘atmospheric river’ range from 1 to 1.5 inches in the North Bay, a half-inch to 1 inch around San Francisco, and one-quarter to three-quarters of an inch in the South Bay, according to Matt Mehle, a meteorologist with the weather service.

Since the start of the water year Oct. 1, San Francisco has received 23.27 inches of rain, just shy of its annual average of 23.65 inches. San Jose has received 14.82 inches (annual average is 14.90) and Oakland 18.57 inches (20.81 average).

Tuesday’s snowpack measurement at the Phillips Station in El Dorado County off Highway 50 was done in front of journalists and broadcast live on the DWR Facebook page. Officials measured 106.5 inches of snow depth; the snow water equivalent, meaning the amount of water in any given area if the snow was all melted, was 51 inches.

“Our April survey is very significant because this is typically when we see the deepest snowpack with the most water content,” said Chris Orrock, a dwr spokesman. “Our water managers use that to judge what type of melt off we’re going to get as we get into the warmer, drier summer months.”

Since 1980, the statewide snowpack through April 1 has measured 160 percent of normal or higher just four times, according to data from the DWR, which conducted Tuesday’s measurement. The largest snowpack over the last nearly 40 years was 1983 at 227 percent; in 2017, the snowpack measured 159 percent of average.

The snowpack is an important factor in determining how the DWR manages California’s water supply. The Sierra snowpack supplies about 30 percent of California’s water needs as it melts in the spring and early summer to meet water demands in the summer and fall.

Every winter, around the start of each month, state water officials and other scientists from more than 50 local, state and federal agencies collect data from more than 300 locations throughout California.

State water officials said this year’s snowpack was fueled by more than 30 “atmospheric river” storms, including six in February that blanketed the Sierra Nevada with as much as 25 feet of snow.

In some years, California only sees six such storms, according to Kristopher Tjernell, the deputy director for integrated watershed management.

“These heavily water-laden storms combined with below average temperatures” have this winter’s robust snowpack, Tjernell said.

However, Tjernell offered a reminder that California is just four years removed from the driest April 1 snowpack on record, just 5 percent of the historic average in 2015.

“These highs and lows are anticipated to be even more extreme as climate change increasingly affects our communities,” Tjernell said.

Tuesday, the state’s largest six reservoirs currently were at between 81 percent (Oroville and Don Pedro) and 132 percent (Melones) of their historical average capacities for this date.

Lake Shasta, California’s largest surface reservoir, was 89 percent full, or 112 percent of its historical average.

Tuesday, state water managers for the first time opened the doors of the newly rebuilt main spillway on the Oroville Dam, which failed two years ago and caused the evacuation of 188,000 people in Butte County.

Lake Oroville, which is nine miles long and a key water supply for California cities and farms, was 81 percent full Tuesday (106 percent above normal for this time of year) and rising. With a series of storms this week, including a weak atmospheric river, state water managers released water to keep space in the lake for additional rainfall and melting Sierra snows later this spring.

Just before 11 a.m., water from Oroville reservoir was released down the main spillway at an estimated rate of about 8,300 cubic feet per second.

Staff writer Paul Rogers contributed to this report.


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