Storms that soaked California during the first half of January did more than bring tons of snow to Sierra Nevada ski resorts. They also helped to significantly boost the state’s water supplies.

Over the three weeks from Jan. 1 until this Tuesday, 47 key reservoirs that state water officials closely monitor added 580 billion gallons of water — as much as roughly 9 million people use in a year, according to an analysis by this newspaper.

The combined storage in the reservoirs, which include critical components of California’s water supply like Shasta Lake, Folsom, Hetch Hetchy and San Luis Reservoir, has expanded from 15.96 million acre feet on New Year’s Day to 17.74 million acre feet now. Each acre foot is enough water to flood an acre of land a foot deep, or 325,851 gallons.

Nearly all of the major reservoirs around California are now at or above their historical averages, swelled by runoff that continues to pour in from brimming creeks, rivers and rising water tables.

Meanwhile, the statewide Sierra Nevada snow pack, which provides about one-third of California’s water, on Tuesday was at 114 percent of normal, up from just 69 percent on Jan. 1.

“January brought us some good snow and precipitation,” said Chris Orrock, a spokesman for the state Department of Water Resources. “We’re hoping for more of the same in February and March.”

How much water has been captured? Shasta Lake near Redding, the state’s largest reservoir, at 35 miles long, has risen 25 feet since Jan. 1, and is now 61 percent full, or 92 percent of its historical average for this time of year.

To be sure, the rain has stopped. Dry, balmy weather is forecast over the next week, with more rain possible the first few days of February. This month’s wet weather could give way to sunny, warm conditions through the rest of the winter.

But for now, the powerful atmospheric river storm that roared in from the Pacific Ocean last week, following other storms and dumping half a foot of rain in some places, and five feet of snow in the Sierra, has washed away a disappointing November and December. The trend is boosting the spirits of water managers, whose memories are still fresh of California’s brutal five-year drought from 2012 to 2017. Across the Bay Area, water agencies say their supplies are in good shape, and they are not expecting summer shortages.

“It looks like a normal winter now. And normal is good,” said Toby Goddard, water conservation manager for the city of Santa Cruz. “January has been a nice boost.”

This week is an important winter milestone, water experts say.

On average, half of California’s annual precipitation falls in December, January and February. With that span now half over for this winter season, Jay Lund, director of the Center for Watershed Sciences at UC Davis, noted that one key water indicator is exactly halfway full.

The eight-station Sierra Nevada Index, a daily report that measures how much precipitation has fallen in eight key watersheds near some of the largest reservoirs in Northern California, has so far received 26 inches this winter, Lund noted. The historic average is 52 inches.

“We are on track to average, which is good. The historical average is better than the recent average,” Lund joked, referring to the five-year drought.

Lund noted that one of the state’s primary reservoirs, Oroville, still hasn’t fully recovered from a disaster two years ago. In February, 2017, the spillway at Oroville Dam, the tallest dam in the United States, crumbled during heavy storms, and authorities evacuated 188,000 residents, fearing an uncontrolled release of water.

On Tuesday, despite the lake level having risen 39 feet in the past three weeks, Oroville was only 37 percent full, or 57 percent of its historical average. State officials have kept it low during repairs.

One trend of note this year: Southern California has received significant rain after several years of lagging behind the north. While San Francisco’s rainfall was 87 percent of the historical average through Tuesday, Los Angeles’ was at 147 percent.

There’s plenty of winter left, said Orrock, with the state Department of Water Resources.

“We’re only about halfway through our three wettest months,” he said. “We have to wait and see what Mother Nature brings us for the rest of the winter. The only thing constant about California’s climate is that it’s so variable.”

Mercury News researcher Leigh Poitinger contributed to this report.