Hammered with record rainstorms and blizzards, nearly half of California is no longer in a drought, and the rest of the state saw dramatic improvement over the past week, federal scientists reported Thursday.

Overall, 49 percent of the state is now drought free, the highest level since April 2013, according to the U.S. Drought Monitor, a weekly study by the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture and the University of Nebraska, Lincoln.

A year ago, only 5 percent of the state was classified as not in drought conditions.

Of importance: Just 2 percent of California remains in “exceptional drought,” down from 24 percent a week ago, and 64 percent at this time last year.

“We’re finally seeing enough precipitation falling to make a significant dent,” said Richard Tinker, a meteorologist with the National Weather Service who compiled the report. “These are tangible improvements. There’s a difference between above-normal and ridiculous, which is what you’ve seen recently.”

The entire northern half of California, from San Francisco Bay to the Oregon border, is now classified as drought-free. In Thursday’s report, 6.4 million more acres — an area 20 times the city of Los Angeles, and much of it in the Sierra Nevada south of Lake Tahoe — was declared drought free this week. Large sections of Southern California and the San Joaquin Valley that have been the hardest hit during the five-year disaster and the slowest to emerge also saw drought conditions ease, although not eliminated, according to the report.

The remarkable turnaround has been driven by a blitzkrieg of powerful winter storms rolling in off the Pacific. Over the past week, Los Angeles, Santa Barbara and Orange counties received 5 inches of rain — their wettest week in seven years — sparking flooding, mudslides and power outages.

The Bay Area is seeing its wettest January since 1998, according to the National Weather Service, with many cities at triple their normal monthly precipitation.

Through Thursday morning, Big Sur Station had received 43.41 inches of rain since Oct. 1, more than the area’s entire yearly average rainfall of 41.91 inches. Similarly, Yosemite National Park had received 44.07 inches since Oct. 1, more than its annual yearly average of 36.73 inches, with eight months still left in the rain year.

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In the Sierra Nevada, a new round of blizzards closed highways and piled up immense amounts of snow. Kirkwood ski resort near Lake Tahoe on Wednesday reported receiving 9 feet of new snow over the past week, while Mammoth Mountain said it had been smothered in 20 feet since New Year’s Day, breaking records for the snowiest month there on record.

Overall, the statewide Sierra snowpack — which provides one-third of California’s water supply — was at 189 percent of its historic average on Thursday. More importantly, it is already at 107 percent of the April 1 historic average, considered the key annual measurement, with two months to go.

State officials continued Thursday to urge caution.

“All this rain and snowfall will undoubtedly have a positive effect on the drought, but we just don’t know yet for certain what that total impact will be,” said Doug Carlson, a spokesman for the California Department of Water Resources. “Something we do know is that groundwater takes a long time to replenish once it has been depleted, and it has been severely depleted in some areas of the Central Valley.”

With major reservoirs around California nearly full or already spilling — the two largest in California, Shasta and Oroville, are both 81 percent full, and Hetch Hetchy is 88 percent full — Silicon Valley’s largest water provider on Tuesday night loosened some of its drought rules.

The board of the Santa Clara Valley Water District, based in San Jose, voted to continue asking the public to conserve water by 20 percent compared to 2013 levels. But with its 10 reservoirs in Santa Clara County at 161 percent of the historic average for January and its groundwater levels steadily recovering, the board dropped its call for the 13 cities and private water companies that it supplies to use mandatory rules to meet the drought target.

Instead, the district said that voluntary rules are acceptable now, and it no longer expects cities and water companies in Santa Clara County, such as San Jose Water Company, to impose drought surcharges or penalties on customers who use large amounts of water, given the improving conditions.

John Tang, a spokesman for San Jose Water Company, said Thursday that the company is drafting a request to the state Public Utilities Commission to drop its drought surcharge program and it expects approval in a matter of weeks, rather than months.

Meanwhile, the administration of Gov. Jerry Brown, who declared a statewide drought emergency in January 2014, has said it will likely wait until April to decide whether to lift that declaration or amend it to apply only to the driest parts of the state.

Tinker, in writing this week’s Drought Monitor, noted that despite the deluges of January, some parts of California, such as the east side of the San Joaquin Valley, continue to have badly depleted groundwater.

“The deepest wells may not respond to the recent inundation for many more months,” he noted.

Each week, the scientists who write the drought monitor assign six levels of drought intensity: no drought, abnormally dry, moderate drought, severe drought, extreme drought and exceptional drought. They analyze soil moisture, stream levels, rainfall totals, snowpack, reservoir levels and other measurements in all 50 states, along with reported observations from more than 350 expert contributors around the country.

Much of California will receive a break from the rain through the weekend. The Bay Area forecast calls for sunny skies and temperatures in the 50s and 60s every day until Tuesday, with more storms likely Wednesday through Friday.

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