(Adds comment, updated reservoir measurement from U.S. Bureau of Reclamation, paragraphs 6-7)
By Steve Gorman
LOS ANGELES, Feb 10 (Reuters) - The most powerful winter storm to hit California in more than a year dumped several feet of snow in the high Sierras and soaked lower elevations with rain over the weekend, easing drought conditions but leaving the state thirsting for more, officials said on Monday.
The Pacific storm doubled the moisture content of the snowpack in the Sierra Nevada mountain range, a key gauge of the state's principal source of surface water, said Dave Rizzardo, the chief of snow surveys for the state Department of Water Resources.
But the 3 inches (8 cm) of additional water content measured on Monday still leaves California at just 20 percent of where the state's snowpack should be by Apri1 1, the traditional end of the winter rainy season, Rizzardo said.
Neither Rizzardo nor other experts would venture to say precisely how much more precipitation the state needs to substantially relieve the drought. But several more storms of similar magnitude would be required.
"We've dug ourselves quite a hole during the past couple of years, so it takes more than one good storm to get us out of it," Rizzardo said.
Louis Moore, a spokesman for the federal Bureau of Reclamation, said, "We'd need another one of these storms weekly for the next several weeks to get back to where we want to be with our water supply." The bureau controls irrigation supplies to much of California's vast Central Valley heartland.
The rainfall runoff likewise added to the state's network of man-made water storage, with one badly depleted northern California reservoir, Folsom Lake, rising 17 feet (5 metres) by Monday, Moore said. But that still leaves it less than a quarter full.
The state's two largest reservoirs, at Shasta Dam and Lake Oroville, each grew by a percentage point, leaving both at just over one-third of their respective capacities.
"It makes a dent, but there's still a long way to go," state Water Resources Department spokesman Ted Thomas said.
Coming off its driest year on record, California began 2014 with surface and groundwater supplies extremely low and no sign of a break in the weather, prompting Governor Jerry Brown to declare a statewide drought emergency.
He urged citizens to reduce water consumption by 20 percent voluntarily, while irrigation districts and municipalities up and down California braced for the sharpest cutbacks in deliveries from state and federal water projects.
Just last week, agriculture officials said farmers in California, which grows half the nation's fruits and vegetables, were expecting to idle some 500,000 acres (200,00 hectares) of cropland this year in what would be a record production loss for the top U.S. farm state.
One storm alone cannot turn the tide, but it was a welcome development, officials said.
Snowfall from the storm, which began late on Friday and ran through the weekend, averaged 3 to 5 feet (.9 to 1.5 metres)across the Sierras and neighboring mountains, with up to 6 feet (1.8 metres) measured at the highest peaks, meteorologists said.
The Sacramento Valley received 2 to 4 inches (5 to 10 cm) of rain, with 10 to 16 inches (25 to 40 cm) soaking the foothills and lower elevations of the Sierra. The coastal mountains north of San Francisco got 10 to 12 inches (25 to 30 cm), forecasters said.
Before Friday, any precipitation moving in from the Pacific had been diverted north into Canada by a ridge of high pressure that was parked for more than two months along the U.S. West Coast, meteorologists for the National Weather Service said.
The latest storm, which they described as an "atmospheric river," crashed through the barrier as it was starting to weaken and pushed it to the south in a shift that forecasters believe may pave the way for wetter weather in the short term.
"There's always the possibility of the ridge returning, but the latest model that we can see kind of keeps the ridge relatively weak and not re-establishing itself to the extent that it was through December and January," said Roger Gass, a National Weather Service meteorologist in Monterey.
Eric Kurth, a meteorologist in Sacramento, concurred, saying another Pacific storm expected to sweep the Pacific Northwest by midweek could reach well into northern and central California this weekend. (Reporting by Steve Gorman; Editing by Cynthia Johnston and Mohammad Zargham)