Sierra snowpack survey prompts drought watchers to be optimistic

A series of sopping storms taking aim on drought-ravaged California this week will provide much needed relief as state reservoirs continue to rise, and the once-abysmal Sierra snowpack is on track to return to normal.

While the snow at Phillips Station off Highway 50 was more than 5 inches below normal on Tuesday, snow surveyors for the California Department of Water Resources were optimistic when they trudged up to manually measure the snow’s water content.

At the measuring site about 90 miles east of Sacramento, a core sample of snow had a water content of 6 inches - 53 percent of average for that location. Statewide, snowpack levels were at 70 percent of average, despite a wetter-than-normal October and December.

But that’s all set to change after a particularly favorable string of wet-weather systems began walloping the Sierra with heavy snow this week.

Tracks left behind after the California's Department of Water Resources conducted a snow survey at Phillips Station, California, on Tuesday January 3, 2017. Frank Gehrke the chief of cooperative snow surveys reported that the samples taken revealed that the snowpack is at 53% of average for this time of year at this location. less Tracks left behind after the California's Department of Water Resources conducted a snow survey at Phillips Station, California, on Tuesday January 3, 2017. Frank Gehrke the chief of cooperative snow surveys ... more Photo: Michael Macor, The Chronicle Buy photo Photo: Michael Macor, The Chronicle Image 1 of / 14 Caption Close Sierra snowpack survey prompts drought watchers to be optimistic 1 / 14 Back to Gallery

“It looks like we’ve got a series of wet, cold storms stretching out through the next week,” said Frank Gehrke, the chief of cooperative snow surveys for the department. “I can see us being at average once the storms move though.”

One week ago, the measuring site at Phillips Station was completely bare, and two years ago, the site had no accumulated snow for the entire year.

The statewide Sierra snowpack — California’s frozen water supply that usually provides about 30 percent of the state’s water requirements — was more than a quarter of the way to its peak average, measured on April 1.

Forecasters are still waiting to see what the final two of the three wettest months of the year may bring.

One of those months is already delivering.

The Bay Area is poised to get pounded with rain over the next week as several storms come in behind Tuesday’s soaker. Sustained winds were set to peak around 30 mph with gusts up to 45 mph during the downpours.

On Tuesday night and Wednesday morning, forecasters were expecting 5 to 8 inches of rain in the area of the massive Soberanes Fire that burned in Monterey County from July to October. They were also forecasting 4 to 5 inches of precipitation in the area of the Loma Fire that swept through parts of the Santa Cruz Mountains in September and October, warning that the rain could fall at a rate of an inch per hour during the heaviest period before dawn.

Flash-flood watches were issued for both burn areas, and officials warned of possible mudslides.

A brief dry period will occur late Thursday and Friday before a larger storm blows in over the weekend, said Brian Garcia, a meteorologist for the National Weather Service.

“That one could be a doozy,” Garcia said.

In the weekend storm, the Big Sur coast and the mountains near Santa Cruz and Napa could get from 15 to 17 inches of rain, Garcia said.

In the Santa Cruz Mountains, the Soquel, Aptos and Corralitos creeks “can expect to get close to flood stage, if not a little over,” Garcia said.

The weekend drenching could produce flooding around the Bay Area, the Central Valley and in the Sierra foothills, forecasters said.

All the rain moving into the state translates to several feet of snow around Lake Tahoe.

Four to 5 feet of fresh powder may fall at the 6,000-foot lake level. More than 2 feet are expected to blanket some mountain passes, producing near whiteout conditions for drivers on Highway 50 and Interstate 80.

“It’s really wringing out the rain over the Sierra,” said Jan Null, a meteorologist with Golden Gate Weather Services.

Parts of the mountains could get a quarter of the season’s average rainfall this week alone. And because the coming storms are relatively warm, they’ll hold more moisture — producing about twice the water content in snow than a colder arctic blast.

Some computer models, Null said, show 15 inches of rain in the Sierra, or about an inch of rain for every half-foot of snow.

But the snowpack is only one indicator of the state’s hydrological health.

Climatologists also look at reservoir levels, groundwater and overall average rainfall for the year.

“We’ll have to wait and see what is recorded,” said Doug Carlson with the Department of Water Resources.

As of Tuesday, Lake Shasta — California’s largest water reservoir — was 118 percent of average and Lake Oroville was at 91 percent of average.

But this week’s rain will need to continue for any chance of stopping a sixth year of drought.

“It may be raining hard, but the big picture is we need to see sustained improvement up and down the mountain range,” Carlson said.

Evan Sernoffsky and Steve Rubenstein are San Francisco Chronicle staff writers. Email: esernoffsky@sfchronicle.com and srubenstein@sfchronicle.com Twitter: @EvanSernoffsky @SteveRubeSF