Two storm systems are aiming a one-two punch at the Mother Lode and the rest of the Central Sierra this weekend and early next week, with the second, colder storm capable of bringing snow down to 1,000-foot elevations.

The first storm system moving in Friday, Saturday and Sunday is coming from the central Pacific Ocean, so it’s a bit warmer, and forecasters are calling it a moderate atmospheric river storm, Karleisa Rogacheski, a meteorologist with the National Weather Service in Sacramento, said Thursday in a phone interview.

The second storm is coming down from the Gulf of Alaska, and it’s expected to be much colder, bringing snow levels down as low as 2,000 feet to 1,000 feet Monday. Mapmakers say Sonora’s elevation is 1,785 feet, Columbia is 2,139 feet, and Jamestown is 1,427 feet.

Both storms are expected to track far enough south to bring precipitation to Calaveras and Tuolumne counties, including 3 to 5 feet of snow at Carson, Ebbetts and Sonora passes, and 3 to 4 feet at Tioga Pass, by late Sunday or early Monday, Rogacheski said.

Later Monday the second storm system could bring 1 to 4 inches from 2,500 feet down to as low as 1,000 feet, 4 to 10 inches above 2,500 feet, and 1 to 2 feet at Carson, Ebbetts and Sonora passes, and 6 to 8 inches at Tioga Pass, Rogacheski said.

There’s a winter storm watch from Friday evening to late Sunday night for the Central Sierra and Northern Sierra above 5,000 feet. And there’s a flash flood watch from Friday evening to late Friday night for Yosemite Valley, other parts of Mariposa County, and the Bay Area.

Overnight lows forecast for Sonora help show how the incoming storms are expected to go from chilly to freezing cold: Friday night’s low is expected around 48, Saturday night 43, Sunday night 39, and Monday night it’s expected to get as cold as 29 degrees Fahrenheit.

The Mother Lode’s primary watersheds, including the Stanislaus River and Tuolumne River basins, have received 18.5 inches of precipitation since the current water year began Oct. 1, which equals 91 percent of average for Jan. 31, according to a five-station state index that includes Calaveras Big Trees and Hetch Hetchy.

In addition, snow survey results Thursday at Phillips Station, about 90 miles northeast of Sonora, showed 50 inches of snow depth and snow water equivalent of 18 inches, which is 98 percent of average. Current snow water equivalent readings from Central Sierra sensors are 102 percent of average for Jan. 31.

Snowpack in the Sierra Nevada is on par with historical averages this time of year, Karla Nemeth, the state Department of Water Resources director. The next two to three months will determine what it means for reservoirs and overall water supply.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or 588-4585. Follow him on Twitter at @GuyMcCarthy.

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