It’s supposed to rain and snow up and down the Central Sierra later this week, and forecasters are calling it “a major pattern change” as well as “the first widespread rain event of the season” with cooler temperatures.

From Thursday morning to Saturday morning, some parts of the Central Sierra could see as much as 0.25 inch of precipitation, depending on how far south the approaching storm tracks. The weather system is likely to be cooler because it’s coming out of the Gulf of Alaska.

Predictions call for rain and snow starting Thursday, with the heaviest precipitation Thursday night into early Friday, forecasters in Sacramento say. Snow levels are expected to be above major pass levels initially, with some snow possible at Ebbetts Pass and Sonora Pass by late Thursday.

“It’s a trough of low pressure moving into the region from the Gulf of Alaska,” Jim Mathews, a National Weather Service meteorologist, said Monday. “So it’ll be a little cooler, and temperatures will be a little more like autumn in the foothills. Up at the pass elevations they may get a little snow.”

The new water year started Oct. 1 and, as of Monday afternoon, records showed zero precipitation for major Mother Lode watersheds that include Calaveras Big Trees State Park in the Stanislaus River basin and Hetch Hetchy in the Tuolumne River basin.

The 2016-17 water year was the second-wettest on record with 72.7 inches of precipitation. Water year 1982-83 was the wettest with 77.4 inches, and 2014-15 was one of the driest with 19 inches total.

What “cooler temperatures” will mean this week depends on how high you are in the foothills or the mountains.

In the Sonora area this week, elevation 1,785 feet, daytime highs are expected to drop from the low 80s today to the low 60s by Friday. At elevations like Mi-Wuk Village, 4,675 feet, it will go from the low 70s to the low 50s. At Sonora Pass, more than 9,600 feet above sea level, expect highs in the mid-50s today falling to the mid-30s by Friday.

Overnight lows in the Mother Lode and the rest of the Central Sierra will fluctuate from the mid-50s tonight in Sonora to the mid-40s Friday night, the high 40s tonight to the low 40s Friday night in the Mi-Wuk area, and from 30 degrees today to low 20s Friday night at Sonora Pass.

Some Mother Lode residents and travelers have already seen signs of the approaching winter. Early snowstorms Sept. 20, which came before the first day of autumn, and Oct. 3 prompted Caltrans District 10 personnel to close Sonora Pass temporarily.

Weather should warm up for the weekend in the Mother Lode foothills, with Saturday sunny and mostly clear and highs in the high 60s, and more sunshine with highs in the 70s for Sunday.

Most reservoirs on major Mother Lode rivers were holding more than half of capacity as of Monday. Pardee on the Mokelumne River was 92 percent full and Camanche was 75 percent full. Donnell Reservoir on the Stanislaus River was 61 percent full and Beardsley was 56 percent full.

New Melones, the fourth-largest capacity reservoir in California, was holding 2,019,320 acre-feet from the Stanislaus watershed, making it 84 percent full.

Hetch Hetchy on the Tuolumne River in Yosemite National Park was close to 90 percent full. Don Pedro, the sixth-largest capacity reservoir in the state, was holding 1,662,631 acre-feet from the Tuolumne watershed and it was 82 percent full. McClure on the Merced River was 70 percent full.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or (209) 588-4585. Follow him on Twitter @GuyMcCarthy.

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