Late-season storm systems coming to the Central Sierra will bring chances of significant rain and snow in as many as three episodes over the next 10 days: Wednesday night to Friday morning, Saturday to Monday, and Tuesday to Thursday next week.

A moisture-laden Pacific jet stream building off the California coast packs the potential to unleash #StormsForDays, a forecaster posted to Twitter before noon Tuesday.

Sonora could get 2 to 3 inches of rain by Friday, then 1 to 2 inches of rain over the weekend, and Climate Prediction Center forecasters say more wet weather is possible May 21 through 23.

Asked how far south the first storm system will track into the Central Sierra, Idamis Del Valle, a National Weather Service meteorologist in Sacramento, said it will bring snow along Interstate 80, expecting 6 to 8 inches, with greater amounts farther south, like a foot at Carson Pass, and 12 to 18 inches at Sonora Pass.

A winter storm watch has been issued for snowstorms above 6,000 feet, including along highways 4, 108 and 120, from late Wednesday to early Friday. From the foothills to the high east edges of Calaveras and Tuolumne counties, the heaviest precipitation amounts are expected between Wednesday night and Thursday evening.

In Sonora, the late spring weather change could see daytime highs drop from the mid-70s Tuesday to the mid-50s by Thursday. Overnight lows in Mother Lode foothill towns along Highway 49 are expected to drop into the 40s through at least the end of this week.

Forecasters are warning motorists to be prepared for slick pavement and ponding on roads, heavy rainfall at times, breezy conditions and gusting winds that could bring down tree limbs and contribute to power outages, and possible chain controls at elevations down to 6,000 feet. Thunderstorms will be possible Thursday.

The approaching systems are expected to be freezing cold at higher elevations, and therefore unlikely to speed up springtime snowmelt cycles that have begun charging Central Sierra creeks, rivers and waterfalls. Overnight lows at Sonora Pass this week include cold-enough-to-kill temperatures as low as 21 degrees.

Significant rain-and-snow storms are rare in the primary watersheds of Calaveras and Tuolumne counties this time of year. Most winter-season precipitation comes to the Central Sierra between October and April. May has been comparatively dry over the past five decades, except the 1997-98 water year.

As of Tuesday, the Stanislaus River and Tuolumne River watersheds had received 44.1 inches of rain since the water year began Oct. 1, according to a five-station index that includes Calaveras Big Trees and Hetch Hetchy. That’s 118 percent of average for the date May 14.

Most major reservoirs in the Mother Lode were more than four-fifths full as of Tuesday. Donnells and Beardsley on the Middle Fork Stanislaus River were 94 percent full and 83 percent full, respectively.

Farther down the Stanislaus River, New Melones, the Golden State’s fourth-largest capacity reservoir, was 82 percent full. In the Tuolumne River watershed, Hetch Hetchy in Yosemite National Park was 70 percent full, Cherry was 77 percent full, and Don Pedro, the state’s sixth-largest reservoir, was 89 percent full.

The high Central Sierra passes connecting the Mother Lode with U.S. 395 and the eastside Sierra remain closed. Expected opening dates for Ebbetts Pass on Highway 4 and Sonora Pass on Highway 108 have not been announced.

Caltrans District 10 crews previously hoped to get Ebbetts and Sonora passes open by May 24, the Friday before Memorial Day, but late-season storms like the ones possible over the next 10 days will likely push pass openings back.

Our goal each year is to try to open the 4 and 108 routes before Memorial Day weekend if possible given the importance to the local economy to communities on both sides of the Sierra,” Warren Alford with Caltrans District 10 in Stockton said Tuesday afternoon. “That said, we needed everything to go our way to make it this year and Mother Nature is not playing nice right now, so there is no current ETO — Estimated Time of Opening.”

Highway 120 in Yosemite National Park, also known as Tioga Road and Tioga Pass, remain closed with no opening dates in sight.

State Water Resources staff say the 2018-19 winter snowpack peaked March 31 and it was the fifth-largest on record. As of Monday, snowpack in the Central Sierra was 133 percent of normal for the date May 13. Precipitation for the 2018-19 winter season is still far below the two wettest winters on record, 1982-83 and 2016-2017.

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

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