A storm system coming out of Idaho and Nevada is bringing chances of rain and thunderstorms to the Mother Lode on Friday and into the weekend, forecasters say.

As of Thursday afternoon, forecasts for the 10 a.m. Saturday start of the 62nd Annual Mother Lode Roundup Parade showed Sonora highs expected in the 70s and a 30 percent chance of showers or thunderstorms after 11 a.m. Saturday. There are also chances of rain during the Roundup Rodeo at Mother Lode Fairgrounds on Saturday afternoon, and again after the Roundup Mothers Day Breakfast, Cowboy Church, Kiddie Rodeo and Rodeo events Sunday.

The best chances for precipitation in Calaveras and Tuolumne counties are on Friday, said Brendon Rubin-Oster, a National Weather Service meteorologist in Sacramento. Rubin-Oster described the storm system as an inside slider moving southwestward over the Sierra Crest and then south down the length of the state toward northern Baja California. Remnants of the storm may contribute to unsettled weather, rain showers and thunderstorms in the Central Sierra foothills this weekend.

At higher elevations in east Tuolumne County, including Pinecrest, Long Barn, Hetch Hetchy and Yosemite Valley, there’s a winter weather advisory for possible thunderstorms, gusting winds, and rain or snow showers above 8,000 feet through 11 p.m. Friday. Rain showers are likely Saturday at Hetch Hetchy and in Yosemite Valley.

“People out there in the mountains should probably start their hikes in the morning to be on the safe side,” Rubin-Oster said. “Thunderstorms will start popping up in the early afternoon if they do develop.”

Passes on Highways 4 and 108 are still closed. Glacier Point Road in Yosemite opens Friday.

Caltrans is still targeting May 24, two weeks from Friday, to have Ebbetts Pass and Sonora Pass open. There are no target dates for opening Tioga Pass and Tioga Road, also known as Highway 120, inside Yosemite National Park. An undated photo posted this week shows snow removal machines on deep snow on Tioga Road near Siesta Lake, about 15 miles east of Crane Flat.

Yosemite National Park staff say workers intend to open Glacier Point Road at 9 a.m. Friday and the store at Glacier Point is expected to open at 10 a.m. Four Mile Trail remains closed from Yosemite Valley to Glacier Point due to snow, ice and other hazards.

The springtime weekend crush is on in Yosemite, where visitors last weekend endured bumper-to-bumper traffic and full parking lots by 1 p.m. Saturday. Park staff are advising visitors to arrive before 9 a.m. or after 4 p.m. Parking areas are expected to fill early each day.

“Visitors coming to Yosemite from Friday through Sunday will experience periods of heavy visitation,” Scott Gediman with park public affairs said. “Visitors should be prepared to wait in long lines at the park entrance stations. All park visitors are encouraged to pack your patience.”

Continuing snowmelt combined with extra rain is expected to bring the Merced River to within a half-foot of its 10-foot flood stage at Pohono Bridge by Friday morning. Forecasters say possible combinations of locally heavy rainfall and rapid snowmelt at higher elevations in the Central Sierra could accelerate runoff and generate daily water rises on creeks and rivers down into the foothills Friday and Saturday.

Hikers, boaters and campers up and down the Mother Lode should remain alert this weekend for rising waters on creeks, streams and rivers, and they should be prepared to move to higher, drier ground if flooding happens.

“Keep in mind that rivers pose dangerous, life-threatening hazards at this time of year due to their swift currents and very cold water temperatures,” National Weather Service forecasters said Thursday. “Fifteen minutes or less exposure to the icy cold waters of a river can lead to hypothermia. Strong undercurrents can carry even an experienced swimmer into dangerous parts of the river. Don’t become the next drowning victim or the victim of a water rescue!”

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