A moisture-laden storm with high-elevation rains is heading toward the Mother Lode, and it could bring as much as 4 inches of rain to Sonora and 7.5 inches to Yosemite Valley by Thursday.

Rain levels are expected to rise as high as 8,000 feet by Wednesday, prompting concerns for melting snowpack and flooding in Calaveras and Tuolumne counties. A flood watch has been issued for the Central Sierra foothills from 4 a.m. Wednesday to 4 p.m. Thursday and it includes the Mother Lode.

The approaching storm is a Gulf of Alaska system dropping down and tapping a lot of moisture across the eastern Pacific, tapping the good moisture feed from the ocean, Karl Swanberg, a National Weather Service forecaster in Sacramento, said in a phone interview.

“For Sonora, starting tonight, we have slight chances of rain developing in the evening, and more steady rain after midnight,” Swanberg said Tuesday afternoon. “Upper 40s for a low.”

Overnight lows for the Sonora area will be 10 to 20 degrees warmer than they were during low-elevation snow storms early this week and last week. It’s supposed to be about 45 degrees early Wednesday, 50 degrees early Thursday, 40 degrees early Friday and back down to 35 degrees early Saturday.

Travel could be treacherous in parts of the Stanislaus National Forest during this possible rain-on-snow event, Forest Service communications staff said Tuesday.

Forest administrators urge people to be prepared for heavy rain and snow if they go anywhere in the Stanislaus National Forest this week.

“The National Weather Service is forecasting five to six inches of precipitation across the forest,” Forest Supervisor Jason Kuiken said in an announcement. “This heavy rain event should begin as snow and rain, but freezing levels are expected to rise to as much as 7,000 feet, which means some of the snowpack could thaw and cause some pretty significant flooding.”

With the wet start to February, forest surface soils are already saturated and many trees’ root systems are vulnerable. Strong winds and accumulations of rain and snow increase the risk of falling trees, both hazard trees and green trees. Falling trees may impact roadways, trails and parking areas.

Kuiken also reminded visitors to beware when traveling near burned areas of the forest, including the Donnell Fire and Ferguson Fire scars. These areas cause concern due to lack of vegetation to help hold soils in place, Kuiken said.

Moderate river rises are expected starting midweek. The Merced River in Yosemite Valley is not expected to reach flood stage at Happy Isles or Pohono Bridge, according to the federal California Nevada River Forecast Center.

More precipitation could come to the Central Sierra later this week and this weekend. Chances of showers in the Sonora area are 80 percent Thursday night and into Friday.

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