Snowfall Thursday along the Ortega (74) Highway between Lake Elsinore and San Juan Capistrano prompted the California Highway Patrol to shut down the 30-mile corridor.
The hard closure went into effect about 4:20 p.m., after CHP officers encountered heavy snow on segments of the two-lane route, which connects Riverside and Orange counties via the Cleveland National Forest.
Vehicles coming through the passage and entering San Juan Capistrano were caked in snow, according to the CHP.
The National Weather Service issued a winter storm warning early in the afternoon, advising that frozen precipitation could drop as low as 1,500 feet.
The Ortega crosses multiple hills above that elevation.
Several accidents were reported, and the CHP said a number of motorists spun out while driving the route, leaving them stranded on the shoulder of the road, unable to get traction in the slush.
Caltrans had not established snow-chain requirements for travel on the highway, which has been closed in the past due to inclement weather causing rock and mudslides.
Officers established closures at Grand Avenue in Lake Elsinore and Nichols Road on the Orange County side.
The corridor was expected to reopen after Caltrans crews run snow plows over the road to clear it, probably sometime after 8 p.m., according to reports from the scene.