The freeway was closed between Gorman and Frazier Mountain Park Road Thursday afternoon and traffic was backed up for miles in each direction after as much as 5 inches of snow fell in the region.
The California Highway Patrol reopened the southbound lanes about 8 a.m. Friday but, as a precaution, CHP officers escorted traffic through the portion of the freeway known as the Grapevine.
CHP said it would begin to escort vehicles in northbound lanes but officials were not sure what time that would be, the Los Angeles Times reported.
In the Angeles National Forest, Mountain High Resort in Wrightwood reported 3-5 inches of snow and Running Springs in San Bernardino County reported 4 inches, the National Weather Service said.
Along the coast, high tides and strong winds Friday morning could cause minor flooding in low-lying areas, forecasters said.
Looking at a broader weather picture, AccuWeather.com said part of the United States is targeted for a cold wave during the second half of January as waves of frigid air move southward across North America from the North Pole.
Forecasters said the timing and location of the cold conditions is uncertain but they expected to see cold air spreading from central Canada into the third week of January.
"The indications are that the initial thrust of the cold will be directed over the West and northern Plains first, with subsequent thrusts of cold air pointed toward the Northeast and the Upper Midwest later in January and into February," said Paul Pastelok, a long-range weather expert with AccuWeather.com.