A late barrage of snow forced the shutdown of a 35-mile stretch of Highway 58 between Mojave and Bakersfield.
The snow brought hundreds of vehicles to a stop and sent the patrol and road crews scrambling to the scene, where they were helping drivers to slowly exit near Tehachapi. No injuries were reported.
The highway remained closed early Wednesday and officials don't know when it will reopen.
"We've had no reports of people being stuck overnight. Between us and Caltrans, we've been patrolling all night," said Steve Dutcher, a public safety dispatcher for the California Highway Patrol.
A similar scene played out shortly about 300 miles to the north, where dozens of cars were either stuck in the snow or involved in accidents near the rural community of Sonora. About 50 to 75 vehicles became stranded or were in collisions on Highway 49 and nearby roadways when it started snowing heavily in the Sierra Nevada foothills, said CHP Lt. Scott Clamp.
"Travelers were just not prepared," Clamp said. There were a handful of minor injuries, but no major injuries, he said.
Interstate 5 in the Grapevine area was closed early Wednesday and the Cajon Pass on Interstate 15 was open despite sporadic snow.
The storm from the Gulf of Alaska brought the first significant rainfall to the region in several weeks, the National Weather Service said.
Periodic showers, including hail, hit the San Francisco Bay area in time for the morning commute, while new snow fell in the Sierra Nevada, where ski resorts around Lake Tahoe were expecting up to 8 inches.
Flurries were spotted high in the hills in Oakland and in neighboring Berkeley, said Rick Canepa, a weather service meteorologist in Monterey. The top of Mt. Hamilton near San Jose and the tips of Mt. Diablo in the East Bay also got a dusting.
In the Sacramento area, a tornado with wind speeds between 40 to 70 mph was spotted north of Red Bluff shortly after 1:30 p.m., according to the weather service. It caused little or no damage.
More rain was forecast for the San Francisco Bay area and for parts of Southern California. Authorities warned of more hazardous conditions for drivers on mountain and interior valley roads into early Wednesday, when the storm was expected to move east.
Even though San Francisco saw highs in the 70s last week, California has had a colder-than-normal winter overall.
"We went from about 10 degrees above normal this past weekend to 10 degrees below today," said Austin Cross, another weather service meteorologist based in Monterey. "We're usually somewhere in the 60s, temperature-wise, at this time of year."
San Francisco has gotten nearly 14 inches of rain since October, or about 85 percent of its normal rainfall during the fall-winter season, Cross said. Oakland received 83 percent and San Jose had about 80 percent, he added.
The far-reaching storm brought snow and colder-than-average temperatures to Arizona. Flagstaff public schools and Northern Arizona University were closed Wednesday because of the winter storm.
The metropolitan Phoenix area was expected to get up to a half-inch of rain as the storm moves inland, and temperatures were forecast to drop to the mid-50s, about 15 degrees below average for this time of year.
The storm's eastward march also was expected to drop as much as nearly a foot of snow as it moves into parts of Nevada, Colorado and Utah.
Associated Press writers Shaya Tayefe Mohajer in Los Angeles and Terry Collins and John S. Marshall in San Francisco contributed to this report.