SAN DIEGO » The California Highway Patrol today partially reopened a 40-mile stretch of Interstate 5 north of Los Angeles that was closed by snow as a cold snap sent temperatures plummeting throughout the state.

The CHP began escorting southbound cars through the Grapevine section of the major north-south route after a 16-hour shutdown that prompted hundreds of truckers and other motorists to pull over and spend the night by the side of the road.

"There must have been 1,000 Mack trucks lined up," Heidi Blood, 40, said early today.

Blood and her three youngsters had been visiting Orange County and set out at 4:30 a.m. today to head back north to their home in Kentfield when they found the road shut down.

"I usually watch the news but I went to a spin class instead. I learned my lesson," she said.

Blood had to give an insulin shot in the car to her 13-year-old blind, diabetic dog, Barney.

To pass the time, the family watched movies and read on their iPads, turning on the car every 30 minutes to use the heater.

"We’re watching ‘Nannie McPhee’ in the car right now," Blood said. "I only have enough juice for another three hours."

The shutdown severed a key link between California’s agricultural Central Valley and Los Angeles.

The highway through Tejon Pass rises to 4,100 feet in the Tehachapi Mountains and frequently is shut down in winter by blowing snow and treacherous black ice on the roadway.

California remained in the grip of a cold snap that’s expected to last through the weekend.

Morning temperatures fell into the 20s and 30s in many areas, and much lower in the mountains. A low of 12 degrees was recorded in the Big Bear mountain resort east of Los Angeles.

Strawberry growers covered their crops, while San Diego zookeepers turned on heaters for the chimpanzees.

In Sonoma County, homeless shelters started handing out extra warm clothes to protect the least fortunate from below-freezing overnight temperatures.

Central Valley citrus growers braced for three days of sub-freezing temperatures. In Modesto, temperatures dipped into the 20s early today, several degrees colder than usual for this time of year.

Napa, in the wine country, and Sacramento, farther north, both recorded 27.

High temperatures in the Central Valley over the weekend were forecast for 50 degrees, 3 degrees lower than normal for mid-January, according to the National Weather Service.

In the south, forecasters warned that a low pressure trough sinking over San Diego County and parts of neighboring Orange County could keep nightly temperatures below the freezing point in coastal areas, the low deserts and inland valleys, threatening orange and avocado orchards and other sensitive plants. The coldest nights were expected to hit today and Saturday.

Farmers were prepared to pull out giant fans to circulate the air and keep it from settling on their citrus trees, said Eric Larson of the San Diego County Farm Bureau.

"These guys are going to be up all night watching thermometers," Larson said.

Workers at SeaWorld in San Diego planned to crank up the heat for their macaws, toucans and parrots. San Diego zookeepers were also heating rooms for chimpanzees, apes and other tropical animals.

"They’ll probably be huddling together and not be in areas where people will be able to see them," said zoo spokeswoman Christina Simmons.

Associated Press writer Jason Dearen in San Francisco and Robert Jablon in Los Angeles contributed to this report.