SAN DIEGO -- California was going from cold to colder Friday as a chill expected to bring the biggest mercury dips in years descended on much of the state.

While the coldest spots of the Bay Area could drop into the high 20s -- meaning a frost warning and precautions for those with tropical outdoor plants -- forecasters with the National Weather Service said Friday evening that they do not anticipate wet weather that would add to problems.

The lowest temperatures in the area are forecast for Morgan Hill, Concord and Livermore, all of which may dip into the 20s through the weekend. Temperatures in Walnut Creek are anticipated to drop to 30 degrees, while San Jose and Fremont should be a few degrees warmer as the Peninsula and Oakland remain in the mid-30s.

The weather service issued a freeze watch in effect through Sunday morning, which is expected to be the chilliest morning of the cold snap. "A clear and cold night will allow temperatures to drop into the upper 20s and lower 30s across the Santa Clara Valley," the advisory says.

Forecaster Steve Anderson said temperatures in the Bay Area will start warming up after Monday, and "should be back to the normal for this time of the year by Thursday."

That normal is a high of 55 to 60 degrees, with overnight lows in the low 40s, Anderson said.

In Southern California, the Grapevine section of Interstate 5 north of Los Angeles was partially reopened Friday after being 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 Friday.

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 subfreezing temperatures. In Modesto, temperatures dipped into the 20s early Friday, 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 Friday 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.

Staff writer Eric Kurhi contributed to this report.