By Brandon Lowrey
LOS ANGELES, Jan 11 (Reuters) - An Arctic air mass sent temperatures plunging across California, forcing the 17-hour closure of a key interstate highway through the mountains north of Los Angeles and threatening citrus crops in the state's vast central valleys, authorities said on Friday.
Temperatures throughout the state fell by as much as 20 degrees Fahrenheit (11 degrees Celsius) below normal, allowing snow to accumulate at elevations as low as 1,500 feet (457 meters), the National Weather Service reported.
Although no further snow was expected to fall over the weekend, temperatures were expected to continue to drop on Saturday before gradually warming into next week, the weather service said.
About 4 inches (10 cm) of snow fell on Thursday on a winding stretch of the Interstate 5 known as the Grapevine, which passes through mountains between Los Angeles and Bakersfield, prompting authorities to shut down the north-south artery for 17 hours beginning Thursday afternoon.
Stranded motorists packed motels on either side of the Grapevine overnight. California Highway Patrol officers reopened the roadway at about 9 a.m. local time on Friday and began escorting cars along the treacherous route, CHP Officer Mike Harris said.
Precise weather conditions along the Grapevine during the freeze were not recorded, but neighboring areas posted temperatures in the mid-20s Fahrenheit, said Stuart Seto, a forecaster for the National Weather Service.
In the San Joaquin Valley, a major agricultural area, low temperatures in the teens threatened to kill citrus crops, which are in danger of perishing whenever the mercury falls below 28 degrees, said meteorologist Jeff Barlow said.
The Weather Service alerted farmers to the danger so they could take precautions, but there may still be a heavy loss.
"They won't be able to save all of the crops," Barlow said. "This is going to be a pretty significant freeze event for the central California citrus crops."
In normally temperate San Diego, temperatures were expected to reach 39 degrees on Friday night, closer to the record low of 34 degrees set in 1888 than the normal 59 degrees, said Robert Balfour, a National Weather Service forecaster.
"The rest of the country is probably laughing at us, saying, 'You call that cold?'" Balfour said.