A Siberia Express artic cold front sent thermometers plummeting Friday night throughout California, beginning three days of hard freezes threatening $1 billion worth of citrus still on the trees in both coastal and central Valley groves.

The temperature was expected to dip to the upper teens in the coldest areas of the valley Friday. In the citrus areas it was projected to drop to the middle 20s.

However, the coldest temperatures are expected Saturday and Sunday night when the thermometer may dip into the single digits.

It is the coldest weather to hit the state in eight years. A Christmas 1998 freeze inflicted more than $550 million in losses to the state’s citrus crop.

According to Dennis Plann, Fresno County deputy ag commissioner said the freeze forecast this weekend mirrors the 1998 freeze.

California has had very little rain in the past month, and the ground is dry. Humidity levels of more than 80 percent and fog are the norm for this time of year. With no rain to dampen the ground, the humidity levels were as low as 14 percent Thursday ahead of the storm.

Citrus growers are irrigating to try and stave off the frost. Wind machine are also running, however with no inversion expected wind machines and helicopters can increase the damage rather than help, according to one University of California grower advisory. Growers are also burning things like peach pits and other trash along the edge of citrus groves to generate heat.

Joel Nelsen, California Citrus Mutual president, estimated that 75 percent of the navel orange crop is still on the trees, along with 72 percent of the Valley lemon crop. Harvest of some mandarin varieties has not yet begun, he said, and all young Valencia orange crop — the summer orange — is still unharvested.