CALIFORNIA (BLOOMBERG) - Southern California utilities could cut power to more than 200,000 homes and businesses on Christmas Eve as a possible wind storm raises wildfire risks from the Central Valley to the Mexican border.

The blackouts, which have nearly doubled in scope since Monday (Dec 21), could affect more than 600,000 people based on the size of the average household. California utilities have increasingly resorted to switching off power lines in advance of high winds rather than risk live wires falling into dry brush and sparking fires.

Sempra Energy's utility serving San Diego County warned that it may need to switch off electricity to about 31,000 homes and businesses starting Wednesday, with the blackouts perhaps lasting into Christmas morning. Edison International's Southern California utility may cut power to about 170,000 homes and businesses across seven counties, from the hills north of Los Angeles east to the Mojave Desert.

A large winter storm that will bring snow across the Great Plains and drive warm air and rain into the Northeast is leaving a vast area of high pressure in its wake across Nevada and Utah. That's creating the perfect conditions for dry, gusty winds - known as Santa Ana winds - to sweep across Southern California, potentially fanning wildfires.

A red flag fire warning has been issued for Ventura and Los Angeles counties, extending south toward the Mexican border, the National Weather Service said. Nearly 7.2 million people will face critical fire danger through Thursday, including in Riverside, Oxnard and San Bernardino, the US Storm Prediction Centre said.

While December is typically one of California's wettest months, a La Nina in the Pacific Ocean has shifted weather patterns, starving Los Angeles of rain through the fall and keeping fire danger high.

Los Angeles International Airport has had only 0.11 of an inch of rain since October, according to the National Weather Service. All of California is abnormally dry, and drought covers more than 95 per cent of it, according to the US Drought Monitor.

At the same time, Southern California is suffering through its worst wave yet of coronavirus infections, with officials begging residents to stay home as much as possible. Many of those homes may now lose power leading into the holiday, if the weather forecast holds.