Firefighters struggling with a wind-driven brush fire that has forced thousands from their homes in Santa Barbara County Tuesday are hoping that a bout of rain from a cold front moving across the state will help bolster their efforts by evening.

The Cave Fire broke out just after 4 p.m. Monday near East Camino Cielo and Painted Cave Road in the Los Padres National Forest. The blaze ignited amid erratic sundowner winds that sent flames rushing downhill toward communities in Santa Barbara and Goleta, spurring evacuations.

Steep, rocky terrain and critically dry grass and brush have stymied firefighters’ efforts. The fire behavior overnight was so erratic that when the winds – gusting up to 50 mph – would let up, the blaze would change direction and race back uphill toward firefighters, Los Padres National Forest Fire Chief Jim Harris said.

“The Cave Fire is burning under some of the toughest firefighting conditions anywhere in the world,” he said. “We’ve experienced several offshore wind events at this point, and that has just dried the fuel bed out to the point where we’re seeing the fire behavior we saw last night.”

The blaze had swelled to 4,262 acres with no containment late Tuesday morning. About 600 firefighters were on scene, defending homes from the advancing flames. Ten fixed-wing tankers and nine helicopters were expected to arrive by midday to aid in the effort, said Santa Barbara County Fire Capt. Daniel Bertucelli.

As the fire grew late Monday, mutual aid started to arrive from neighboring counties to help the local and national forest firefighters. The Ventura County Fire Department sent two strike teams — about 10 fire engines — Monday evening, and the Los Angeles County Fire Department sent a Firehawk helicopter, which is capable of performing nighttime water drops.

Engine teams were embedded in neighborhoods to defend homes in the fire’s path, and bulldozers and hand crews were at work throughout the night digging into the dirt to create containment lines around the perimeter of the blaze.

The fire prompted Santa Barbara County officials to declare a local emergency and request that Gov. Gavin Newsom proclaim a state of emergency for the region. The fire is “causing conditions of extreme peril to the safety of persons and property within Santa Barbara County,” the county wrote.

About 2,400 homes bordered by East Camino Cielo, Ontare Road, Foothill Road/Cathedral Oaks Road and Fairview Avenue were placed under mandatory evacuation orders. An evacuation warning was issued for the area north of Foothill Road and from Ontare Road east to Gibraltar Road.

Shortly after noon, about 4,000 residents in the far east area of the evacuation zone – the largest section of displaced homeowners – will be allowed to return. Officials, however, warned that many would be arriving to homes without electricity. Large swaths of Santa Barbara County, including unincorporated areas between Goleta and Santa Barbara, along Highway 154 through Mission Canyon and parts of Summerland and Carpinteria, are experiencing power outages because of the fire.

It was not clear when evacuation orders for the rest of the region would be lifted.

No injuries have been reported, and no homes have been destroyed, but one outbuilding was damaged, officials said. It was not clear how the blaze started.

Firefighters expect their efforts throughout the day will be made more difficult by significant southwest winds blowing through the region. The fire has chewed through dense, old brush in an area that hasn’t burned since the Painted Cave Fire in 1990. That fire, which authorities said was the result of arson, destroyed hundreds of homes in the area and caused $250 million in damage. Harris called the area around San Marcos Pass where the winds funnel through the hillsides a “nightmare spot” for a blaze.

“We’ve had a lot of fires in Santa Barbara County in the front country over the last 15 years or so,” Harris said. “This is one of the last slots of old vegetation. It’s something we’ve all talked about for years and years.”

Fire crews may be getting some help from Mother Nature by Tuesday evening, when a storm is expected to arrive in the area, Bertucelli said.

“What’s working in our favor is we’re getting rain tonight,” he said. “It’s definitely going to affect our fire behavior. ... It’s going to diminish the fire.”

The storm, which is expected to drop about an inch of precipitation on the fire area, brings its own challenges in the burn area in the form of possible debris flows. During an early morning briefing, fire officials warned crews to be cautious when the rain started and to have a plan in case roadways were washed out.