Officials urged residents of about 500 homes in rural mountain communities to evacuate Wednesday as they battled a huge California wildfire roughly the size of the city of Chicago.

The so-called Day Fire burning much of Lockwood Valley is so big, it's actually creating its own weather — fiery twisters known as 'fire tornadoes.' As of midday Wednesday, the fire was 42 percent contained. It's charred over 144,880 acres — about 226 square miles — and cost the state of California more than $45 million to combat.

Nearly 4,000 firefighters, aided by bulldozers and aircraft, concentrated on the northwestern edge of the fire. It was ignited on Labor Day by a homeless man burning debris and dry brush in Los Padres National Forest.

Firefighters were taking advantage of low winds Wednesday and were dug in at the fire line trying to control the blaze. Every available federal aircraft — including 19 air tankers and 16 helicopters — were dropping water and fire retardant on the flames to aid the ground effort.

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"The problem is we've had extremely dry fuels," fire spokesman Dan Bastion said Wednesday. "Brush and trees will be ignited by the heat of the fire, so there's a domino effect going on."

After jumping fire lines on Tuesday, the blaze slowed in the cool overnight weather. But erratic winds could cause it to flare later, said U.S. Forest Service spokesman Larry Comerford.

"They'll come from one direction one time and then they'll whip the other direction," he said.

Fire trucks, bulldozers and water tankers guarded homes within sight of the massive fire. No occupied homes had been lost to the fire, which burned some 70 miles northwest of Los Angeles.

Six unoccupied buildings were destroyed, including a modular home, a cabin, barns and camp trailers, Bastion said.

Authorities recommended evacuation of several remote Ventura County communities arrayed along roads west of Interstate 5.

"It sounded like a jet engine," Forest Service firefighter Greg Valencia said of a towering wall of flame he saw blow past a home, leaving it unscathed.

On the ground, crews staged equipment at the widely spaced homes for structure protection. At almost every house there was at least one engine and a few firefighters clearing brush, hosing down roofs and decks. A bulldozer plowed a firebreak around one home.

The new fire activity was a surprise setback for firefighters. The blaze had been moving relatively slowly with the dying of weekend Santa Ana winds that had the potential to greatly spread flames but did not.

But on Tuesday, the fire suddenly jumped lines and burned 1,000 more acres.

The run prompted authorities to recommend evacuation of Lockwood Valley, Pinon Pines, Pine Mountain Club and Lake of the Woods — remote Ventura County communities west of Interstate 5.

Two barns, an unoccupied cabin and two camp trailers burned, Comerford said.

No occupied homes have been lost to the fire about 70 miles northwest of Los Angeles, which has become one of the largest and longest-burning wildfires in state history.

Pat Martin, 61, fled her home in Lake of the Woods. Spending the night at a Red Cross shelter set up at a high school gymnasium, she feared her home would be damaged or destroyed.

"When you're this old, how do you just start over?" Martin said.

Dale King, 57, was helping firefighters clear brush around his home when flames roared across the edge of his 5-acre property.

"The house was vibrating. It sounded like a freight train in the yard," said King.

"It was scary. I've never seen a wall of fire 200 feet high moving right at you and there's nothing you can do," he said. "It makes you believe in the Lord, I'll tell you."

Gov. Arnold Schwarzenegger has declared a state of emergency for Ventura County, clearing the way for government assistance with costs related to the fire.

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FOX News' William LaJeunesse and The Associated Press contributed to this report.