The fire appears to have begun around noon at a home on Butts Canyon Road just north of the Aetna Springs Winery, said Scott Brown, a vineyard manager who lives a few hundred feet from the fire's origin.

Neighbors told him some young men accidentally started the blaze while cutting weeds using a tool with a metal blade, Brown said.

"I don't even let my guys mow the grass when it's like this," Brown said.

He was working about 8 miles away at a vineyard when he got word of the fire, and by the time he got home it had already raced across the hillside and destroyed some Quonset huts, he said.

In addition to being the driest year he can remember, it was 112 degrees in the shade on Monday, said Brown, who's lived in the Pope Valley for 45 years.

Berlant confirmed the person who reported the fire said it may have started with someone cutting weeds. He said investigators were looking into that possibility but had not confirmed that's how the fire began.

General contractor John Taylor lives off Snell Valley Road and received an evacuation order from Cal Fire.

"It's stressful, but that's what you get when you live out in the country," he said. He added he wasn't too scared by the blaze, saying he thought Cal Fire crews were doing an excellent job fighting it.

"Cal Fire is on it," he said around 7:30 p.m., moments before a spotter plane buzzed overhead guiding a large air tanker to the spot where it dropped a plume of red retardant near a house Taylor built in 2008.

"This has been a very fast-moving wildfire that has been driven by the terrain, hillsides, dry conditions and fanned by winds," Berlant said.

Fire destroyed or damaged at least five structures, one of them a home. One building near the sign for Snell Valley Ranch was fully engulfed in flames around 6 p.m.

At one point Cal Fire had been fighting to keep the fire south of Snell Valley Road, but erratic winds and steep terrain helped the fire jump the road and continue north.

That caused Cal Fire officials to temporarily cancel the evacuation of Berryessa Estates, instead telling residents to "shelter in place" until they could safely leave.

Massive plumes of black smoke billowed into the sky from the area of mainly steep, rugged grassland and mixed oak and pine woods. Three helicopters and a half-dozen air tankers attacked the fire from above as several hundred firefighters worked on the ground. The smoke could be seen miles away in Lake and Sonoma counties.

Napa fire officials midday called for two immediate strike teams from Sonoma County. Because of the required rush, 10 local engines immediately headed out from their stations to meet up in Pope Valley. Typically strike teams, of five engines each, meet and then head for their destination together.

One team equipped for wildland fires was assigned to hose down utility poles to keep them from burning all the way through and falling across the road. The team, made up for units from Kenwood, Rohnert Park, Windsor, Bennett Valley and Santa Rosa departments, later was assigned to attack an exposed flank of the fire off Butts Canyon Road.

The other, made up of engines better equipped for urban firefighting from Santa Rosa, Rohnert Park and Forestville, was assigned to protect the Berryessa Estates subdivision, located at the extreme northwestern corner of Lake Berryessa.

Cal Fire called for reinforcements overnight, and by Wednesday morning the number of firefighters on the blaze had grown from several hundred to about 1,000. That included Cal Fire firefighters as well as personnel from numerous county and city agencies.

Authorities closed Butts Canyon Road between Snell Valley Road to the north and Aetna Springs Road to the south, Napa County Sheriff's Capt. Doug Pike said. Deputies helped shut roads and evacuate about a dozen residents from the Butts Canyon area.

The Red Cross began setting up an evacuation center at Pope Valley Union Elementary School, but as the fire forced additional evacuations at Berryessa Estates near Putah Creek north of Lake Berryessa, the evacuation center was moved to a larger venue — Middletown High School.

Temperatures reached the low 90s Tuesday — milder compared to recent days, Berlant said. The winds blew at about 12 mph with gusts up to 15 mph. These conditions normally would not pave the way for such a massive fire, according to Berlant, and it was the extremely dry vegetation that allowed the blaze to get so large.

"It doesn't take much to spark a fire right now," Berlant said. "It's the dry conditions. The brush and trees are very, very dry."

Firefighters used the relatively cool and humid nighttime conditions to create new fire lines to hold back the blaze, Cal Fire officials said. "The weather definitely cooperated with us," Cal Fire Deputy Chief Gabrielle Avina said Wednesday morning.

Despite the favorable conditions, light winds drove the fire north/ northeast, across the Lake County line toward Snell Peak.

Around 6 p.m., surrounded by a moonscape of smoldering ponderosa pines, oaks and manzanita, Mike Elson, a battalion chief with Central Fire, said the size and intensity of the fire meant his crew had a long night ahead.

"We'll be up all night, no doubt," he said.

Staff Writer Randi Rossmann contributed to this report. You can reach Staff Writer Jamie Hansen at 521-5205 or jamie.hansen@pressdemocrat.com and Julie Johnson at 521-5220 or julie.johnson@pressdemocrat.com.