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.