(BCN) Hundreds of firefighters are continuing to battle three large vegetation fires in Sonoma and Napa counties this afternoon.

The biggest of the three blazes is the Yellow Fire in Sonoma County, which has burned at least 125 acres of oak woodland and rolling hills east of Healdsburg, according to Cal Fire.

It started around 1:20 a.m. today as two separate fires in the area of Yellow Jacket Ranch Road and state Highway 128 in Knights Valley, Cal Fire spokeswoman Karen Stasko said. The two small fires later merged.

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There are 114 firefighters battling the Yellow Fire, which was 50 percent contained as of 9 a.m. and is expected to be fully contained by Thursday morning. The blaze damaged about a third of a home in the area, Cal Fire spokeswoman Suzie Blankenship said.

The response includes crews from Cal Fire, Bennett Valley, Sebastopol, Healdsburg, Geyserville, Gold Ridge, Knights Valley, Graton, Kenwood Valley and the Central Fire Authority.

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The Silverado Fire in Napa County has charred about 75 acres of grass and oak woodland north of Yountville and has destroyed one structure, Stasko said. The fire was reported near the 7300 block of Silverado Trail, close to Oakville Cross Road, at 11:44 p.m. Tuesday. It was 75 percent contained as of midday, according Cal Fire.

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— Amy Hollyfield (@amyhollyfield) May 1, 2013

Blankenship said 80 firefighters are at the scene and that the Silverado Fire expected to be contained by about 5 p.m. today.

A third blaze, dubbed the Summit Fire, has burned about 2 acres near the Schramsberg Vineyards south of Calistoga, also in Napa County.

Blankenship said midday that 46 firefighters were battling that blaze, which started at about 5:30 a.m. today near the 1400 block of Schramsberg Road.

No structures have burned and there are no injuries. Firefighters are unable to get engines close to the fire because of the steep terrain and are hiking in with hoses, Blankenship said.

The cause of the fire is under investigation and there is no estimate when it will be contained.

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Wildfires Scorch 200 Acres in Napa and Sonoma Counties