By Curtis Skinner

SAN FRANCISCO (Reuters) - A wildfire running rampant through drought-parched vegetation in northern California grew by several thousand acres overnight into Wednesday, as firefighters worked to stop the flames from spreading toward communities.

The so-called Jerusalem Fire, which was sparked on Sunday, has grown to cover 16,500 acres (6,677 hectares) of dry and rural ranch land north of Napa Valley wine country and is only 6 percent contained, the California Department of Forestry and Fire Protection said.

The fire has displaced about 150 people and threatens at least 50 more houses and ranches, department spokesman Steve Swindle said.

He said that while the burn area is sparsely populated, firefighters were working to keep flames from spreading south and threatening the communities around Lake Berryessa.

Overnight, firefighters merged part of the fire's northeastern edge with a southwestern stretch of a behemoth 69,636-acre (28,180 hectare) blaze nearby known as the Rocky Fire, as a way of burning through combustible vegetation, Swindle said.

"There is a lot of unburned fuel between those two fires," he said.

Letting the vegetation burn through between the two fires will create a safe zone, Swindle said.

The Rocky Fire, the state's fiercest wildfire so far this season, stayed at around 88 percent containment on Wednesday morning, state fire officials said. That inferno has destroyed almost 100 buildings, roughly half of which were homes, and sent hundreds of people fleeing.

Strong and erratic winds were expected to increase the risk of fire in Northern California for several days this week, according to state forestry officials.

The two blazes were among at least two dozen burning across the American West.

The Wolverine Fire in the northern Cascade mountains of Washington state covers 34,500 acres (13,961 hectares) and could grow amid drying, warming conditions through the day, according to the InciWeb fire information center.

That blaze forced about 360 people to be evacuated from a camping area in the year round Lutheran retreat of Holden Village, fire officials said.

(Reporting by Curtis Skinner; Editing by Daniel Wallis)