By Steve Gorman
(Reuters) - California's biggest and fiercest wildfire of the year grew larger as it raged for a seventh day through drought-parched chaparral and ranch lands north of Napa Valley wine country after jumping a highway that authorities hoped would slow its advance.
The blaze, dubbed the Rocky Fire, has charred some 65,000 acres, destroyed more than 50 buildings and displaced thousands of residents since erupting last week in the rugged canyons and foothills east of the town of Clearlake, about 110 miles north of San Francisco.
On Tuesday, nearly 7,000 structures, mostly homes, were listed as threatened, with more than 13,000 people placed under evacuation orders or advisories, according to the California Department of Forestry and Fire Prevention (Cal Fire).
What sparked the Rocky Fire was uncertain, but the blaze is one of nearly two dozen conflagrations that erupted across the state following thousands of dry lightning strikes in recent days, the bulk of them in northern California.
All of the blazes have been fed by thickets of vegetation left desiccated by four years of drought, and fire managers said they were bracing for hot, dry conditions to persist through summer.
Cal Fire resources already were stretched with more than 10,000 firefighters on the front lines statewide on Tuesday and all off-duty personnel called back to work.
At least 3,200 firefighters were assigned to the Rocky Fire alone, backed by 19 water-dropping helicopters, four air tankers and nearly 60 bulldozers, Cal Fire reported.
Firefighters had begun to gain some ground against the blaze as triple-digit temperatures and high winds abated on Monday, helping crews establish containment lines around 12 percent of the perimeter. But late that day, flames roared across a two-lane highway that fire managers were counting on to help block its advance.
By Tuesday morning another 5,000 acres had been scorched, marking a significant growth spurt for the fire but far less than the record 20,000 acres devoured in one five-hour period at its peak intensity on Saturday night. Smoke from the blaze was visible up to 80 miles to the south in Napa, one of California's renowned wine-making regions.
Ground crews focused Tuesday on carving additional fire breaks through chaparral and maintaining what perimeter control they had achieved, officials said.
Rising temperatures and lower humidity were forecast for Wednesday, followed by the possibility of more thunderstorms across the Sierras and coastal mountain ranges on Thursday.
President Barack Obama was briefed on the fires on Tuesday, the White House said. Governor Jerry Brown declared a state of emergency late last week.
Property losses from the Rocky Fire, by far the state's biggest this season, stood at 24 homes and 26 outbuildings that were destroyed last week, and portions of two highways through the area remained closed.
No serious injuries have been reported from the Rocky Fire. But a U.S. Forest Service ranger from South Dakota died last Thursday in a separate, smaller fire in the Modoc National Forest near California's border with Oregon.
(Reporting and writing by Steve Gorman from Los Angeles; Editing by Bill Trott and Cynthia Osterman)