A colossal wildfire raging on the edge of Yosemite National Park has produced dangerous weather patterns by fuelling thunderous pyrocumulus clouds that can alter the wind direction rapidly, potentially trapping firefighters, forest officials say.
The so-called Rim Fire grew to 134,000 acres (54,000 hectares) by early Sunday morning, up 9000 acres from the day before, with smoke columns rising more than 30,000 feet (9 km), said US Forest Service spokesman Dick Fleishman.
"That's a real watch-out situation for our firefighters when they see that kind of activity, they know that the wind could actually move that fire right back on them," Fleishman said. "That's been happening every afternoon."
The fire was threatening power and water supplies to San Francisco, about 320 km to the west, and scorched part of Yosemite, a park known for its waterfalls, giant sequoia groves and other scenic wonders.
Started on August 17 in the Stanislaus National Forest, the fire remained largely unchecked with extreme terrain and increased wind hampering efforts at containment, Yosemite National Park spokesman Tom Medema said. Seven per cent of the wildfire is now contained, he said.
The fire blackened just over 12,000 acres in the northwest corner of Yosemite by Sunday (local time), up marginally from the day before, consuming brush, oaks and pines and threatened some giant sequoia trees in the park, Medema said.
Officials have closed parts of the park's northwestern edge throughout the week, including the Hetch Hetchy Reservoir area, Lake Eleanor, Lake Cherry and the Tuolumne and Merced giant sequoia groves.
The fire by Sunday burned within 5km of the reservoir and was still 32km from Yosemite Valley, the park's main tourist centre, Medema said. About 2700 firefighters were expected to be on the front lines on Sunday to fight the fire.
Officials said they have no plans to shut down the entire park or its top attractions.
California Governor Jerry Brown on Friday declared a state of emergency for San Francisco, saying the fire had damaged the electrical infrastructure serving the city and forced the Public Utilities Commission to shut down power lines.
The Hetch Hetchy Reservoir provides water to 2.6 million customers in the San Francisco area and Brown in his declaration said the city's water supply could be affected if the blaze harms the reservoir, most likely by contaminating its water with ash.
There was no evidence of ash in the reservoir by Saturday evening, said Tyrone Jue, spokesman for the San Francisco Public Utilities Commission.
San Francisco could draw on water from neighbours if the supply is compromised, he said.
Yosemite, one of the nation's major tourist destinations, attracted nearly 4 million visitors last year. The park has been posting updates and alerts on its website.
The blaze in the western Sierra Nevada Mountains is now the fastest-moving of 50 large wildfires raging across the drought-parched US West. The blazes have strained resources and prompted fire managers to open talks with Pentagon commanders and Canadian officials about possible reinforcements.
A 111,000-acre fire near the resort town of Sun Valley in central Idaho was 82 per cent contained on Sunday as the number of firefighters assigned to the blaze was reduced to several hundred from a high of 1800, officials said.
At its height a week ago, the blaze forced the evacuation of 2250 homes in upscale developments in a scenic river valley known for a world-class ski resort and for premier hiking and biking trails that wind through the Sawtooth Mountains.
The Rim Fire had destroyed 11 homes, 12 outbuildings and four commercial properties by Sunday.
Evacuation advisories were lifted for roughly 2500 residences in two Tuolumne County communities on Saturday, but at least 2000 households were under evacuation advisories, Fleishman said.
Highway 120, one of four access routes to Yosemite, was temporarily closed. The highway leads to the west side of the 750,000-acre (300,000-hectare) national park.
The 2013 fire season has already drained US Forest Service fire suppression and emergency funds, causing the agency to redirect US$600 million (NZ$767m) meant for other projects like campground and trail maintenance and thinning of trees to reduce wildfire risks, agency spokesman Mike Ferris has said.