The numbers are staggering and the prospects are absolutely scary as a massive California wildfire menaces Yosemite National Park and San Francisco's water supply. The tourist hotspot Yosemite Valley and its iconic attractions, including the El Capitan rock formation, currently are safe, miles from the Rim Fire's reach. However, the blaze continues to spread toward the Hetch Hetchy Reservoir, which serves 2.6 million customers in the Bay Area. (30160)

YOSEMITE NATIONAL PARK, Calif. — These are scary times for the folks of Tuolumne, a picturesque northern California community threatened by a historic wildfire.

Fire crews are doing their best, but just a spark or two could send the inferno raging down their canyon and into their town.

“It is dangerous for them right now,” said Cal Fire Division Chief Mike Ramirez. “When you’re on a ridge like this and the fire is below you, that is a recipe for disaster.”

Tuolumne is on the northern edge of the Rim Fire, a monster of a wildfire that has charred more than 187,000 acres. It’s the seventh-largest blaze in California history, requiring the efforts of almost 4,200 firefighting personnel.

The blaze was 27% contained as of midday Wednesday, Ramirez said, and that number is expected to rise after firefighters set “back fires” Tuesday night, burning out about four miles on the northwestern edge of the fire.

Teams were waiting for the smoke that had settled overnight to rise before determining how effective the back fires had been, he said.

While humidity was higher Wednesday, roughly 24%, Ramirez said fire crews were concerned that thunderstorms could create erratic wind patterns that might complicate firefighting efforts.

“To be this close is to be scared,” said Laura Murphy, a wildlife rehabilitator who lives in Tuolumne with her husband. “But we’re ready, we’re packed, we’ve already moved the animals.

Despite the threat, the fire is fascinating.

“We will leave when we have to, but its amazing to watch,” she said.

A threat to the Bay Area

Nearby Yosemite Valley and its iconic attractions, including the El Capitan rock formation, currently are safe, miles from the Rim Fire’s reach. However, the blaze was on the edge of the Hetch Hetchy Reservoir, which serves 2.6 million customers in the San Francisco Bay Area.

The fire is wrapping around the south side of the reservoir and extends about 3/4 of a mile past O’Shaughnessy Dam to the south and east. Firefighters are at Hetch Hetchy protecting critical facilities, the San Francisco Public Utilities Commission said Wednesday.

Water quality remains unchanged, despite ash that has fallen on the 459-square-mile reservoir, because the water is drawn from a depth of 260 feet, the commission said.

The city was already transferring 275 million gallons of water daily from Hetch Hetchy to other reservoirs because Hetch Hetchy is full, but as a precaution, the city increased that amount by 27 million gallons, a Tuesday statement said.

The commission said it was confident the Rim Fire would not affect the reservoir: “Due to the rocky, granite terrain and limited brush along the perimeter of the reservoir, there is little risk for direct water quality impacts.”

The fire could threaten the area’s hydroelectric generators, which provide much of San Francisco’s electricity.

Because of the approaching flames, officials shut down the generators, and the city — more than 120 miles to the west — temporarily is getting power from elsewhere.

“All of San Francisco’s municipal electric customers continue to be fully supplied; there will be no interruption in electric service,” the commission said.

More evacuations possible

The Tuolumne County Sheriff’s Office, meanwhile, issued evacuation advisories for Tuolumne and nearby Ponderosa Hill, said InciWeb, a federal website that collects information from agencies like the U.S. Forest Service and Bureau of Land Management. It was not clear how many residents were covered by the evacuation advisory.

Evacuations also remained in place for areas south of Highway 120 (including Scotts Ridge), north of Old Yosemite Road and along the Highway 108 corridor between Tuolumne City and Pinecrest.

Authorities say the Rim Fire started on August 17. The cause is under investigation.

“There’s a lot of concern, and there’s a lot of work to be done,” U.S. Forest Service spokesman Lee Bentley said.

Firefighters on Tuesday were able to build fire lines in several locations. The forecast is also looking favorable as daytime highs cool into the 70s over the Labor Day weekend — a break from the temperatures in the high 80s firefighters were facing Wednesday.

“Access and difficult terrain remain concerns for crews and equipment,” said an incident report. “Rapid fire growth and extreme fire behavior are hampering suppression efforts.”

As many as 20 helicopters and air tankers were aiding the efforts.

A top priority is stopping the fire from spreading farther in Yosemite National Park.

A portion of Tioga Road in the park will be closed for several days beginning Wednesday so that firefighters can perform fire suppression.

The park has closed a few other roads, campgrounds and wildlife trails while restricting smoking and building camping or cooking fires in wilderness areas.

The Merced and Tuolumne giant sequoia groves, while not in imminent danger, were closed as firefighters conduct preventative measures to protect the majestic trees.

“The work that will be performed over the next few days is instrumental in suppressing the Rim Fire within Yosemite. The safety of the firefighters working along the road is our paramount concern,” park Superintendent Don Neubacher said in a statement.

Tamarack Flat and Yosemite Creek campgrounds, both along Tioga Road, will remain closed.

Yosemite, with hundreds of campground sites and lodging units, had nearly 4 million visitors last year, the National Park Service said. The park typically has 15,000 visitors on a busy summer weekend.

While the Yosemite Conservancy says the Rim Fire has consumed tens of thousands of acres in the park, it has had little or no direct impact on Yosemite Valley, a popular spot for tourists and home to many of the famous cliffs and waterfalls in the park.

About 4,500 structures, many of them vacation homes, were under threat, InciWeb said. The fire has cost more than $20 million, Bentley said.

Thirty-one residences and 80 outbuildings have been destroyed.

CNN’s Eliott C. McLaughlin and Christopher Laible contributed to this report.

Gary Tuchman and Ed Payne |CNN