The 11-day-old massive wildfire burning forest lands in the western Sierra Nevada, including parts of Yosemite National Park, has scorched 288 square miles as of Tuesday night. Officials said they expected the blaze to continue to grow overnight.
Flames are pressing into Yosemite National Park from the west. Fire officials have closed Tioga Pass Road, which is the main access road from the east. It will be closed through Labor Day, which means thousands of would-be visitors will be affected.
Weather was set to play a big role on the blaze as firefighters say the major problem on their hands remains protecting water and power sources.
According to Matt Mehle, a National Weather Service meteorologist assigned to the fire, an expected increase in humidity could help supress flames Tuesday.
"The next couple of days are really going to be key for us," said California fire spokesman Daniel Berlant. "If the weather cooperates, and we see an increase in containment, we could really turn a corner on this stubborn fire."
Families remain on edge and fear the danger is not over yet. If the flames spread across the Tuolumne River Canyon, the homes would be among the first to go.
"They did say you'd better be ready to go when we tell you," said Ed Neri of Tuolumne.
On Monday, the fire edged closure to the Hetch Hetchy Reservoir, the source of San Francisco's famously pure drinking water.
Utility officials monitored the clarity of the Hetch Hetchy Reservoir and used a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the big city.
The fire - now the seventh-largest California wildfire in records dating to 1932 - was threatening about 4,500 structures and has destroyed at least 111. At least 31 residences were among those lost.
Firefighting efforts have already cost more than $44 million since the fire erupted Aug. 17. That's only a fraction of what's estimated to be spent during this year's fire season.
The cause of the fire remains under investigation.
The Associated Press contributed to this report.