SOUTH LAKE TAHOE, Calif. (AP) - Flames raced across treetops and through drought-stricken vegetation as firefighters scrambled Wednesday to keep a growing California wildfire from reaching a resort city at the southern tip of Lake Tahoe after evacuation orders were expanded to neighboring Nevada.

Thick smoke from the Caldor Fire enveloped the city of South Lake Tahoe, which was all but deserted during a summer week usually bustling with tourists.

The National Weather Service warned that critical weather conditions through Wednesday could include extremely low humidity, dry fuel and gusts up to 30 mph.

“With those winds, as it ran through the forest it created what’s called an active crown fire run, where the fire actually goes from treetop to treetop,” said Stephen Vollmer, a fire behavior analyst for the California Department of Forestry and Fire Protection.

He said embers were being cast up to a mile out in front of the fire, creating new ignition points, including in some parts of the dense forest that haven’t burned since 1940 or before.

The blaze was 3 miles outside of South Lake Tahoe by Tuesday afternoon, Cal Fire Battalion Chief Henry Herrera told KGO-TV.

A day earlier, roughly 22,000 residents jammed the city’s main artery for hours after they were ordered to leave as the fire advanced.

South Lake Tahoe city officials said only a handful of residents defied Monday’s evacuation order. But nearly everyone worried Tuesday about what the fire would do next.

Tom O’Connell and his wife, Linda, awaited the fate of their home while anchored on their sailboat in Ventura Harbor. The two-bedroom they’ve owned for 40 years survived the Angora Fire that destroyed about 250 houses in 2007. They didn’t know if they’d be lucky again.

“You worry about the things you can have some control over,” O’Connell said. “We’ve no control over this.”

Pushed by strong winds, the Caldor Fire crossed two major highways and swept down slopes into the Tahoe Basin, where firefighters working in steep terrain were protecting remote cabins.

Cal Fire Division Chief Erich Schwab said some homes burned, but it was too early to know how many.

“The fire burned through there extremely fast, extremely hot. And we did the best that we could,” he said Tuesday night.

Thick smoke prevented air firefighting operations periodically last week. But since then, nearly two dozen helicopters and three air tankers dumped thousands of gallons of water and retardant on the fire, fire spokesman Dominic Polito said.

As flames moved toward the Heavenly ski resort on the California-Nevada border, officials turned on the mountain’s snow-making machines. Cal Fire Battalion Chief Jed Gaines told KPIX-TV that spraying the slopes with water was “increasing the humidity level, it’s getting everything wet” so that if the fire starts climbing “it’s able to slow it down.”