(SOUTH LAKE TAHOE, Calif.) — A California wildfire is inching closer to a resort town as fire conditions worsen in the region.

The Caldor Fire has now scorched more than 204,390 acres and is just 20% contained as it burns southeast of Lake Tahoe — raging nearer to the popular ski resort town of South Lake Tahoe.

It is now the 15th-largest fire in California history, destroying or damaging at least 772 structures, including nearly 550 homes, according to officials. Nearly 35,000 structures are still threatened.

Overnight, the fire remained “very active” due to poor humidity — down to 5% in some spots — according to the California Department of Forestry and Fire Protection. But slow growth allowed firefighters to get more containment over the flames.

Those on the front lines are in an uphill battle now, with 35 to 40 mph wind gusts forecast through Wednesday night. Red flag warnings have been issued in California and Nevada.

Nearly 60,000 residents in Amador, El Dorado, Alpine and Douglas counties in California are under evacuation orders, which also expanded to Nevada on Wednesday as a result of the gusty winds and flames moving east.

The air quality in the Lake Tahoe area is nearly 40 times what the World Health Organization deems unhealthy due to the heavy plumes of smoke emitting from the Caldor Fire, according to IQ Air, a data tool that measures and ranks air quality in cities around the world.

There are currently 20 fires burning in California alone, with more than 1.88 million acres burned and 2,700 structures destroyed in the state.

The Dixie Fire — the second-largest and 14th most destructive fire in California history — is still burning in Lassen County in Northern California after it sparked on July 13. The fire has burned through 844,081acres, an area the size of Rhode Island, and is only 52% contained.

The Monument Fire in Trinity County, California — the 20th-largest in the state — has scorched through 174,706 acres and is 29% contained.

More than 15,000 fire personnel have been assigned to the front lines of the wildfire, while more than 59,000 people in California have been evacuated, according to Cal Fire.

About 6.3 million gallons of fire retardant have been dropped from the air this year — a record set by the McClellan Air Tanker Base set for most fire retardant delivered in a single year, Cal Fire reported.

ABC News’ Max Golembo and Jenna Harrison, Kayna Whitworth and Haley Yamada contributed to this report.

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