The spread of the Caldor Fire near Lake Tahoe accelerated this weekend and Monday, eluding the work of firefighters to keep the blaze in check. It could move into the Tahoe Basin early this week as the fire danger escalates regionwide in Northern California amid challenging weather.

Mandatory evacuation orders expanded Monday over the Sierra Crest to the shores of the lake and east to the Nevada border. The order includes all of South Lake Tahoe, a city of nearly 22,000. Residents were instructed to evacuate east on Highway 50 toward Nevada.

The Caldor Fire, a massive wildfire that has consumed more than 270 square miles in Northern California, threatened to move into the Lake Tahoe area on Aug. 30. (The Washington Post)

After a surge in federal firefighting resources, officials had hoped to gain the upper hand on the northeastern side of the blaze, which has been a looming threat to the Tahoe area and other Sierra communities.

But on Sunday, the flames moved aggressively amid gusty winds and extremely low humidity. As of Monday morning, the fire had consumed more than 177,000 acres and was 14 percent contained.

“Today it let loose, and we have more predicted weather coming,” Jeff Marsolais, Eldorado National Forest supervisor, said at a community meeting Sunday evening. “I think it’s really critical that the public do what they can to be a part of an orderly evacuation.”

A red-flag warning for critical fire weather is in effect through 11 p.m. Wednesday, with winds from the west and southwest forecast to strengthen. The entire region could see gusts of 25 to 35 mph. Even stronger winds, though, could emerge in the higher reaches of the Sierra Nevada, where the Caldor Fire and parts of the Dixie Fire are burning.

“New or existing wildfires will have the potential for rapid spread and growth under these conditions,” the National Weather Service warned.

Several small towns along Route 50 between Sacramento and Lake Tahoe are also threatened. Firefighters were actively defending structures in this area and had burned the slopes above the highway in an effort to divert the fire around the town of Strawberry.

A number of elements will come together this week that could cause the fire to move quickly into the Lake Tahoe Basin.

Winds, low humidity and critically dry vegetation will be in “full alignment” Monday and Tuesday, according to the Predictive Services fire-weather outlook for Northern California.

Fires tend to burn faster up-slope, and westerly winds lining up with the slopes of the Sierra will accelerate fire spread.

The Caldor Fire has been difficult to contain in part because embers cast from the main fire are easily igniting new fires some distance away, a process known as “spotting.” The California Department of Forestry and Fire Protection (Cal Fire) reported spotting distances of up to one mile Sunday. Given the tinder-dry vegetation, there is a high probability of such ignitions, particularly during high winds.

With a summer of extreme heat and drought, fires are growing quickly even when winds are light. “Fires burning in northern California are exhibiting extreme fire growth based on critical fuel conditions,” according to an advisory issued by the National Interagency Fire Center. “Fire spread is fuel driven and does not depend on wind to spread.”

The 765,000-acre Dixie Fire was “very active” Sunday and “stood up a sizable and very visible column,” according to an InciWeb report. Other large blazes continue to burn in Northern California forests, including the River Complex, the Antelope Fire, the Monument Fire and the McCash Fire.

Last week, the U.S. Forest Service closed nine national forests through Sept. 6 “to better provide public and firefighter safety” amid the extreme fire conditions in Northern California and strained firefighting resources across the country.

Fire activity has also increased in Southern California, driven by hot and dry weather over the past week. The season in the south had been slower than in the north, due in part to more-moderate temperatures and rain driven by the Southwest monsoon over inland mountains.

This year, 1,683,620 acres have been consumed by fire in California, surpassing the 1,633,659 acres burned by this time last year.

“Fires are burning in ways that nobody has seen before,” Cal Fire Director Thom Porter said at a news conference Aug. 23. “Yes, I keep saying that … but it is absolutely true.”

Diana Leonard is a science writer covering natural hazards in California.