Afternoon winds gusting to 30 mph (48 kph) on Monday pushed the Dixie Fire within a few miles of Susanville and prompted evacuation orders for Janesville, a small nearby mountain community, fire officials said.
Susanville, with about 18,000 people, is the county seat of Lassen County and the largest city that the blaze has approached. The former Sierra Nevada logging and mining town has two state prisons, a nearby federal lockup and a casino.
Ash fell from the advancing fire and a Police Department statement urged residents "to be alert and be ready to evacuate" if the fire threatens the city, although no formal evacuation warning had been issued.
Bulldozers had cut fire lines in the path of the northward-trending blaze but "a lot of our lines are getting tested now," fire spokesman David Janssen said.
The National Weather Service issued a fire weather watch through Thursday in the area because of afternoon winds that could gust to 35 mph (56 kph) at times, propelling flames.
The weather forecast prompted Pacific Gas & Electric to warn that it might cut off power to 48,000 customers in portions of 18 counties from Tuesday evening through Wednesday afternoon to prevent winds from knocking down or hurling debris into power lines and sparking new wildfires. Most of those customers are in Butte and Shasta counties, which have seen a number of deadly and devastating wildfires in recent years, including the Dixie Fire.
Investigations are continuing, but Pacific Gas & Electric has notified utility regulators that the Dixie and Fly fires may have been caused by trees falling into its power lines. The Dixie Fire began near the town of Paradise, which was devastated by a 2018 wildfire ignited by PG&E equipment during strong winds. Eighty-five people died.
The Dixie Fire has scorched more than 900 square miles (2,331 square kilometers) in the northern Sierra Nevada and southern Cascades since it ignited on July 13 and eventually merged with a smaller blaze called the Fly Fire. It's less than a third contained by fire lines.
Ongoing damage surveys have counted more than 1,100 buildings destroyed, including 625 homes, and more than 14,000 structures remained threatened. Numerous evacuation orders were in effect.
The small lumber town of Westwood was still under evacuation orders and protective lines were holding but the blaze remained a threat.
"There are still some people staying in there," Janssen said. "We’re hoping it won’t turn bad in there and we have to change our mission from protecting structures to saving lives ... Our biggest concern right now is that people aren’t taking the evacuation seriously."
California was dealing with several other massive fires, including one that started on Saturday southeast of the Dixie Fire in El Dorado County that had grown to about 3.5 square miles (9 square kilometers) and prompted evacuations.
The Dixie Fire is the largest of nearly 100 major wildfires burning across more than a dozen Western states that have seen historic drought and weeks of high temperatures and dry weather that have left trees, brush and grasslands as flammable as tinder.
Two dozen fires were burning in Montana and nearly 50 more in Idaho, Washington and Oregon, according to the National Fire Interagency Center.
In Montana, a fire near Hays that began on Monday had burned about 8 square miles (20 square kilometers) and residents in and around the tiny enclave of Zortman were told to be on notice for possible evacuation.
The U.S. Forest Service said last week that it is operating in crisis mode, with more than double the number of firefighters deployed than at the same time a year ago. More than 25,000 firefighters, support personnel and management teams were assigned to U.S. blazes.
Climate change has made the U.S. West warmer and drier in the past 30 years and will continue to make the weather more extreme and wildfires more destructive, according to scientists.