Northern California's Mosquito Fire in the Sierra Nevada foothills swelled to 63,776 acres on Wednesday evening after "critically dry fuels" drove rapid growth, per Cal Fire.

Threat level: Evacuation orders and warnings remained in place for affected communities northeast of Sacramento as the blaze burned across El Dorado and Placer counties at 20% containment and became the largest recorded in California this year.

• As poor air quality looms, schools in the area halted in-person classes, including in the Washoe County School District, The University of Nevada, Reno and Truckee Meadows Community College, AP reports.

State of play: Lighter southwesterly winds than Tuesday helped clear some of the air in fire-affected areas and firefighters were holding the Mosquito Fire inside control lines with the help of cooler temperatures and higher humidity, according to Cal Fire's incident report.

Yes, but: "The fire remains active on this eastern front, steadily burning and advancing in heavy unburned fuels," Cal Fire said.

• The priority was "strengthening and securing the southwest corner" of the fire that ignited Sept. 6 "to protect and defend the communities of Foresthill and Todd Valley."

What they're saying: Cal Fire fire behavior analyst Jonathan Pangburn said during a briefing earlier this week that authorities had expected an increase in fire behavior in the area due to "historically dry fuels."

• Pangburn compared the dry fuel conditions firefighters were facing in tackling the Mosquito Fire to the King Fire, east of Sacramento, which burned more than 97,000 acres in 2014 — when they deployed emergency fire shelters in some areas due to the rapidly moving blaze.
• "Those events were similar to this, not wind-driven events, just fuel that was primed and ready and decided to get up with a hurry and put some people in some dangerous situations," Pangburn said, per the Los Angeles Times.
• "We're ready for an absolute blowup out here."

The big picture: California Gov. Gavin Newsom last week declared a state of emergency for counties threatened by the Mosquito and Fairview Fires.

• The Fairview Fire, near Hemet, which killed two people earlier this month, has destroyed more than 28,300 acres in Riverside County and was 75% contained as of Wednesday, according to Cal Fire.

By the numbers: 91 large fires were burning on Wednesday across eight states in the Western U.S. — including 10 in California, according to the latest information from the National Interagency Fire Center.

• The McKinney Fire, near the border with Oregon, which was previously the largest wildfire to burn in California this year, has razed 60,138 acres and was 99% contained on Wednesday, according to Inciweb.

Context: Most of the U.S. West is experiencing drought conditions — particularly the Southwest, where an ongoing megadrought is the most severe such event in at least 1,200 years, per Axios' Andrew Freedman.

• Climate change is ratcheting up the severity of extreme weather events, Freedman notes.

Go deeeper: One extreme climate event can worsen others, studies show

Editor's note: This article has been updated with new details throughout.