Heavy wind gusts in Northern California led the Bear Fire to spread explosively Tuesday and early Wednesday, forcing expanded evacuation orders of at least 20,000 residents, including in parts of Butte and Yuba counties.

“Today was a critical fire day with extreme fire behavior,” the U.S. Forest Service wrote in a Tuesday night incident update.

The Butte County Sheriff’s Office has issued a flurry of evacuation orders and warnings near the south corner of Lake Oroville, extending west along Highway 162 as far as Foothill Boulevard on the outskirts of Oroville. Evacuations are also mandatory in a less-populated area north of the Feather River, east of Cherokee Road and west of the lake.

The entire city of Oroville, except the immediate area of Oroville Municipal Airport that’s located across Highway 70 from the rest of the city, was under a voluntary evacuation warning as of Wednesday morning. The Butte County Courthouse in Oroville has closed because of the fire.

A vast patch of sparsely populated land east of Highway 70 extending south nearly to the Butte-Yuba county line is also under a warning.

Oroville has a population of close to 20,000. The evacuation warning came around 2 a.m. and was not upgraded to a mandatory order as of 6:45 a.m.

In Yuba County, mandatory evacuation orders were ordered Tuesday for the area of La Porte and New York Flat roads, north of Brownsville. The area includes the communities of Forbestown, Woodleafe, Clipper Mills and Strawberry Valley.

The Bear Fire is part of the North Complex, a lightning-ignited group of fires that started more than three weeks ago in the Plumas National Forest area. The cluster of fires stayed concentrated in Plumas County prior to this week, but the western side of the complex flared up dramatically Tuesday, producing a massive, dark smoke cloud that blanketed the area.

Wind gusts exceeded 50 mph in Northern California throughout much of Tuesday, according to the National Weather Service.

The fire isn’t immediately threatening Paradise, the Butte County town that was devastated in November 2018 by the state’s deadliest-ever wildfire, the Camp Fire. Paradise, along with neighboring Magalia and Concow, are all located a few miles north of the mandatory and voluntary evacuation zones as of Wednesday morning.

But extreme smoke from the Bear Fire, mixed with the morning sun, have turned the skies orange or even blood red in those areas, as well as near Chico. One camera maintained by PG&E located in Jarbo Gap, just west of Highway 70 and a couple of miles east of the Concow Reservoir, showed thick orange smoke blanketing the surrounding hills shortly after sunrise. It’s an unquestionably frightening sight for locals affected by the Camp Fire tragedy.

Cal Fire’s Butte Unit posted photos around 1:30 a.m. from Highway 162 and Forbestown Road, showing the fire “has crested the hill across the lake” from that location. The photos show deep orange flames lighting up the hill.

The Bear Fire jumped the Middle Fork of the Feather River around 10 a.m. Tuesday, the Forest Service said in an evening incident report.

“A dry cold front passing over the fire with very high winds and low humidities rapidly drove the high-intensity crown fire to the southwest,” the Forest Service said.

The Forest Service says the North Complex is collectively 58,404 acres and 37 percent contained as of early Wednesday. It has burned brush and timber since sparking in Plumas National Forest Aug. 17. The eastern half of the fire complex held within its containment lines Tuesday, authorities said.

More than 1,300 firefighters are assigned to the North Complex.

A separate incident in Yuba County first reported Wednesday morning, the Willows Fire, caused the evacuation of approximately 3,000 residents from Loma Rica, Cal Fire said.