“It was a first for us,” said Shane Snyder, meteorologist with the National Weather Service in Reno, which issued the warning shortly before 3 p.m.

Multiple videos posted to social media showed twister-like formations in the path of the Loyalton fire, which started Friday evening in the Tahoe National Forest near California’s border with Nevada.

The fire quickly grew to 20,000 acres and was zero per cent contained as of Sunday morning.

Authorities were performing updated flight mapping and expected the acreage to rise, said Joe Flannery, public affairs officer for the national forest.

“Our resources on the ground are facing extreme fire behavior, rugged terrain and warm temperatures,” Flannery said.

Evan Bentley, severe weather meteorologist with the National Weather Service Storm Prediction Center, wrote on Twitter that radar data showed at least four “distinct anticyclonic circulations” associated with the fire on Saturday.

One was present for more than an hour and traveled about four miles, he wrote.

The extreme weather phenomenon is believed to have been sparked by the rapid growth and intensity of the blaze.

“It was hot; it was very unstable atmospherically,” Snyder said in an interview, “and that allowed the fire, which is burning very hot and (through) lots of fuel, to really explode up in a vertical sense, up into the atmosphere.”

The hotter the air, the more rapidly it rises, he said.

“Hot air wants to rise, and if it’s very hot it wants to rise dramatically.

“It’s allowed to rise because the temperature of the air the fire makes is much warmer than the air around it.

“So it keeps rising until it’s not warmer than the air around it,” Snyder said.

That can send a column of smoke up tens of thousands of feet into the atmosphere, he said.

And as it rises, the air underneath it needs to be replaced, creating a vortex that pulls in air from all around it.

At the same time, changes in wind speed that occur higher up in the atmosphere cause the air to spin as it accelerates upward.

“With that rapidly rising, then you spin up your fire tornado,” Snyder said, adding: “It starts spinning as it rises.”

Fire tornadoes are rare but not unheard of in California.

A spinning vortex that barreled into Redding during the Carr fire in July 2018 was roughly 1,000 feet in diameter and reached speeds of 136-165 mph, equivalent to a twister with a rating of EF-3 on the five-level Enhanced Fujita scale, according to the California Department of Forestry and Fire Protection.

The fire tornado killed a firefighter as he raced toward a neighborhood in flames, an investigation later found.

In 2008, a whirl caught firefighters by surprise in the remote Indians fire, which was burning in extremely dry chaparral in the Los Padres National Forest.

It caused serious injuries and forced the deployment of fire shelters.

Most recently, on Wednesday, a flaming tornado was seen spinning out from the Lake fire, hours after the blaze erupted in the Angeles National Forest  above  Lake Hughes.

“The fire grew from 400 to 10,000 acres very, very quickly, in the span of a couple hours,” Jake Miller, public information officer for the Lake fire, said Sunday.

“So that’s when you start seeing those fire tornados and fire whirls starting out,” he added.

The fire had grown to 17,862 acres and was 12 per cent contained as of Sunday morning.

At least 12 homes and commercial buildings had been destroyed, and about 250 people remained under evacuation orders.

Though its growth had slowed considerably, challenges remained. Triple-digit temperatures, low relative humidities and heavy fuels combined to create the potential for continued spread, Miller said.

He added that the most active flank of the fire was the northwestern portion, which was burning into a remote area largely inaccessible by roads.

“It’s an area that just hasn’t burned in about 100 years, so there’s a lot of active fuel.

“That’s why we keep getting this afternoon activity – it heats up and kind of sparks the fire up again,” Miller said.

“Then we have the mixture of that with the fuel out there, and the combination of those two kind of reignites the fire.”

As the fire chews through the sunbaked brush dried out by the afternoon heat, it sends up a large plume of smoke, which in turn creates its own problems by generating erratic gusty winds that fling embers into the air, he said.