JEAN, Nevada (AP) — A brief but heavy downpour Tuesday helped firefighters battling a massive blaze in California and Nevada, but meteorologists warned of the potential for sudden and erratic wind shifts that could endanger crews later on.

The York Fire was partially contained by Tuesday morning after igniting last week in the Mojave National Preserve in California and spreading into Nevada, while flames scorched tens of thousands of acres of desert scrub, juniper and Joshua tree woodland.

Now the largest wildfire of the season in California, the blaze was mapped at roughly 125 square miles (323.7 square kilometres) on Tuesday, with 23 per cent containment.

While the 15-minute downpour early Tuesday helped firefighting efforts, thunderstorms could pose problems if they pass over the area, said Clay Morgan, a National Weather Service meteorologist in Las Vegas.

If the storms miss the flames entirely, crews could face unstable wind conditions — with gusts up to 40 mph (64 kph) — and risk having the fire blown back at them, Morgan said.

The blaze erupted on Friday near the remote Caruthers Canyon area of the vast wildland preserve, crossed the state line into Nevada on Sunday and sent smoke further east into the Las Vegas Valley. The York Fire started on private land within the preserve, according to authorities, and remains under investigation.

Smoky conditions delayed flights at Harry Reid International Airport on Monday, but normal flight schedules appeared to resume by Tuesday morning.

Firefighters battled “fire whirls” on Monday in their struggle to get the flames under control.

A fire whirl — sometimes called a fire tornado — is a “spinning column of fire” that forms when intense heat and turbulent winds combine, according to the National Park Service.

The vortexes — which can be anywhere from a few feet tall to several hundred feet high, with varying rotational speeds — were spotted Sunday on the north end of the York Fire.

“While these can be fascinating to observe they are a very dangerous natural phenomena that can occur during wildfires,” the park service wrote.

Significant portions of the US population have been subject to extreme heat in recent weeks. Worldwide, July was so steamy that scientists calculate it will be the hottest month ever recorded and likely the warmest to hit human civilization.

Experts say plants like blackbrush scrub, pinyon-juniper woodlands and the famous Joshua trees in the New York Mountains in San Bernardino County are at-risk of taking centuries to regrow naturally, if they are ever able to come back.

To the southwest, the Bonny Fire burned about 3.6 square miles (9.3 square kilometres) in the rugged hills of Riverside County. The blaze was about 40 per cent contained on Tuesday morning.