"Historic" and "extreme" weather conditions could fan a wildfire in New Mexico which is already the second biggest ever seen in the US state.
The so-called Hermits Peak Fire has been burning for more than a month and has torn through an area larger than the city of Chicago.
Many families have been left homeless and thousands have been evacuated.
Winds, near-record high temperatures and dry conditions are now expected to stoke the blaze further.
The National Weather Service in Albuquerque tweeted that its forecasters are "using exceedingly rare language" in its warning for a "long duration and extreme fire weather event".
State Governor Michelle Lujan Grisham called on people in mandatory evacuation areas to leave immediately.
"Tonight we will enter an exceptionally dangerous period of extreme fire weather. As severe winds pick up, conditions may worsen and air support may be limited," she tweeted.
US President Joe Biden this week declared a major disaster in New Mexico, unlocking federal resources including financial aid for affected individuals.
Restaurants and grocery stores in Las Vegas, a New Mexico city of 13,000 people, have been closed, while schools have either closed or moved to remote-only options.
"It's literally like living under a dark cloud. It's unnerving," Liz Birmingham, a resident of the city, told CBS News.
Elmo Baca, chairman of the Las Vegas Community Foundation, said: "There's uncertainty and there's fear about how the winds are going to affect the fire from day to day.
"Once the people are evacuated out of an area, they can't go back, so they're just stuck worrying."
The fire has blackened more than 267 sq miles (691 sq km).
It is believed to have started on 6 April and has been traced, in part, to a preventive fire initiated by the US Forest Service to reduce flammable vegetation. But the blaze then merged with another wildfire.
The frequency of large wildfires has increased dramatically in recent decades.
Compared with the 1970s, fires larger than 10,000 acres (40 sq km) are now seven times more common in the west of the US, according to Climate Central, an independent organisation of scientists and journalists.