ST. LOUIS — The region was greeted Monday morning by more than a heat warning. Opaque, gray skies obscured the sun and the tops of tall buildings, and the stench of smoke permeated the air.

For some, it prompted a question that has become a sign of the times in 2023: Was it just fog? Or another round of wildfire smoke that has choked much of North America throughout the summer as fires raged across the continent?

Turns out that it was both fog and smoke, albeit from a fire closer to home, said the local forecast office of the National Weather Service. A fire that took off Sunday at a sprawling Madison salvage yard, in the Metro East, billowed smoke into the air from tires and cars and even forced the closure of a nearby highway ramp.

And the conflagration roughly coincided with a period of stagnant air conditions as well as the formation of fog blanketing the region, said Lydia Jaja, a NWS meteorologist.

“The air is just so stagnant and trapped near the surface that that smoke doesn’t have anywhere to go,” she said.

Clearer, sunny conditions emerged Monday afternoon. And the fire remained stubbornly difficult to put out: Officials in Madison said the blaze at 111 Salvage — which has 10 acres of cars and trucks, according to its website — was still burning as of the early afternoon.

An air quality alert was in effect for the area Monday, warning of unhealthy conditions for certain groups, including children, the elderly and those with heart or lung problems. “Less intense” outdoor activities were encouraged, according to the NWS.

Air quality wasn’t the only threat in play Monday. The area faced a second straight day of dangerous heat and high temperatures soaring around 100 degrees. Meanwhile, the heat index, which takes humidity into account to register how hot it feels, has reached far higher, with Monday approaching Sunday’s staggering heat index of 117 degrees — the fourth-highest ever recorded in St. Louis and the hottest since 1995.

The run of scorching temperatures aren’t just a bad roll of the meteorological dice but an expected consequence as climate change makes heat extremes increasingly likely to occur.

The overseer of the region’s power grid said Monday that it is bracing for near-record electricity demand.

The heat wave has prompted warnings not only for its extreme highs but for its unrelenting persistence, with overnight lows hovering around 80 degrees — threatening to deprive the body of breaks from the heat.

With similar conditions expected to last until at least Thursday, the St. Louis area could be in for its longest recorded stretch of temperatures that never dip below 80 degrees, Jaja said.

“Don’t underestimate the nighttime temperatures,” she warned. “Duration is one of the top reasons that heat is a killer.”